1. INTRODUCTION

1.1 Introduction to Communication

Since the dawn of civilization communication has been of primary importance to

human beings. “Communication” is the process of conveying intelligence or message from

one place to another. At first, communication was achieved by sound through voice. As the

distance of communication increased, various devices such as line telegraphy, line telephony,

visual signaling was introduced. It has been only very recently in human history that the

electromagnetic spectrum, outside the visible region, has been employed for communication,

through the use of radio.

Figure 1.1 Electromagnetic spectrum

Communication can be further classified into two types i.e. communication through

wires and cables, the other one being wireless communication. In earlier days communication

was possible through landlines that are connected using cables. But wireless communication

is being used extremely because it is the most fascinating and popular system of

communication these days. In wireless communication system people can communicate

through radios, cell phones, internet etc.

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The wireless communication is also called as radio communication. The

electromagnetic waves are utilized in this type of communication. Electromagnetic waves

consist of electric and magnetic fields always perpendicular to each other which are

perpendicular to the direction of propagation. For this system, the antenna is considered to

be one of the most critical components. An antenna in advanced wireless system is usually

required to optimize the radiation energy in some directions and suppress it in others.

1.2 Historical Advancement

The history of antennas dates back to James Clerk Maxwell who unified the theories

of electricity and magnetism, and eloquently represented their relations through a set of

profound equations best known as Maxwell’s Equations. His work was published in 1873.

The first antennas were employed by Hertz in 1887 in his classic demonstration of

electromagnetic waves.

In 1897, Guglielmo Marconi described a complete system for wireless telegraphy. It

was not until 1901 that Marconi was able to send signals over large distances. He performed

the first transatlantic transmission in 1901.

From Marconi’s inception through the 1940s, antenna technology was primarily

centered on wire related radiating elements and frequencies up to about UHF. It was not until

World War ІІ that modern antenna technology was launched and new elements (such as

waveguide aperture, horns, reflectors) were primarily introduced. A contributing factor to this

newεra was the invention of microwaves sources (such as the klystron and magnetron) with

frequencies of 1 GHz and above.

World War ІІ launched a newεra in antennas, which had a major impact on the

advance of modern antenna technology. They are expected to have an even greater influence

on antenna engineering into the twenty-first century.

1.3 Microwave Frequency Bands

The microwave signals exist at microwave frequencies. The term “microwave”

indicates the wavelengths in the micron region. This means microwaves frequencies are up to

infrared and visible light regions means 1GHz to 10^6 GHz. Microwaves are nothing but

electromagnetic waves.

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The various microwave frequency band designations are listed in the table. This

classification has been given according to the Institute of Electrical and Electronics Engineers

(IEEE).

Band Designation Frequency range in GHz

L band

S band

C band

X band

Ku band

K band

Ka band

Millimeter

Sub millimeter

1.000- 2.000

2.000- 4.000

4.000- 8.000

8.000- 12.000

12.000- 18.000

18.000- 27.000

27.000- 40.000

40.000- 300.000

>300.000

3 | P a g eTable 1.1 IEEE Microwave frequency bands

2. FUNDAMENTALS OF ANTENNA

2.1 Basics of Antenna

In large distance communication the use of wires or waveguide is impossible,

impractical or uneconomic. Under these circumstances antennas find their applications and

play a vital role in the process of communication. Antenna helps to convey electromagnetic

energy from one place to another.

Antenna Definition

Antennas are the metallic structures designed for radiating and receiving

electromagnetic energy. The official definition of the antenna according to the IEEE is simply

“a part of a transmitting or receiving system that is designed to radiate or receive

electromagnetic waves”.

Antenna provides a means for transmitting and receiving radio waves. Thus, an

antenna is defined as “a transitional structure between free space and a guided device”. The

guided device or the transmission line may take the form of a coaxial line or a hallow pipe

(waveguide), and it is used to transport EM energy from transmitting source to the antenna or

from antenna to the receiver.

Antennas are reciprocal devices i.e. they behave the same way while transmitting or

receiving radio waves. In receiving mode, antennas collect incoming waves and direct to a

common feed point where transmission line is attach. An important part of any antenna sub-

system is the transmission line used to connect the transmitter or receiver to the antenna. The

line connects to the antenna at its input terminals or input port.

2.2 History of Antenna

2.2.1 Origin of Antennas:

The origin of the word antenna relative to wireless apparatus is attributed to

Guglielmo Marconi. In 1895, while testing early radio apparatuses in the Swiss Alps at

Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless

equipment. A 2.5 meter long pole, along which was carried a wire, was used as a radiating

and receiving aerial element. In Italian a tent pole is known as antenna centrale, and the pole

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with a wire alongside it used as an aerial was simply called antenna. Until then wireless

radiating transmitting and receiving elements were known simply as aerials or terminals.

Marconi's use of the word antenna (Italian for pole) would become a popular term for what

today is uniformly known as the antenna.

A Hertzian antenna is a set of terminals that does not require the presence of a ground

for its operation. A loaded antenna is an active antenna having an elongated portion of

appreciable electrical length and having additional inductance or capacitance directly in

series or shunt with the elongated portion so as to modify the standing wave pattern existing

along the portion or to change the effective electrical length of the portion. An antenna

grounding structure is a structure for establishing a reference potential level for operating the

active antenna. It can be any structure closely associated with (or acting as) the ground which

is connected to the terminal of the signal receiver or source opposing the active antenna

terminal.

In colloquial usage, the word antenna may refer broadly to an entire assembly

including support structure, enclosure (if any), etc. in addition to the purely functional

components. Antennas have practical uses for the transmission and reception of radio

frequency signals such as radio and television. In air, those signals travel very quickly and

with a very low transmission loss. The signals are absorbed when moving through more

conductive materials, such as concrete walls or rock. When encountering an interface, the

waves are partially reflected and partially transmitted through.

A common antenna is a vertical rod a quarter of a wavelength long. Such antennas are

simple in construction, usually inexpensive, and both radiate in and receive from all

horizontal directions (omni directional). One limitation of this antenna is that it does not

radiate or receive in the direction in which the rod points. This region is called the antenna

blind cone or null.

2.2.2 Types of Antennas:

There are two fundamental types of antenna directional patterns, which, with

reference to a specific two dimensional plane (usually horizontal [parallel to the ground] or

[vertical perpendicular to the ground]), are either:

1. Omni-directional (radiates equally in all directions), such as a vertical rod (in the

horizontal plane) or

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2. Directional (radiates more in one direction than in the other).

In colloquial usage "Omni-directional" usually refers to all horizontal directions with

reception above and below the antenna being reduced in favor of better reception (and thus

range) near the horizon. A "directional" antenna usually refers to one focusing a narrow beam

in a single specific direction such as a telescope or satellite dish, or, at least, focusing in a

sector such as a 120° horizontal fan pattern in the case of a panel antenna at a cell site.

2.2.3 Basic Models of Antennas:

There are many variations of antennas. Below are a few basic models. More can be

found in Radio frequency antenna types.

The isotropic radiator is a purely theoretical antenna that radiates equally in all

directions. It is considered to be a point in space with no dimensions and no mass.

This antenna cannot physically exist, but is useful as a theoretical model for

comparison with all other antennas. Most antennas' gains are measured with reference

to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic

radiator).

The dipole antenna is simply two wires pointed in opposite directions arranged either

horizontally or vertically, with one end of each wire connected to the radio and the

other end hanging free in space. Since this is the simplest practical antenna, it is also

used as a reference model for other antennas; gain with respect to a dipole is labeled

as dBd. Generally, the dipole is considered to be omnidirectional in the plane

perpendicular to the axis of the antenna, but it has deep nulls in the directions of the

axis. Variations of the dipole include the folded dipole, the half wave antenna, the

ground plane antenna, the whip, and the J-pole.

The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements

added which are functionality similar to adding a reflector and lenses (directors) to

focus a filament light bulb.

The random wire antenna is simply a very long (at least one quarter wavelength) wire

with one end connected to the radio and the other in free space, arranged in any way

most convenient for the space available. Folding will reduce effectiveness and make

theoretical analysis extremely difficult. (The added length helps more than the folding

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typically hurts.) Typically, a random wire antenna will also require an antenna tuner,

as it might have random impedance that varies non-linearly with frequency.

The horn is used where high gain is needed, the wavelength is short (microwave) and

space is not an issue. Horns can be narrow band or wide band, depending on their

shape. A horn can be built for any frequency, but horns for lower frequencies are

typically impractical. Horns are also frequently used as reference antennas.

The parabolic antenna consists of an active element at the focus of a parabolic

reflector to reflect the waves into a plane wave. Like the horn it is used for high gain,

microwave applications, such as satellite dishes.

The patch antenna consists mainly of a square conductor mounted over a ground

plane. Another example of a planar antenna is the tapered slot antenna (TSA), as the

Vivaldi-antenna.

2.3 Basic Antenna Characteristics

An antenna is a device that is made to efficiently radiate and receive radiated electromagnetic

waves. There are several important antenna characteristics that should be considered when

choosing an antenna for your application as follows:

Antenna radiation patterns

Gain

Power Gain

Directivity

Polarization

2.3.1 Radiation pattern

Practically any antenna cannot radiate energy with same strength uniformly in all directions.

The radiation from antenna in any direction is measured in terms of field strength at a point

located at a particular distance from antenna. Radiation pattern of an antenna indicates the

distribution of energy radiated by the antenna in the free space. In general radiation pattern is

a graph which shows the variation of actual field strength of electromagnetic field of all the

points equidistant from antenna. The two basic radiation patterns are field strength radiation

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pattern which is expressed in terms of field strength E (in V/m) and power radiation pattern

expressed in terms of power per unit solid angle.

Field radiation pattern is a 3-dimensional pattern. To achieve this it requires representing the

radiation for all angles of Φ and θ which give E-plane (vertical plane) and H-plane

(horizontal plane) pattern respectively.

2.3.2 Gain

Antenna gain relates the intensity of an antenna in a given direction to the intensity

that would be produced by a hypothetical ideal antenna that radiates equally in all directions

(isotropically) and has no losses. Since the radiation intensity from a lossless isotropic

antenna equals the power into the antenna divided by a solid angle of 4π steridians, we can

write the following equation:

Gain = 4π * Radiation Intensity/Antenna Input Power

2.3.3 Power Gain

The ratio of the power radiated in a particular to the actual power input to the antenna

is called power gain of the antenna. The maximum power gain can be defined as the ratio of

the maximum radiation intensity to the radiation intensity due to isotropic lossless antenna.

2.3.4 Directivity

The directive gain of the antenna is the measure of the concentration of radiated

power in a particular direction. It may be regarded as the ability of the antenna to direct

radiated power in a given direction. It is usually a ratio of radiation intensity in a given

direction to the average radiation intensity.

2.3.5 Polarization

Polarization is the orientation of the electromagnetic waves far from the source. There

are several types of polarization that apply to antennas. They are Linear (which comprises

vertical and horizontal), oblique, Elliptical (left hand and right hand polarizations), circular

(left hand and right hand) polarizations.

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2.4 History of Microstrip Antennas:

A microstrip antenna consists of conducting patch on a ground plane separated by

dielectric substrate. This concept was undeveloped until the revolution in electronic circuit

miniaturization and large-scale integration in 1970. After that many authors have described

the radiation from the ground plane by a dielectric substrate for different configurations. The

early work of Munson on micro strip antennas for use as a low profile flush mounted

antennas on rockets and missiles showed that this was a practical concept for use in many

antenna system problems. Various mathematical models were developed for this antenna and

its applications were extended to many other fields. The number of papers, articles published

in the journals for the last ten years, on these antennas shows the importance gained by them.

The micro strip antennas are the present day antenna designer’s choice.

Low dielectric constant substrates are generally preferred for maximum radiation. The

conducting patch can take any shape but rectangular and circular configurations are the most

commonly used configuration. Other configurations are complex to analyze and require

heavy numerical computations. A microstrip antenna is characterized by its Length, Width,

Input impedance, and Gain and radiation patterns. Various parameters of the microstrip

antenna and its design considerations were discussed in the subsequent chapters. The length

of the antenna is nearly half wavelength in the dielectric; it is a very critical parameter, which

governs the resonant frequency of the antenna. There are no hard and fast rules to find the

width of the patch.

2.5 Waves on Microstrip

The mechanisms of transmission and radiation in a microstrip can be understood by

considering a point current source (Hertz dipole) located on top of the grounded dielectric

substrate (fig. 1.1) This source radiates electromagnetic waves. Depending on the direction

toward which waves are transmitted, they fall within three distinct categories, each of which

exhibits different behaviors.

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Fig. 2.1 Hertz dipole on a microstrip substrate

2.5.1 Surface Waves

The waves transmitted slightly downward, having elevation angles θ between π/2and

π -arcsin (1/√εr), meet the ground plane, which reflects them, and then meet the dielectric-to-

air boundary, which also reflects them (total reflection condition). The magnitude of the field

amplitudes builds up for some particular incidence angles that leads to the excitation of a

discrete set of surface wave modes; which are similar to the modes in metallic waveguide.

The fields remain mostly trapped within the dielectric, decaying exponentially above the

interface (fig1.2). The vector α, pointing upward, indicates the direction of largest

attenuation. The wave propagates horizontally along β, with little absorption in good quality

dielectric. With two directions of α and β orthogonal to each other, the wave is a non-uniform

plane wave. Surface waves spread out in cylindrical fashion around the excitation point, with

field amplitudes decreasing with distance (r), say1/r, more slowly than space waves. The

same guiding mechanism provides propagation within optical fibers. Surface waves take up

some part of the signal’s energy, which does not reach the intended user. The signal’s

amplitude is thus reduced, contributing to an apparent attenuation or a decrease in antenna

efficiency. Additionally, surface waves also introduce spurious coupling between different

circuit or antenna elements. This effect severely degrades the performance of microstrip

filters because the parasitic interaction reduces the isolation in the stop bands. In large

periodic phased arrays, the effect of surface wave coupling becomes particularly obnoxious,

and the array can neither transmit nor receive when it is pointed at some particular directions

(blind spots). This is due to a resonance phenomenon, when the surface waves excite in

synchronism the Floquet modes of the periodic structure. Surface waves reaching the outer

boundaries of an open microstrip structure are reflected and diffracted by the edges. The

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diffracted waves provide an additional contribution to radiation, degrading the antenna

pattern by raising the side lobe and the cross polarization levels. Surface wave effects are

mostly negative, for circuits and for antennas, so their excitation should be suppressed if

possible.

Fig 2.2 Surface waves on Microstrip

2.5.2 Leaky Waves

Waves directed more sharply downward, with θ angles between π - arcsin (1/√εr) and

π,are also reflected by the ground plane but only partially by the dielectric-to-air boundary.

They progressively leak from the substrate into the air (Fig 1.3), hence their name laky

waves, and eventually contribute to radiation. The leaky waves are also non-uniform plane

waves for which the attenuation direction α points downward, which may appear to be rather

odd; the amplitude of the waves increases as one moves away from the dielectric surface.

This apparent paradox is easily understood by looking at the figure 1.3; actually, the field

amplitude increases as one move away from the substrate because the wave radiates from a

point where the signal amplitude is larger. Since the structure is finite, this apparent divergent

behavior can only exist locally, and the wave vanishes abruptly as one crosses the trajectory

of the first ray in the figure.

In more complex structures made with several layers of different dielectrics, leaky

waves can be used to increase the apparent antenna size and thus provide a larger gain. This

occurs for favorable stacking arrangements and at a particular frequency. Conversely, leaky

waves are not excited in some other multilayer structures.

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Fig 2.3 Leaky waves on Microstrip

2.5.3 Guided Waves

When realizing printed circuits, one locally adds a metal layer on top of the substrate,

which modifies the geometry, introducing an additional reflecting boundary. Waves directed

into the dielectric located under the upper conductor bounce back and forth on the metal

boundaries, which form a parallel plate waveguide. The waves in the metallic guide can only

exist for some Particular values of the angle of incidence, forming a discrete set of waveguide

modes. The guided waves provide the normal operation of all transmission lines and circuits,

in which the electromagnetic fields are mostly concentrated in the volume below the upper

conductor. On the other hand, this buildup of electromagnetic energy is not favorable for

patch antennas, which behave like resonators with a limited frequency bandwidth.

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3. MICROSTRIP ANTENNA

3.1 Introduction to Microstrip Antenna

Microstrip antennas are most popular and attractive for low profile applications at

frequencies above 1 GHz due to their light weight, conformability and low cost. Micro strip

antennas are attractive due to their light weight, conformability and low cost. These antennas

can be integrated with printed strip-line feed networks and active devices. This is a relatively

new area of antenna engineering. The radiation properties of micro strip structures have been

known since the mid 1950’s.

The application of this type of antennas started in early 1970’s when conformal

antennas were required for missiles. Rectangular and circular micro strip resonant patches

have been used extensively in a variety of array configurations. A major contributing factor

for recent advances of micro strip antennas is the current revolution in electronic circuit

miniaturization brought about by developments in large scale integration. As conventional

antennas are often bulky and costly part of an electronic system, micro strip antennas based

on photolithographic technology are seen as an engineering breakthrough.

In its most fundamental form, a Micro strip Patch antenna consists of a radiating patch

on one side of a dielectric substrate which has a ground plane on the other side as shown in

Figure 2.1. The patch is generally made of conducting material such as copper or gold and

can take any possible shape. The radiating patch and the feed lines are usually photo etched

on the dielectric substrate.

Figure 3.1 Structure of a Micro strip Patch Antenna

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In order to simplify analysis and performance prediction, the patch is generally

square, rectangular, circular, triangular, and elliptical or some other common shape as shown

in Figure 2.2. For a rectangular patch, the length L of the patch is usually 0.3333λo< L < 0.5

λo, where λo is the free-space wavelength. The patch is selected to be very thin such that t <<

λo (where t is the patch thickness). The height h of the dielectric substrate is usually 0.003

λo≤h≤0.05 λo. The dielectric constant of the substrate (εr) is typically in the range 2.2 ≤ εr≤ 12.

Figure 3.2 Common shapes of micro strip patch elements

3.2 Basic Principle of operation

Microstrip antenna are essentially suitably shaped discontinuties that are designed to

radiate. The discontinuities represents abrupt changes in the microstrip line geometry e.g. a

step change in width, an open end or a microstrip bend. Discontinuties alter the electric and

magnetic field distributions. These results in energy storage and sometimes radiation at the

discontinuity. As long as the physical dimensions and relative dielectric constant of the line

remains constant, virtually no radiation occur. However the discontinuity introduced by the

rapid change in line width at the junction between the field line and patch radiates. The other

end of the patch where the metalization abruptly ends also radiates. When the fields on a

microstrip line encounter an abrupt change in width at the input to the patch,electric fields

spread out. It creates fringing fields at this edge, as indicated. After this transition the

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pathlooks like another micro strip line. The fields propogate down this transition line untill

the other edge is reached. Here the abrupt ending of the line again creates fringing fields as

for the open end discontinuity. The fringing fields stores energy. The edges appear as

capacitors to ground since the changes in electric field are greater than that for the magnetic

field. Because the patch is much wider than a typical microstrip line, the fringing fields also

radiate,which is represented by capacitance, which accounts for power lost due to radiation.

Fig3.3 Electric field distributions in microstrip cavity

Micro strip patch antennas radiate primarily because of the fringing fields between the

patch edge and the ground plane. For good antenna performance, a thick dielectric substrate

having a low dielectric constant is desirable since this provides better efficiency, larger

bandwidth and better radiation. However, such a configuration leads to a larger antenna size.

In order to design a compact Micro strip patch antenna, substrates with higher dielectric

constants must be used which are less efficient and result in narrower bandwidth. Hence a

trade-off must be realized between the antenna dimensions and antenna performance.

3.3 Advantages

Micro strip patch antennas are increasing in popularity for use in wireless applications

due to their low-profile structure. Therefore they are extremely compatible for embedded

antennas in handheld wireless devices such as cellular phones, pagers etc... The telemetry and

communication antennas on missiles need to be thin and conformal and are often in the form

of Micro strip patch antennas. Another area where they have been used successfully is in

Satellite communication. Some of their principal advantages are given below:

1. Size and Profile

2. Ease Of Manufacturing

3. Integration

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4. Ease of Forming Arrays

5. Efficient

6. Low fabrication cost.

7. The antennas may be easily mounted on missiles, rockets and satellites

8. Linear, circular polarizations are possible with simple changes in feed position.

9. Feed lines and matching networks are fabricated simultaneously with the antenna

structure.

3.4 Limitations

The main limitations however are

1. Narrow bandwidth

2. Practical limitations on maximum gain

3. Radiate into a half plane

4. Poor endfire radiation performance

5. Low power handling capability

6. Possibility of exitation of surface waves.

3.5 Rectangular Microstrip Antenna

Microstrip antennas are among the most widely used types of antennas in the

microwave frequency range, and they are often used in the millimeter-wave frequency range

as well [1, 2, 3]. (Below approximately 1 GHz, the size of a microstrip antenna is usually too

large to be practical, and other types of antennas such as wire antennas dominate). Also

called patch antennas, Microstrip patch antennas consist of a metallic patch of metal that is on

top of a grounded dielectric substrate of thickness h, with relative permittivity and

permeability εr and μr as shown in Figure 3.1 (usually μr = 1). The metallic patch may be of

various shapes, with rectangular and circular being the most common, as shown in Figure 3.1.

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Figure 3.4 Rectangular & Circular Patch Antennas

Most of the discussion in this section will be limited to the rectangular patch, although

the basic principles are the same for the circular patch. (Many of the CAD formulas presented

will apply approximately for the circular patch if the circular patch is modeled as a square

patch of the same area). Various methods may be used to feed the patch, as discussed below.

One advantage of the microstrip antenna is that it is usually low profile, in the sense that the

substrate is fairly thin.

If the substrate is thin enough, the antenna actually becomes “conformal,” meaning

that the substrate can be bent to conform to a curved surface (e.g., a cylindrical structure). A

typical substrate thickness is about 0.02 λ0. The metallic patch is usually fabricated by a

photolithographic etching process or a mechanical milling process, making the construction

relatively easy and inexpensive (the cost is mainly that of the substrate material).

Other advantages include the fact that the microstrip antenna is usually lightweight

(for thin substrates) and durable. Disadvantages of the microstrip antenna include the fact that

it is usually narrowband, with bandwidths of a few percent being typical. Some methods for

enhancing bandwidth are discussed later. Also, the radiation efficiency of the patch antenna

tends to be lower than some other types of antennas, with efficiencies between 70% and 90%

being typical.

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3.6 Basic Mechanism of RMSA

The metallic patch essentially creates a resonant cavity, where the patch is the top of

the cavity, the ground plane is the bottom of the cavity, and the edges of the patch form the

sides of the cavity. The edges of the patch act approximately as an open-circuit boundary

condition. Hence, the patch acts approximately as a cavity with perfect electric conductor on

the top and bottom surfaces, and a perfect “magnetic conductor” on the sides. This point of

view is very useful in analyzing the patch antenna, as well as in understanding its behavior.

Inside the patch cavity the electric field is essentially z directed and independent of the z

coordinate. Hence, the patch cavity modes are described by a double index (m, n). For the (m,

n) cavity mode of the rectangular patch the electric field has the form

Where L is the patch length and W is the patch width. The patch is usually operated in

the

(1,0) mode, so that L is the resonant dimension, and the field is essentially constant in the y

direction. The surface current on the bottom of the metal patch is then x directed, and is given

by:

For this mode the patch may be regarded as a wide microstrip line of width W, having

a resonant length L that is approximately one-half wavelength in the dielectric. The current is

maximum at the centre of the patch, x = L/2, while the electric field is maximum at the two

“radiating” edges, x = 0 and x = L. The width W is usually chosen to be larger than the length

(W= 1.5 L is typical) to maximize the bandwidth, since the bandwidth is proportional to the

width. (The width should be kept less than twice the length, however, to avoid excitation of

the (0, 2) mode.)

At first glance, it might appear that the microstrip antenna will not be an effective

radiator when the substrate is electrically thin, since the patch current will be effectively

shorted by the close proximity to the ground plane. If the modal amplitude A10 were constant,

the strength of the radiated field would in fact be proportional to h. However, the Q of the

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cavity increases as h decreases (the radiation Q is inversely proportional to h). Hence, the

amplitude A10 of the modal field at resonance is inversely proportional to h. Hence, the

strength of the radiated field from a resonant patch is essentially independent of h, if losses

are ignored. The resonant input resistance will likewise be nearly independent of h. This

explains why a patch antenna can be an effective radiator even for very thin substrates,

although the bandwidth will be small.

3.7 Resonant Frequency

The resonance frequency for the (1, 0) mode is given by

Where, c is the speed of light in vacuum. To account for the fringing of the cavity

fields at the edges of the patch, the length, the effective length Le is chosen as

Le= L + 2ΔL

Where,

3.8 Radiation Patterns

The radiation field of the microstrip antenna may be determined using either an

“electric current model” or a “magnetic current model”. In the electric current model, the

current in (2) is used directly to find the far-field radiation pattern. Figure 3.3a shows the

electric current for the (1, 0) patch mode. If the substrate is neglected (replaced by air) for the

calculation of the radiation pattern, the pattern may be found directly from image theory. If

the substrate is accounted for, and is assumed infinite, the reciprocity method may be used to

determine the far-field pattern.

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(a) Electric Current for (1, 0) patch

(b) Magnetic Current for (1, 0) patch

Fig. 3.5 Electric & Magnetic Current Distributions

In the magnetic current model, the equivalence principle is used to replace the patch

by a magnetic surface current that flows on the perimeter of the patch. The magnetic surface

current is given by:

Where E is the electric field of the cavity mode at the edge of the patch and n is the

outward pointing unit-normal vector at the patch boundary. Figure 3.2b shows the magnetic

current for the (1, 0) patch mode. The far-field pattern may once again be determined by

image theory or reciprocity, depending on whether the substrate is neglected or not. The

dominant part of the radiation field comes from the “radiating edges” at x = 0 and x = L. The

two non-radiating edges do not affect the pattern in the principle planes (the E plane at φ = 0

and the H plane at φ = π/2), and have a small effect for other planes.

It can be shown that the electric and magnetic current models yield exactly the same

result for the far-field pattern, provided the pattern of each current is calculated in the

presence of the substrate at the resonant frequency of the patch cavity mode. If the substrate

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is neglected, the agreement is only approximate, with the largest difference being near the

horizon.

According to the electric current model, accounting for the infinite substrate, the far-

field pattern is given by:

Where,

kx = k0 sinθ cosφ

ky = k0 sinθ sinφ

and Eih is the far-field pattern of an infinitesimal (Hertzian) unit-amplitude x- directed electric

dipole at the centre of the patch.

This pattern is given by:

The radiation patterns (E- and H-plane) for a rectangular patch antenna on an infinite

substrate of permittivity εr = 2.2 and thickness h /λ0= 0.02 are shown in Figure 3.3. The patch

is resonant with W/ L = 1.5. Note that the E-plane pattern is broader than the H-plane pattern.

The directivity is approximately 6 dB.

Figure 3.6 Radiation Pattern (E & H plane)

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3.9 Radiation Efficiency

The radiation efficiency of the patch antenna is affected not only by conductor and

dielectric losses, but also by surface-wave excitation - since the dominant TM mode of the

grounded substrate will be excited by the patch. As the substrate thickness decreases, the

effect of the conductor and dielectric losses becomes more severe, limiting the efficiency. On

the other hand, as the substrate thickness increases, the surface-wave power increases, thus

limiting the efficiency. Surface-wave excitation is undesirable for other reasons as well, since

surface waves contribute to mutual coupling between elements in an array, and also cause

undesirable edge diffraction at the edges of the ground plane or substrate, which often

contributes to distortions in the pattern and to back radiation.

For an air (or foam) substrate there is no surface-wave excitation. In this case, higher

efficiency is obtained by making the substrate thicker, to minimize conductor and dielectric

losses (making the substrate too thick may lead to difficulty in matching, however, as

discussed above). For a substrate with a moderate relative permittivity such as εr = 2.2, the

efficiency will be maximum when the substrate thickness is approximately λ0 = 0.02. The

radiation efficiency is defined as

Where Psp is the power radiated into space, and the total input power Ptotal is given

as the sum of Pc - the power dissipated by conductor loss, Pd- the power dissipated by

dielectric loss, and Psw - the surface-wave power. The efficiency may also be expressed in

terms of the corresponding Q factors as

A plot of radiation efficiency for a resonant rectangular patch antenna with W / L =

1.5 on a substrate of relative permittivity εr = 2.2 or εr = 10.8 is shown in Figure 2.5. The

result is plotted efficiency versus normalized (electrical) thickness of the substrate, which

does not involve frequency.

23 | P a g e

Figure 3.7 Radiation Efficiency for a rectangular patch Antenna

The conductivity of the copper patch and ground plane is assumed to be ζ = 3.0×107

[S/m] and the dielectric loss tangent is taken as tanδd = 0.001. The resonance frequency is 5

GHz. However, a specified frequency is necessary to determine conductor loss. For h / λ0 <

0.02, the conductor and dielectric losses dominate, while for h /λ0 > 0.02, the surface-wave

losses dominate. (If there were no conductor or dielectric losses, the efficiency would

approach 100% as the substrate thickness approaches zero.)

3.10 Bandwidth

The bandwidth increases as the substrate thickness increases (the bandwidth is

directly proportional to h if conductor, dielectric, and surface-wave losses are ignored).

However, increasing the substrate thickness lowers the Q of the cavity, which increases

spurious radiation from the feed, as well as from higher-order modes in the patch cavity.

Also, the patch typically becomes difficult to match as the substrate thickness increases

beyond a certain point (typically about 0.05 λ0). This is especially true when feeding with a

coaxial probe, since a thicker substrate results in a larger probe inductance appearing in series

with the patch impedance. However, in recent years considerable effort has been spent to

improve the bandwidth of the microstrip antenna, in part by using alternative feeding

schemes. The aperture-coupled feed of Figure 2.2c is one scheme that overcomes the problem

of probe inductance, at the cost of increased complexity.

Lowering the substrate permittivity also increases the bandwidth of the patch antenna.

However, this has the disadvantage of making the patch larger. Also, because of the patch

24 | P a g e

cavity is lowered, there will usually be increased radiation from higher-order modes,

degrading the polarization purity of the radiation.

By using a combination of aperture-coupled feeding and a low-permittivity foam

substrate, bandwidths exceeding 25% have been obtained. The use of stacked patches (a

parasitic patch located above the primary driven patch) can also be used to increase

bandwidth even further, by increasing the effective height of the structure and by creating a

double-tuned resonance effect. A CAD formula for the bandwidth (defined by SWR < 2.0) is

Where the terms used have been defined in the previous section on radiation

efficiency. The result should be multiplied by 100 to get percent bandwidth. Note that

neglecting conductor and dielectric loss yields a bandwidth that is proportional to the

substrate thickness h.

Figure 3.8 Calculated & Measured Bandwidth

It is seen that bandwidth is improved by using a lower substrate permittivity, and by

making the substrate thicker.

3.11 Input Impedance

25 | P a g e

A variety of approximate models have been proposed for the calculation of input

impedance for a probe-fed patch. These include the transmission line method [9], the cavity

model, and the spectral-domain method. These models usually work well for thin substrates,

typically giving reliable results for h / λ0 < 0.02.

Commercial simulation tools using FDTD, FEM, or MoM can be used to accurately

predict the input impedance for any substrate thickness. The cavity model has the advantage

of allowing for a simple physical CAD model of the patch to be developed.

Figure 3.9 Equivalent Circuit of Patch Antenna

In this model the patch cavity is modeled as a parallel RLC circuit, while the probe

inductance is modeled as a series inductor. The input impedance of this circuit is

approximately described by

3.12 Linear Polarization

Antenna Polarization is a very important parameter when choosing and installing an

antenna. It helps to have a good grasp of all the aspects of this subject. Most communications

systems use either vertical or horizontal or circular polarization. Knowing the difference

between polarizations and how to maximize their benefit is very important to the antenna

user. A linear polarized antenna radiates wholly in one plane containing the direction of

propagation. In a circular polarized antenna, the plane of polarization rotates in a circle

making one complete revolution during one period of the wave. An antenna is said to be

vertically polarized (linear) when its electric field is perpendicular to the Earth's surface. A

circular polarized wave radiates energy in both the horizontal and vertical planes and all

planes in between.

26 | P a g e

The difference, if any, between the maximum and the minimum peaks as the antenna

is rotated through all angles, is called the axial ratio or ellipticity and is usually specified in

decibels (dB). If the axial ratio is near 0 dB, the antenna is said to be circular polarized. If the

axial ratio is greater than 1-2 dB, the polarization is often referred to as elliptical.

3.12.1 Important Considerations

Polarization is an important design consideration. The polarization of each antenna in

a system should be properly aligned. Maximum signal strength between stations occurs when

both stations are using identical polarization. On line-of-sight (LOS) paths, it is most

important that the polarization of the antennas at both ends of the path use the same

polarization.

In a linearly polarized system, a misalignment of polarization of 45 degrees will

degrade the signal up to 3 dB and if misaligned 90 degrees the attenuation can be 20 dB or

more. Likewise, in a circular polarized system, both antennas must have the same sense. If

not, an additional loss of 20 dB or more will be incurred.

Linearly polarized antennas will work with circularly polarized antennas and vice

versa. However, there will be up to a 3 dB loss in signal strength. In weak signal situations,

this loss of signal may impair communications. Cross polarization is another consideration. It

happens when unwanted radiation is present from a polarization which is different from the

polarization in which the antenna was intended to radiate. For example, a vertical antenna

may radiate some horizontal polarization and vice versa. However, this is seldom a problem

unless there is noise or strong signals nearby.

3.12.2 Typical Applications

Vertical polarization is most often used when it is desired to radiate a radio signal in

all directions such as widely distributed mobile units. Vertical polarization also works well in

the suburbs or out in the country, especially where hills are present. As a result, nowadays

most two way Earth to Earth communications in the frequency range above 30 MHz use

vertical polarization.

Horizontal polarization is used to broadcast television in the USA. Some say that

horizontal polarization was originally chosen because there was an advantage to not have TV

reception interfered with by vertically polarized stations such as mobile radio. Also, man

27 | P a g e

made radio noise is predominantly vertically polarized and the use of horizontal polarization

would provide some discrimination against interference from noise.

3.12.3 Other Considerations

If your antenna is to be located on an existing tower or building with other antennas in

the vicinity, try to separate the antennas as far as possible from each other. In the UHF range,

increasing separation even a few extra feet may significantly improve performance from

problems such as desensitization.

When setting up your own exclusive communications link, it may be wise to first test

the link with vertical and then horizontal polarization to see which yields the best

performance (if any). If there are any reflections in the area, especially from structures or

towers, one polarization may outperform the other.

On another note, when radio waves strike a smooth reflective surface, they may incur

a 180degree phase shift, a phenomenon known as specular or mirror image reflection. The

reflected signal may then destructively or constructively affect the direct LOS signal. Circular

polarization has been used to an advantage in these situations since the reflected wave would

have a different sense than the direct wave and block the fading from these reflections.

3.12.4 Diversity Reception

Even if the polarizations are matched, other factors may affect the strength of the

signal. The most common are long and short term fading. Long term fading results from

changes in the weather (such as barometric pressure or precipitation) or when a mobile

station moves behind hills or buildings. Short term fading is often referred to as "multipath"

fading since it results from reflected signals interfering with the LOS signal. Some of this

fading phenomenon can be decreased by the use of diversity reception. This type of system

usually employs dual antennas and receivers with some kind of "voting" system to choose the

busiest signal.

However, for best results, the antennas should be at least 20 wavelengths apart so that

the signals are no longer correlated. This would be 20-25 feet at 880 MHz, quite a structural

problem. Nowadays we are inundated with mobile radios and cellular telephones. The

polarization on handheld units is often random depending on how they are held by the user.

This has led to new studies which have found that polarization diversity can be an advantage.

28 | P a g e

The most important breakthrough in this area is that the antennas at the base station do not

have to be separated physically as described above.

They can be collocated as long as they are orthogonal and well isolated from each

other. Only time will tell if these systems are truly cost effective.

Figure 3.10 Linear Polarization

In our project we designed the Rectangular Microstrip Antenna with following specifications.

Specifications

PARAMETERS VALUES

1. Operating Frequency

2. Dielectric Substrate

3. Dielectric constant

4. Height of the Substrate

5. Connector

6. Polarization

7. VSWR

2270 MHz(s-band)

FR-4

4.4

0.16cm

SMA

Linear

2:1

29 | P a g e

Table 3.1 Design Specifications

4. DESIGN OF RECTANGULAR MICROSTRIP ANTENNA

4.1 Selection of substrate

The first step in designing is to select an appropriate substrate. The selection of

substrate material is a balance between the required electrical, mechanical and environmental

performance required by a design versus economic constraints.

Generally, if one has the available design volume to use air as a substrate for a micro

strip antenna, this is a good choice. The antenna efficiency is high, the gain is maximized as

is the impedance bandwidth of a conventional micro strip antenna. The surface wave loss

when air is used as a substrate is minimal.

When a dielectric substrate is selected, one is interested in a material with the lowest

loss tangent (tan) available. The loss tangent is a metric of the quality of electrical energy

which is converted to heat by dielectric. The lowest possible loss tangent maximizes the

antenna efficiency (decreases the losses) and is expanded.

The relative dielectric constant εr of the substrate determines the physical size of

patch antenna. The larger the dielectric constant the smaller the element size, but also the

smaller the impedance bandwidth and directivity, and the surface wave loss increases. The

use of substrates with higher dielectric constants also tightens fabrication tolerances.

The tolerance of the dielectric value is also significant importance in manufacturing

yield. A Monte-Carlo type analysis using the cavity model is a good method of estimating

antenna manufacturing yield for a rectangular microstrip antenna when etching tolerance,

substrate thickness tolerance, feed point location tolerance and dielectric tolerances are

known. Substrate electrical and physical parameters also vary with temperature. Recent work

by Kabacik and Bialkowski indicates that Teflon/fiberglass substrates can have a significant

variation of dielectric constant for many airborne and space borne applications. The dielectric

constant and loss tangent of Teflon fiberglass often differed from what was quoted by

manufacturers in their data sheets true range than encountered in many aerospace

applications. The performance variations are due to changes in the material dielectric

properties –thermal expansion had a minor effect on micro strip antenna performance.

30 | P a g e

Material εr tanδ

Teflon

Rexolite 1422

Noryl

FR 4

Alumina

2.1

2.55

2.6

4.1

9.8

0.0005

0.0007

0.0011

0.02

0.0003

Table: 4.1 Dielectric constants of different materials

Generally the metal cladding attached to the dielectric substrate material is copper.

Two types of copper foil are used as cladding, rolled foil and electrodeposited foil. Rolled

foil is passed through a rolling mill a number of times until the desired physical dimensions

are obtained and bonded to the substrate. Rolled copper has a polished mirror like

appearance. Electrodeposited foil is created electro deposition of copper on to an inert form.

A thin layer of copper is continuously removed from the form then bonded to the substrate.

The computation of characteristic impedance and losses of a micro strip transmission

line depend on the copper foil thickness. The copper cladding is described in terms of weight

per square yard. The thickness of the cladding may then be delivered and is listed.

Generally, dielectric constant εr and loss tangent tanδ increases with temperature. In

space applications moisture out gassing produces a lower dielectric constant and loss tangent.

Teflon (polytetrafluoroethylene) has very desirable electrical qualities but is not

recommended for many space applications. An extensive discussion of PTFE substrates and

their fabrication may be found in the literature.

Rexolite is a very good material for space applications and has many desirable

mechanical properties. Rexolite is easily machined. And its dielectric constant remains stable

up to100Ghz.

31 | P a g e

Noryl is suitable for many commercial microwave applications. it has a much lower

loss than FR4 and is relatively cost effective, but it is soft and melts at a relatively low

temperature which can create soldering complications, and sometimes has unsuitable

mechanical properties for some applications.

FR4 is inexpensive and finds use in many commercial below 1Ghz. The material can

be used for some wireless applications, bur great care must be taken to budget and minimize

the losses when it is used as a substrate above 1Ghz. Manufacturer parkNelco has developed

a sandwiched substrate of PTFE and epoxy glass (FR4) which has desirable properties of FR4

with lower loss.

Alumina has desirable microwave properties for applications which require a

relatively high dielectric constant εr=10.0 and a low loss tangent. It drawbacks are the

difficulty involved in machining it and it brittleness. Alumina has good thermal conductivity

and in some aerospace applications it more readily dissipates heat and remains cooler than

other common microwave substrates. In some missile applications where high temperatures

may compromise solder joints alumina is a viable option for the dissipation of heat.

Alumina’s dielectric constant is very sensitive to the processing used to produce the alumina.

All substrates and laminates have different requirements for processing. Details of

fabrication issues and methods may be found in the literature and directly from

manufacturers. Other fabrication options such as screen printing conductive links directly on

substrates have also been investigated.

4.2 Design procedure for Rectangular Microstrip Antenna

4.2.1 Considered Values:

The three essential parameters for the design of a Rectangular Microstrip Patch

Antenna:

Frequency of operation (fo): The resonant frequency of the antenna must be selected

appropriately. The Mobile Communication Systems uses the frequency range from

2100-5600 MHz. Hence the antenna designed must be able to operate in this

frequency range. The resonant frequency selected for my design is 2.27 GHz.

32 | P a g e

Dielectric constant of the substrate (εr): The dielectric material selected for our

design is RT Duroid which has a dielectric constant of 4.4. A substrate with a high

dielectric constant has been selected since it reduces the dimensions of the antenna.

Height of dielectric substrate (h): For the micro strip patch antenna to be used in

cellular phones, it is essential that the antenna is not bulky. Hence, the height of the

dielectric substrate is selected as 1.6 mm.

Hence, the essential parameters for the design are:

• fo = 2.27 GHz

• εr = 4.4

• h = 1.6 mm

4.2.2 Initial Design Values :

The transmission line model will be used to design the antenna.

Step 1: Calculation of the Width (W):

The width of the Micro strip patch antenna is given as:

(4.1)

Where c –Velocity of light

fo Resonant Frequency

εr Relative Dielectric Constant

Of course other widths may be chosen but for widths smaller than those selected

according to equation (4.1), radiator efficiency is lower while for larger widths, the efficiency

are greater but for higher modes may result, causing field distortion. As a result design aid,

equation (4.1) is plotted for the common dielectric substrates. If other materials are employed

equation (4.1) should be used with appropriate value of εr . In this work upon Substituting c

= 3.00e+011 mm/s, εr = 4.4 and fo = 2.15 GHz, we get:

W = 40.21 mm

33 | P a g e

Fig 4.1 Plot for Frequency versus Width

Step 2: Calculating the Length (L):

For calculating the length we have to calculate the following parameters first.

Effective dielectric constant (εreff):

Once W is known, the next step is the calculation of the length which involves several

other computations; the first would be the effective dielectric constant.

The dielectric constant of the substrate is much greater than the unity; the effective

value of εeff will be closer to the value of the actual dielectric constant ε r of the substrate.

The effective dielectric constant is also a function of frequency.

As the frequency of operation increases the effective dielectric constant approaches

the value of the dielectric constant of the substrate is given by :

(4.2)

34 | P a g e

Substituting εr = 4.4, W = 42.45 mm and h = 1.6 mm we get:

εreff = 4.098

Effective length ( Leff):

The effective length is:

(4.3)

`Substituting εreff = 4.4, c = 3.00e+011 mm/s and fo = 2.25 GHz we get:

Leff = 34.408 mm

Length Extension (∆L):

Because of fringing effects, electrically the micro strip antenna looks larger than its

actual physical dimensions. For the principle E – plane (xy plane), where the dimensions of

the path along its length have been extended on each by a distance, ∆L , which is a function

of the effective dielectric constant and the width-to-height ratio (W/h).The length extension

is:

(4.4)

Substituting εreff = 4.4, W = 40.57 mm and h = 1.6 mm we get:

∆L = 0.739 mm

Calculation of actual length of patch (L):

Because of inherent narrow band width of the resonant element, the length is a critical

parameter and the above equations should be used to obtain an accurate value for the patch

length L.

Fig 4.2(a) which is a plot of L versus frequency for the various substrates and for

chosen substrate may then be used to verify the design.

35 | P a g e

The actual length is obtained by:

(4.5)

Substituting Leff = 34.408 mm and ∆L = 0.7391 mm we get:

L = 31.16 mm

Fig 4.2 Plot for Length versus Frequency

4.2.3 Computed Values:

Step 3: Calculation of the Gain (G):

The gain of the micro strip antenna is given by the following formula

(4.6) G=4 π A

λg2

Where A = L*W = 31.16*40.21 =1252.9436 sqcm

36 | P a g e

λg=λ0

√ε r (4.7)

=132.1

√4.4 = 63.004 mm

By substituting the above values we get

G=3.966 or 5.98 dBi

Gain

Fig 4.3 Plot for Frequency versus Gain

37 | P a g e

Step 4: Calculation of the Beam Width (θ):

The beam width of a micro strip element can be increased by choosing a smaller

element, thus reducing W and L. For a given resonant frequency, these dimensions may be

changed by selecting a substrate having a higher relative permittivity. In many applications, a

decrease in physical size is desirable.

Beam Width in H-Plane

(4.8) θBH=2 cos−1( 1

2{1+L K0

2 })1/2

θBH-Beam Width in H- Plane

Substituting L = 31.16 mm and Ko=0.0475 we get:

θBH=115.169

Beam Width in E-Plane

θBE=2cos−1( 7.033K 0

2W 2+K 02h2 )

1/2

θBE-Beam Width in E- Plane

Substituting W = 40.21 mm, h=1.6mm and Ko=0.0475 we get:

θBE=73.498

As beam width increases, element gain and consequently directivity decrease,

however the antenna efficiency remains unaffected.

Step 5: Calculation of the Band Width Percentage (BW%) :

The band width of the given micro strip antenna is as follows

(4.9) BW=100(S−1)

√S8

3 εr

hλ0

Substitutingλ0=132.15 mm, h=1.6mm and S=2:1, εr = 4.4 we get:

38 | P a g e

BW=0.518%

Fig 4.4 Plot for Frequency versus Band Width

4.3 Selection of RF Connectors

For high-frequency operation, the average circumference of a coaxial cable must be

limited to about one wavelength in order to reduce multi modal propagation and eliminate

erratic reflection coefficients, power losses, and signal distortion. Except for the sexless

APC-7 connector, all other connectors are identified as either male (plugs) which have a

center conductor that is a probe or female (jacks) which have a center conductor that is a

receptacle. Sometimes it is hard to distinguish them as some female jacks may have a hollow

center "pin" which appears to be male, yet accepts a smaller male contact. An adapter is an .

zero loss interface between two connectors and is called a barrel when both connectors are

identical. Twelve types of coaxial connectors are described below, however other special

purpose connectors exist, including blind mate connectors where spring fingers are used in

place of threads to obtain shielding (desired connector shielding should be at least 90 dB).

39 | P a g e

Figure 1 shows the frequency range of several connectors and Figure 2 shows most of these

connectors pictorially (actual size).

40 | P a g e

1. APC-2.4 (2.4mm) - The 50 S APC-2.4 (Amphenol Precision Connector-2.4 mm) is

also known as an OS-50 connector. It was designed to operate at extremely high

microwave frequencies (up to 50 GHz).

2. APC-3.5 (3.5mm) - The APC-3.5 was originally developed by Hewlett-

Packard (HP), but is now manufactured by Amphenol. The connector provides

repeatable connections and has a very low VSWR. Either the male or female end of

this 50 S connector can mate with the opposite type of SMA connector. The APC-3.5

connector can work at frequencies up to 34 GHz.

3. APC-7 (7mm) - The APC-7 was also developed by HP, but has been improved and is

now manufactured by Amphenol. The connector provides a coupling mechanism

without male or female distinction and is the most repeatable connecting device used

for very accurate 50 S measurement applications. Its VSWR is extremely low up to 18

GHz. Other companies have 7mm series available.

4. BNC (OSB) - The BNC (Bayonet Navy Connector) was originally designed for

military system applications during World War II. The connector operates best at

frequencies up to about 4 GHz; beyond that it tends to radiate electromagnetic energy.

The BNC can accept flexible cables with diameters of up to 6.35 mm (0.25 in.) and

characteristic impedance of 50 to 75 S. It is now the most commonly used connector

for frequencies under 1 GHz.

5. SC (OSSC) - The SC coaxial connector is a medium size, older type constant 50 ohms

impedance. It is larger than the BNC, but about the same as Type N. It has a

frequency range of 0-11 GHz.

6. C -The C is a bayonet (twist and lock) version of the SC connector.

7. SMA (OSM/3mm) - The SMA (Sub-Miniature A) connector was originally designed

by Bendix Scintilla Corporation, but it has been manufactured by the Omni-Spectra

division of M/ACOM (as the OSM connector) and many other electronic companies.

It is a 50 S threaded connector. The main application of SMA connectors is on

components for microwave systems. The connector normally has a frequency range to

18 GHz, but high performance varieties can be used to 26.5 GHz.

41 | P a g e

8. SSMA (OSSM) - The SSMA is a micro miniature version of the SMA. It is also 50 S

and operates to 26.5 GHz with flexible cable or 40 GHz with semi-rigid cable.

9. SMC (OSMC) - The SMC (Sub-Miniature C) is a 50 S or 75 S connector that is

smaller than the SMA. The connector can accept flexible cables with diameters of

up to 3.17 mm (0.125 in.) for a frequency range of up to 7-10 GHz.

10. SMB (OSMB) - The SMB is like the SMC except it uses quick disconnect instead of

threaded fittings. It is a 50 / 75 S connector which operates to 4 GHz with a low

reflection coefficient and is useable to 10 GHz.

11. TNC (OST) - The TNC (Threaded Navy Connector) is merely a threaded BNC. The

function of the thread is to stop radiation at higher frequencies, so that the connector

can work at frequencies up to 12 GHz (to 18 GHz when using semi-rigid cable). It can

be 50 or 75 S.

12. Type N (OSN) - The 50 or 75 S Type N (Navy) connector was originally designed for

military systems during World War II and is the most popular measurement connector

for the frequency range of 1 to 11 GHz. The precision 50 S APC-N and other

manufacturer’s high frequency versions operate to 18 GHz.

Note: Always rotate the movable coupling nut of the plug, not the cable or fixed connector,

when mating connectors. Since the center pin is stationary with respect to the jack, rotating

the jack puts torque on the center pin. With TNC and smaller connectors, the center pin will

eventually break off. An approximate size comparison of these connectors is depicted below

(not to scale).

42 | P a g e

Figure 4.5 Frequency Range of Microwave Connectors

43 | P a g e

SC Jack-Type N Jack

APC 2.4 Jack- APC 3.5 Jack Type N Jack- TNC Jack

SMA Plug - TNC Plug SSMA Jack - BNC Jack Type N Plug - TNC Jack

Standard Wave Guide Double Ridge Wave Guide

7mm 7mm-3.5mm Plug SMA Jack

Figure 4.6 Microwave Coaxial Connectors (Connector Orientation Corresponds to Name below It)

44 | P a g e

SMC Plug - SMA

Jack

Size Series Coupling Impedance ()

Frequency(GHz)

VSWR(max)

Voltage(V)

Subminiature

Miniature

Medium

Large

SMASMBSMC

BNCTNCSHVBNMC

CNNCQM

QL

ScrewSnap onScrew

BayonetScrewBayonetScrewScrew

BayonetScrewScrewScrew

Screw

505050

5050NC5050

50505050

50

12.4/18410

411NA0.20.5

1111114

5

1.31.411.6

1.31.31.31.31.3

1.351.31.31.3

1.3

500500500

5005005000200200

1500100010005000

5000

Table 4.2 Basic Features of the most Common Connector Series

45 | P a g e

5. ANALYTICAL MODELS & EXCITATION OF

MICROSTRIP PATCH ANTENNA

5.1 Methods of Analysis

The preferred models for the analysis of Micro strip patch antennas are the

transmission line model, cavity model, and full wave model (which includes primarily

integral equations/Moment Method). The transmission line model is the simplest of all and it

gives good physical insight but it is less accurate. The cavity model is more accurate and

gives good physical insight but is complex in nature. The full wave models are extremely

accurate, versatile and can treat single elements, finite and infinite arrays, stacked elements,

arbitrary shaped elements and coupling. These give less insight as compared to the two

models mentioned above and are far more complex in nature.

5.1.1 Analytical Models

There are many methods of analysis for the micro strip antennas. They can be broadly

classified into two categories -

1. Model – Based Analysis Technique

2. Full – Wave Analysis Technique

The various model – based and full – wave analysis techniques that have been used

for the analysis of the Rectangular Micro Strip Antenna are:

Wire Grid Model

Cavity Model

Modal Dispersion Model

Transmission Line Model

Integral Equation Method

Vector Potential Approach

Dyadic Green’s Function Technique

46 | P a g e

Radiating Aperture Method

In Wire Grid Model the antenna is modeled as a fine grid of wire segments. The

currents on the wire segments are solved using the Richmond’s reaction theorem [22] to get

all the antenna characteristics of interest.

The Cavity Model offers both simplicity and physical insight. In this model the

antenna is treated as a cavity whose fields are computed using the full model expansions. The

importance of this model is that it includes the effects of non resonant modes.

The Modal Expansion Method is similar to cavity model but differs in impedance

boundary conditions that are imposed at the four radiating walls to obtain a solution. Though

the method does not lead to an exact solution, it provides a good insight into the physics of

antenna.

The Transmission Line Model [21 , 27] considers the antenna as two radiating slots

perpendicular to the feed line of length ‘L’. This model is easy to use and analyze due to its

simplicity but suffers from some disadvantages. This model is limited to square and

rectangular geometries. It gives less physical insight and ignores field variations along the

radiating edge. It is not adequate for predicting impedance variation with feed location over

the surface of the patch.

The integral equation method is general method and can treat patches of arbitrary

shapes including those with thick substrate. The method requires considerable analytical and

computational efforts and provides little physical insight.

In Vector Potential Approach, the field produced by a horizontal electric dipole is

determined and the antenna characteristics are then evaluated by numerical techniques.

Though the solution obtained is rigorous, it is less attractive due to lack of closed form

expressions.

In Dyadic Green’s Function Method the characteristics of the micro strip antenna are

evaluated and the field from an arbitrary source distribution may be found by means of a

super position integral.

In radiating aperture method the Vector Kirchoff relation is used. This method is

mathematically precise if the aperture fields are known exactly.

47 | P a g e

Transmission model is adapted in this work for the analysis of the rectangular and the

square micro strip antennas.

5.1.2 Transmission Line Model

This model represents the micro strip antenna by two slots of width W and height h,

separated by a transmission line of length L. The micro strip is essentially a non-

homogeneous line of two dielectrics, typically the substrate and air.

Figure 5.1 Micro strip Line Figure 5.2 Electric Field Lines

Hence, as seen from Figure 2.8, most of the electric field lines reside in the substrate

and parts of some lines in air. As a result, this transmission line cannot support pure

transverse-electric-magnetic (TEM) mode of transmission, since the phase velocities would

be different in the air and the substrate. Instead, the dominant mode of propagation would be

the quasi-TEM mode. Hence, an effective dielectric constant (εreff) must be obtained in order

to account for the fringing and the wave propagation in the line. The value of ε reff is slightly

less then εr because the fringing fields around the periphery of the patch are not confined in

the dielectric substrate but are also spread in the air as shown in Figure 3.8 above. The

expression for εreff is given by Balanis [12] as:

Where εreff = Effective dielectric constant

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εr = Dielectric constant of substrate

h = Height of dielectric substrate

W = Width of the patch

Consider Figure 2.9 below, which shows a rectangular micro strip patch antenna of

length L, width W resting on a substrate of height h. The co-ordinate axis is selected such that

the length is along the x direction, width is along the y direction and the height is along the z

direction.

Figure 5.3 Micro strip Patch Antennas

In order to operate in the fundamental TM10 mode, the length of the patch must be

slightly less than λ/2 where λ is the wavelength in the dielectric medium and is equal to

λo/√εreff where λo is the free space wavelength. The TM10 mode implies that the field varies

one λ/2 cycle along the length, and there is no variation along the width of the patch. In the

Figure 2.10 shown below, the micro strip patch antenna is represented by two slots, separated

by a transmission line of length L and open circuited at both the ends. Along the width of the

patch, the voltage is maximum and current is minimum due to the open ends. The fields at the

edges can be resolved into normal and tangential components with respect to the ground

plane.

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Figure 5.4 Top View of Antenna Figure 5.5 Side View of Antenna

It is seen from Figure 2.11 that the normal components of the electric field at the two

edges along the width are in opposite directions and thus out of phase since the patch is λ/2

long and hence they cancel each other in the broadside direction. The tangential components

(seen in Figure 2.11), which are in phase, means that the resulting fields combine to give

maximum radiated field normal to the surface of the structure. Hence the edges along the

width can be represented as two radiating slots, which are λ/2 apart and excited in phase and

radiating in the half space above the ground plane. The fringing fields along the width can be

modeled as radiating slots and electrically the patch of the micro strip antenna looks greater

than its physical dimensions. The dimensions of the patch along its length have now been

extended on each end by a distance ∆L, which is given empirically by Hammerstad [13] as:

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5.2 Excitation Techniques

Micro strip patch antennas can be fed by a variety of methods. These methods can be

classified into two categories- contacting and non-contacting. In the contacting method, the

RF power is fed directly to the radiating patch using a connecting element such as a micro

strip line. In the non-contacting scheme, electromagnetic field coupling is done to transfer

power between the micro strip line and the radiating patch [5]. The four most popular feed

techniques used are the micro strip line, coaxial probe (both contacting schemes), aperture

coupling and proximity coupling (both non-contacting schemes).

5.2.1 Micro strip Line Feed

In this type of feed technique, a conducting strip is connected directly to the edge of

the Micro strip patch as shown in Figure 2.3. The conducting strip is smaller in width as

compared to the patch and this kind of feed arrangement has the advantage that the feed can

be etched on the same substrate to provide a planar structure.

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Figure 5.6 Micro strip Line Feed

The purpose of the inset cut in the patch is to match the impedance of the feed line to

the patch without the need for any additional matching element. This is achieved by properly

controlling the inset position. Hence this is an easy feeding scheme, since it provides ease of

fabrication and simplicity in modeling as well as impedance matching. However as the

thickness of the dielectric substrate being used, increases, surface waves and spurious feed

radiation also increases, which hampers the bandwidth of the antenna [5]. The feed radiation

also leads to undesired cross polarized radiation.

5.2.2 Coaxial Feed

The Coaxial feed or probe feed is a very common technique used for feeding

Microstrip patch antennas. As seen from Figure 2.4, the inner conductor of the coaxial

connector extends through the dielectric and is soldered to the radiating patch, while the outer

conductor is connected to the ground plane.

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Figure 5.7 Probe fed Rectangular Micro strip Patch Antenna

The main advantage of this type of feeding scheme is that the feed can be placed at

any desired location inside the patch in order to match with its input impedance. This feed

method is easy to fabricate and has low spurious radiation. However, a major disadvantage is

that it provides narrow bandwidth and is difficult to model since a hole has to be drilled in the

substrate and the connector protrudes outside the ground plane, thus not making it completely

planar for thick substrates (h > 0.02λo). Also, for thicker substrates, the increased probe

length makes the input impedance more inductive, leading to matching problems [9]. It is

seen above that for a thick dielectric substrate, which provides broad bandwidth, the micro

strip line feed and the coaxial feed suffer from numerous disadvantages. The non-contacting

feed techniques which have been discussed below, solve these issues.

5.2.3 Aperture Coupled Feed

In this type of feed technique, the radiating patch and the micro strip feed line are

separated by the ground plane as shown in Figure 2.5. Coupling between the patch and the

feed line is made through a slot or an aperture in the ground plane.

Figure 5.8 Aperture-coupled feed

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The coupling aperture is usually centered under the patch, leading to lower cross-

polarization due to symmetry of the configuration. The amount of coupling from the feed line

to the patch is determined by the shape, size and location of the aperture. Since the ground

plane separates the patch and the feed line, spurious radiation is minimized. Generally, a high

dielectric material is used for bottom substrate and a thick, low dielectric constant material is

used for the top substrate to optimize radiation from the patch [5]. The major disadvantage of

this feed technique is that it is difficult to fabricate due to multiple layers, which also

increases the antenna thickness. This feeding scheme also provides narrow bandwidth.

5.3.4 Proximity Coupled Feed

This type of feed technique is also called as the electromagnetic coupling scheme. As

shown in Figure 2.6, two dielectric substrates are used such that the feed line is between the

two substrates and the radiating patch is on top of the upper substrate. The main advantage of

this feed technique is that it eliminates spurious feed radiation and provides very high

bandwidth (as high as 13%) [5], due to overall increase in the thickness of the micro strip

patch antenna. This scheme also provides choices between two different dielectric media, one

for the patch and one for the feed line to optimize the individual performances.

Figure 5.9

Matching can be achieved by controlling the length of the feed line and the width-to-

line ratio of the patch. The major disadvantage of this feed scheme is that it is difficult to

fabricate because of the two dielectric layers which need proper alignment. Also, there is an

increase in the overall thickness of the antenna.

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Table 5.2 The characteristics of the different feed techniques

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6. FABRICATION

6.1 Fabrication Procedure

The first step in the fabrication process is to generate the art work from drawings.

Accuracy is vital at this stage and depending on the complexity and dimensions of the

antenna, either full or enlarged scale artwork should be prepared on Stabiline or Rubilith film.

Using the precision cutting blade of a manually operated coordinagraph, the opaque layer of

the Stabiline or Rubylith film is cut to the proper geometry and can be removed to produce

either a positive or negative representation of the Microstrip antenna. The design dimensions

and tolerances are verified on a Cordax measuring instruments using optical scanning.

Enlarged artwork should be photo reduced using high precision camera to produce a

high resolution negative, which is later used for exposing the photo resist. The laminate

should be cleaned using the substrate manufacturer recommended procedure to insure proper

adhesion of the photo resist and the necessary resolution in the photo development process.

The photo resist is now applied to both sides of the laminate using laminator. Afterwards, the

laminate is allowed to stand to normalize to room temperature prior to exposure and

development.

The photographic negative must be now held in very close contact with the

polyethylene cover sheet of the applied photo resist using a vaccum frame copy board or

other technique, to assure the fine line resolution required. With exposure to theproper

wavelength light, a polymerization of the exposed photo resist occurs, making it insoluble in

the developer solution. The backside of the antenna is exposed completely without a mask,

since the copper foil is retained to act as a ground plane.

The protective polythene cover sheet of the photo resist is removed and the antenna is

now developed in a developer which removes the soluble photo resist material. Visual

inspection is used to assure proper development. When these steps have been completed, the

antenna is now ready for etching. This is a critical step and requires considerable care so the

proper etch rates are achieved.

After etching, the excess photo resist is removed using a stripping solution. Visual

and optical inspections should be carried out to insure a good product and to insure

conformance with dimensional tolerances, with final acceptance or rejection being based on

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resonant frequency, radiation pattern and impedance measurement. For acceptable units the

edges are smoothened and the antenna is rinsed in water and dried.

If desired, a thermal cover bonding may be applied by placing a bonding film between

the laminates to be bonded and placing these between tooling plates. Dowel pins can be used

for alignment and the assembly is then heated under pressure until the bond line temperature

is reached. The assembly is allowed to cool under pressure below the melting point of the

bonding film and the laminate is then removed for inspection.

The above procedure comprises the general steps necessary in producing a

Microstrip antenna. The substances used for the various processes example cleaning, etching,

etc., are the tools used for machining, etc., depending on the substrate chosen. Most

manufacturers provide informative brochures on the appropriate choice of chemicals,

cleaners, etchants, etc., for their substrates.

Figure 6.1 Photographic Negative Used For Fabrication Procedure

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W=42.25mm

L=32.93mm

6.2 Step by step design procedure

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DESIGN

MASTER DRAWING

ART WORK LAY OUT

PHOTO REDUCTION

NEGATIVE DEVELOPMENT

LAMINATE CLEANING

RESIST APPLICATION

RESIST EXPOSURE

RESIST DEVELOPMENT

ETCHING

BONDING

FINISHING

INSPECTION

DESIGN

MASTER DRAWING

Figure 6.2 Flow chart showing the fabrication process

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INSPECTION

Drilling hole of diameter 1.3mm by

using precision drilling machine

SOLDERING

Checking with ohm meter for the patch

& centre conductor continuity

Visual inspection of solder point which

should be blister

7. MEASUREMENTS AND TESTING

7.1 Measurements

Testing of antenna involves measurement of electrical and electromagnetic

parameters. Electrical parameters involve measurement of Return loss or VSWR,

Impedance and electromagnetic parameters involves the measurement of Gain and

Radiation pattern.

Network Analyzer has been used to measure the return loss, VSWR and impedance

shown in figures 7.1 & 7.2. Radiation patterns and gain of the antenna at the designed

frequency are preferably done in an anechoic chamber. The measured data is presented in

table-7.1.

From table 7.1, it can be seen that the impedance band width of the microstrip antenna

#1 is about 0.87 %. In applications such as mobile applications larger band widths are

required. Band width can be improved by using the thick dielectric substrate with low

dielectric constant Antenna # 2 and Antenna # 3 of table 7.1 falls under this category and

offer band width of 2.4 % and 9.3 % respectively at their center frequencies. Here the band

width mainly depends on the thickness of the substrate for a given dielectric constant.

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Table 7.1 Comparison of Radiation & Electrical Parameters of Different Antennas

Note:

ANTENNA # 1 : Rectangular Patch with RT Duroid dielectric substrate

ANTENNA # 2 : Rectangular Patch with PUF dielectric substrate with 6mm thickness

ANTENNA # 3 : Circular Patch with PUF dielectric substrate

ANTENNA # 4 : Rectangular Patch with FRP dielectric substrate of thickness 1.6mm

7.2 Testing

Here is a description of some of the components used to test various antenna

parameters Return Loss, VSWR, impedance measurements using Smith Chart has been

obtained using the Vector Network Analyzer. Radiation Patterns can be obtained using the

experimental set up containing Anechoic Chamber.

7.2.1 Testing Using Network Analyzer

The testing of antenna is done using N5230A which is a Two Port Vector Network

Analyzer. The HP / Agilent N5230A PNA-L vector network analyzer provides the best

combination of speed and accuracy for measuring multi-port and balanced components such

as filters, duplexers and RF modules up to 20 GHz. A vector analyzer provides simple and

complete vector network measurements in a compact, fully integrated RF network. N5230A

vector network analyzer offers built-in source, receiver and s-parameter test set covering

frequencies from 10 MHz to 20 GHz.

The N5230A's automatic port extension feature automatically measures and corrects

for fixtures, making measurements of in-fixture devices simple and accurate. The

configurable test set provides access to the signal path between the internal source and the

analyzer's test ports. This option provides the capability to improve instrument sensitivity for

measuring low-level signals, to reverse the directional coupler to achieve even more dynamic

range or to add components or other peripheral instruments for a variety of applications such

as high-power measurements. The extended power range adds a 60 dB step attenuator

internally to the RF source path. This attenuator extends the source output power range to

over 80 dB, allowing for maximum flexibility when stimulating the device under test.

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SPECIFICATIONS OF N5230A

Channels : 2

Frequency Range : 10MHz to 20GHz

Dynamic Range : 110dB

Source Output : -30dBm to 20dBm

Display : Polar, Rectangular, Smith Chart

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Fig 7.1 Vector Network Analyzer

Fig 7.2 Vector Network Analyzer during Measurements

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7.2.2 Measurement of Return loss or VSWR

The return loss is the measure of power reflected and is related to the reflection

coefficient ‘Γ’ given by

Return Loss in dB = 20 log │Γ│

The relation between reflection coefficient and VSWR is given by

VSWR (S) = 1+│Γ│ 1+│Γ│

The experimental setup for measuring return loss of the antenna is shown in the figure

below. Experimental setup for measuring the Return Loss consists of

1. Sweep oscillator

2. Dual directional coupler

3. Network Analyzer

Sweep oscillator: The power is fed to the antenna from the sweep oscillator through the dual

directional coupler as shown in the figure below. The sizes of sweep in a sweep oscillator can

be fixed so that the power will be fed to the antenna between the required ranges of

frequencies from start to top.

Dual directional coupler: The dual directional consists of four ports, an input port at which

the power is incident from the sweep oscillator and a test port at which the antenna whose

return loss is to be measured is connected. In the dual directional coupler a part of the

incident input power is coupled to isolated port. The ratio of the isolated power to couple

power will give us the reflection coefficient ‘Γ’.

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Figure 7.3 Experimental setup for measuring return loss

Network Analyzer: The reflection coefficient ‘Γ’ is fed to the network analyzer which

converts this to dB to get the return loss then the values of return loss at different frequencies

in the sweep range fixed will be displayed on the screen of the network analyzer. A HP

N5230C vector network analyzer is employed in the present measurements.

Before measuring the return loss of the antenna, the network analyzer should be

calibrated as explained below:

1. The terminal at the test port at which the test antenna is to be mounted is short

circuited. Now the power fed to the test port travels back through the short circuits so

that there will be no radiation at all. The reflected power will be equal to the incident

power and so the reflection coefficient is equal to 1, which in turn leads to a return

loss of zero dB, therefore, when the test port terminals are short circuited, we must

get a zero dB line on the display.

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2. The terminals at the test port are now open circuited. The power fed to the test port

cannot be radiated because there is no load. So all the power reflects back. The

reflection coefficient is 1 and therefore leads to a return loss of 0 dB. Hence when the

terminals at the test port are open circuited the screen should display a 0 dB line.

During short circuit of test port terminals the power reflects back with phase reversal.

During the open circuit the reflected power is in- phase with respect to the incident power.

These two settings are stored in memory and the setup is ready for practical measurements.

The antenna is then connected at the test port and the observed plot is the return loss of the

antenna. The percentage bandwidth at -10dB return loss is

% Bandwidth = (f2-f1)/fr × 100

Where (f2-f1) is the frequency band for which the return loss is less than 10 dB.

7.2.3 Radiation Pattern Measurements

The radiation patterns of an antenna is usually represented graphically by plotting the

electric field of the antenna as a function of direction. This electric field strength is expressed

as volts per meter or normalized field in dB.

A complete radiation pattern comprises the radiation for all the angles of and and

really requires three dimensional presentations. This is quite complicated. For the practical

purposes, the pattern is measured in planes of interest. Cross sections in which the radiation

patterns are the most frequently taken are the horizontal (=90 degrees) and vertical

(=constant) planes. These are called the horizontal patterns and vertical patterns

respectively. The terms commonly used are the E- plane and H-plane and they are the planes

passing through the antenna in the direction of beam maximum and parallel to the far-field E

and H vectors. These patterns are known as the ‘Principal Planes’ patterns. The radiation

patterns of the antenna are measured with the scientific Atlanta instrumentation in an

anechoic chamber. The instrumentation consists of the following four major parts as shown in

below figure.

1. Transmitter System

2. Positioning and Controlling System

3. Receiving System

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4. Recording System

Transmitting System:

The transmitting or source instrumentation consists primarily of the RF signal source

and associated transmitting antenna.

Signal Source: The model 2150 signal source provides RF power in the 0.1 to 18 GHz

frequency range. The control unit is located near the operator’s console. The RF oscillators

are installed in the main frame assembly which is mounted near the source antenna.

Source Antenna: Several types of antennas designed especially for the antenna test range can

be used. These include standard gain horns, dipoles, parabolic reflector antennas, log periodic

arrays and circularly polarized antennas depending upon the requirement.

Positioning & Controlling System:

The antenna to be tested is mounted on the turntable of the antenna test positioner.

The speed and direction of the rotation of the test antenna can be controlled from the

operator’s console by a direct current motor. A synchro transmitter is mechanically coupled

to the positioner turntable and electrically to a position indicator. The antenna test positioner

is controlled by the series 4100 positioner control unit. Electrical cables are used to supply

power from control system to test positioner.

Indicator system: A position indicator allows remote angle read out of the test

positioner. The synchro transmitter in the test positioner provides the position data to operate

the position indicator.

Receiving System:

The antenna under test usually tested in the receive mode. Therefore a receiving or

detecting system must be connected to the test antenna to convert RF signals to a low

frequency signals which can drive the pen system of pattern recorder. Thus the antenna must

receive an RF signal i.e modulated with an audio signal. The model 2150 signal source has

an audio oscillator as a standard feature. The two types of detectors commonly used for

making antenna measurements are crystal detector and Bolometer. Scientific Atlanta antenna

pattern recorders will operate crystal detectors or Bolometers detectors directly.

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Antenna Pattern Recorder:

The radiation patterns of the antenna are recorded as relative amplitude and / or phase

as a function of the position (or angle). The synchro position data from the test positioner is

connected to the recorder’s chart servo system. The resultant graph is a plot of the relative

amplitude of the received signal as a function of the antenna position (or angle) .

Fig 7.4 Experimental Set Up For Plotting Radiation Pattern

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Fig 7.5 Anechoic Chamber When Enclosed

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Fig 7.6 Anechoic Chamber

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7.2.4 Gain Measurement

The setup used for measurement of gain is the same as that used for radiation pattern

measurement given in figure (7.3). The gain of the antenna is measured by replacing the test

antenna with a standard antenna (horn antenna in this case) and taking the pattern of the

same. The gain is then calculated by comparing the power level differences of the test

antenna with that of the standard antenna.

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8. SIMULATING THE MICRO STRIP ANTENNA

8.1 Program in MATLAB

8.1.1Merits Of Programming

The design of the micro strip antenna involves many lengthy and tedious calculations

such as width, length, feed locations, dimensions of the feed. As these calculations are

cumber some and time consuming when done by hand a computer programming approach is

adopted to simplify the task.

8.1.2 Program TO Find Width, Length & Feed Point

The width and length of the micro strip antenna are to be calculated from the

corresponding equations as given in chapter 4. The next parameter to be found is the feed

point location. In the project, the coaxial type of feed is chosen to feed the antenna. The

impedance of the feed is 50Ώ. Hence in the program the importance of the antenna is found at

every point along the length of the antenna according to the standard formulae given in the

chapter 4 and the point of feed is hence found.

Thus the program in MATLAB to find the length, width of the micro strip antenna

and also the feed location is given below. It takes the input as frequency of operation(GHz),

substrate thickness (in cm) and dielectric constant.

8.1.3 MATLAB Program

er=4.4

fr=2.27e9

rin=50

c=3e11

h=1.6

ll=0

ul=pi

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i=pi/5

e0=8.8419e-012

m0=4*pi*1e-7

sgm=5.8*1e7

lt=0.0002

et0=120*pi

vswr=2

%ko=2pi/lam

%WIDTH OF THE ANTENNA

w=c/(2*fr)*sqrt(2/(er+1))

ereff=((er+1)/2)+((er-1)/2)*(sqrt(1/(1+(12*(h/w)))))

lam=c/fr;

lamg=lam/sqrt(ereff)

%LENGTH OF THE ANTENNA

u=(w/h)

dell=0.412*h*(ereff+0.3)*(u+0.264)/((ereff-0.258)*(u+8));

l=(lamg/2)-(2*dell)

%FEED POINT CALCULATION

k0=2*pi/lam

ans=0

for p=ll:i:ul

f=(sin(k0*w*cos(p)/2)/cos(p))^2*sin(p)^3

if p==ll

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ans=ans+f

else

ans=ans+2*f

end

end

sum=i/2*ans

gl=sum/(120*pi^2)

ans1=0;

for x=ll:i:ul

f1=(sin(k0*w/2*cos(x)/cos(x))^2)

f2=besselj(0,k0*l*sin(x))

f3=sin(x)^3;

f=f1*f2*f3;

if x==ll

ans1=ans1+f

elseif x==ul

ans1=ans1+f

else

ans1=ans1+2*f

end

end

gl2=1/(120*pi^22)*i/2*ans1

rin=1/(2*(gl+gl2))

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y0=1/pi*cos(sqrt(50/rin))

feedpoint=1/(2*(er^0.5))

%BEAM WIDTH CALCULATION

B_H=2*cos(sqrt(1/(2+k0*w)))*(180/pi)

B_E=2*cos(sqrt(7.03/(3*k0^2*w^2+(k0*h)^2)))*(180/pi)

%DIRECTIVITY CALCULATION

gr=1/rin

dir=4*(k0*w)^2/(pi*et0*gr)

dirdb=10*log10(dir)

%RADIATION EFFICIENCY

pr3=10*k0^2*h^2*(l-l/er+(2/(5*er^2)))

x0=lam/lamg

x1=x0^2-1

x2=er-x0^2

psur1=30*pi*k0^2*er*x1

psur2=er*(1/sqrt(x1)+sqrt(x1)/x2)+k0*h*(1+er^2*x1/x2)

psur3=psur1/psur2

efficiency=pr3/(pr3+psur3)

efficiencyp=efficiency*100

%GAIN CALCULATION

gain=efficiency*dir

gaindb=10*log10(gain)

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8.2 Using IE3D Software

Electromagnetic simulation is a new technology to yield high accuracy analysis and

design of complicated micro wave and RF printed circuit antennas, high speed digital circuits

and other electronic components.

IE3D is an integral equation, method-of-moment, full wave electromagnetic simulator

solving current distribution on 3D metallic substance in multi layered dielectric environment.

It includes lay out editor, electromagnetic simulator, schematic editor and circuit simulator,

near field calculation program, format converter, current and field display program. The

IE3D employs a 3D non uniform triangular and rectangular mixed meshing scheme.

It solves the current distribution, slot field distribution, network s-parameter, radiation

pattern and near field on an arbitrarily shaped and oriented 3D metallic structure in a multi

layered dielectric environment. It solves the Maxwell’s equations in integral form and it

solutions includes the wave effects, discontinuities effects, coupling effects and radiation

effects. The simulation result include s-, y-, z- parameters, VSWR, RLC equivalent circuits,

current distributions, near field, radiation patterns, directivity, efficiency and gain.

8.2.1 IE3D Simulation Capabilities

IE3D is an integrated electromagnetic simulation and optimization package for the

analysis and design of the micro wave circuits, micro wave and millimeter wave integrated

circuits (MMIC), RF printed circuits, HTS circuits and filters, micro strip patch antennas,

strip lines, coaxial lines, rectangular wave guides, 3D interconnects, conical and cylindrical

helix antennas, inverted antennas, wire antennas, various kinds of wireless antennas, IC

transmission lines, IC packaging, electromagnetic applications in medical sciences.

8.2.2 IE3D Features

1. Modeling true 3D metallic structures in layered dielectric environments.

2. High efficiency, high accuracy and low cost electromagnetic simulations tools on PCs

with Windows Based Graphic Interface.

3. The MS Windows based menu driven graphic interface allows interactive

construction of 3D and multi layered metallic structures as a set of polygons.

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4. A built-in library enables the construction of complicated structures such as circles,

rings, spheres, rectangular and circular, spiral, cylindrical and conical vias and

helices.

5. Automatic non uniform mesh generator with rectangular and triangular cells.

6. Flexible de-embedding of circuit parameters.

7. Modeling structures with finite ground planes and differential feed structures.

8. Accurate modeling of true 3D metallic structure and metal thickness.

9. Electromagnetic optimization. Modeling of thin, lossy and high dielectric constant

substrates. Mixed electromagnetic and nodal analysis.

10. Efficient matrix solvers.

11. Cartesian & Smith chart display of the S, Y, Z parameters, VSWR and radiation

patterns.

12. Extracting RLC equivalent circuits for structures.

13. 3D and 2D display of current distribution, radiation patterns and near field. Magnetic

current modeling of slot structures.

14. “Simulate and find Excitation” features allowing monitoring of array power

distribution on network.

Flexible utility features and built-in circuit simulator.

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Figure 8.1 Simulated 3-D Radiation Patterns(Coaxial Feed)

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Figure 8.2 3D Radiation Pattern of Rectangular Patch

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9. ANALYSIS & RESULTS

9.1 Analysis

This section deals with the comparing the measured values with the obtained values.

Thus we can analyze the differences between them. The comparison is as follows:

ANTENNA

PARAMETERSMEASURED OBTAINED

Length 31.16 mm 31.16 mm

Width 40.21 mm 40.21 mm

Thickness 1.6 mm 1.6mm

Gain 5.98 dB 4.48 dB

Beam Width E- plane 73.49(degrees) 82(degrees)

Beam Width H- Plane 115.16(degrees) 120(degrees)

Table 9.1 Comparison between measured and obtained values

From the above we finally conclude that the measured values and the obtained values

are approximately equal. Thus this project has been carried out successfully. The efficiency

can be improved by increasing the dielectric constant of the material. For Aero Space

Vehicles smaller Band Width is required which have been seen in the Micro Strip Antenna.

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9.2 Test Results

9.2.1 Return Loss

Fig 9.1 Plot of Return Loss versus Resonant Frequency

This is the return loss plot obtained from a vector network analyzer. The X-axis

indicate the frequency in GHz and the Y-axis indicates S11 indB.

At 2.2775 GHz the return loss is -11.461 dB

At 2.2585 GHz the return loss is -9.5464 dB

At 2.3017 GHz the return loss is -9.5451 dB

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Fig 9.2 Plot of VSWR versus Resonant Frequency

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9.2.2 Impedance measurement:

From the above setup the input impedance of the microstrip antenna also can be

plotted on a smith chart and the obtained result is shown below:

Fig 9.3 Impedance Measurement

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9.2.3 Gain Measurement

Fig 9.4 Anechoic Chamber during Readings

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Table 9.2 Gain Measurement

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9.2.3 Radiation Pattern

Fig 9.5 Radiation Pattern Horizontal and Vertical Plane

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Fig 9.6 Microstrip antenna top view

Fig 9.7 Patch Antenna Fabricated On 1.6 mm Thin FR4 Substrate

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Fig 9.8 SMA connector on ground plane of Microstrip antenna

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10. APPLICATIONS

Microstrip antennas are the promising candidates for microwave and millimeter wave

applications where low cost, low profile, conformability and ease of manufacture are

required. Some of the principal applications of micro strip antenna are

1. RADARS

2. Missile Telemetry

3. Aircraft

4. Satellite Communications

5. Mobile Communication Base Station

6. Mobile Communication Headsets

7. Navigation

8. GPS(Global Positioning System)

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11. CONCLUSION

A rectangular micro strip antenna is designed using the appropriate design formulae

and is fabricated using the quick fabrication procedure and is tested using the vector network

analyzer N5203A.

The antenna design is worked out at frequency 2270MHz frequency. Even though the

antenna is desired to operate at this frequency, when tested practically it is found that, it is

resonating at 2277MHz.

The dielectric constant plays a major role in the overall performance of a patch

antenna. It affects both the width, in turn the characteristic impedance and the length resulting

in an altered resonant frequency. We have used the fiber glass substrate but the permittivity

(εr) alters from batch to batch sometimes even between different sheets of substrates. In

addition FRP-4 has a high loss tangent and is highly frequency dependent. This has become

an issue for RFID applications above 800MHz.

The bandwidth of the patch antenna depends largely on the permittivity (εr) and

thickness of the dielectric substrate. Ideally a thick dielectric lower permittivity (εr) low

insertion loss is preferred for broad band purpose.

From the result 1 observed that the beam width of the micro strip element can be

increased by choosing a smaller element, thus reducing W and L. For the given resonant

frequency these dimensions will be changed by selecting a substrate having a higher relative

permittivity. The advantages of the micro strip antenna are that they are low cost,

conformable, light weight and low profile, while both linear and circular polarization is easily

achieved. These attributes are desirable when considering RFID RADAR systems.

This antenna material is also ideal for antenna arrays. Longer ranges, larger areas,

faster assembly line speeds will all benefit from the focused energy and directionality

available through antenna array beam forming. The print and etch process of printed circuit

board is very repeatable and highly cost effective. It eliminates the labor and the technician

work required to insure proper phase matching between elements. It also reduces energy

requirements of the system. The reduced side lobe emissions reduce false alarms, reduce

interference between other antennas and minimize emission in unwanted directions.

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12. FUTURE SCOPE

Microstrip antennas find their use in airborne and spacecraft systems mainly due to

their low profile, conformal nature and easy integration with MICs. This trend is likely to

continue with possibility of latest materials like high temperature superconductors,

conducting polymers, electric or magnetic anisotropic materials as patch conductors for

improving the electrical performance of these antennas. Ferrite materials could also be used

for frequency and polarization agility etc.

Future challenges of a Microstrip antenna are:

Bandwidth Extension Techniques

Control of Radiation Patterns

Reducing Losses / increasing efficiency

Improving feed networks

Size reduction techniques

The band width can be increased as follows

By increasing the thickness of the substrate

By use of high dielectric constant of the substrate so that physical dimensions of

the parallel plate transmission line decreases.

By increasing the inductance of the micro strip by cutting holes or slots in it.

By adding reactive components to reduce the VSWR

In order to increase the directivity of the micro strip antennas multiple micro strip

radiators are used to cascade to form an array.

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13. REFERENCES

Books

[1]. R. Garg, P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip Antenna Design Handbook,

ArtechHouse, 2000.

[2]. M. Kulkarni, Microwave and Radar Engineering, Umesh Publication.

[3]. KD. Prasad, Antenna Wave Propagation

[4]. K. F. Lee, Ed., Advances in Microstrip and Printed Antennas, John Wiley, 1997.

[5]. D. M. Pozar and D. H. Schaubert, Microstrip Antennas: The Analysis and Design of

Microstrip Antennas and Arrays, IEEE Press, 1995.

[6]. F. E. Gardiol, “Broadband Patch Antennas,” Artech House.

[7]. S K Behera, “Novel Tuned Rectangular Patch Antenna As a Load for Phase Power

Combining” Ph.D Thesis, Jadavpur University, Kolkata.

[8]. D. R. Jackson and J. T. Williams, “A comparison of CAD models for radiation from

rectangular microstrip patches,” Intl. Journal of Microwave and Millimeter-Wave

Computer Aided Design, Vol. 1, No. 2, pp. 236-248, April 1991.

[9]. D. R. Jackson, S. A. Long, J. T. Williams, and V. B. Davis, “Computer- aided design

of rectangular microstrip antennas”, ch. 5 of Advances in Microstrip and Printed

Antennas, K. F. Lee, Editor, John Wiley, 1997.

[10]. D. M. Pozar, “A reciprocity method of analysis for printed slot and slot-

coupled microstrip antennas,” IEEE Trans. Antennas and Propagation, vol. AP-34, pp.

1439-1446, Dec. 1986.

[11]. C. A. Balanis, “Antenna Theory, Analysis and Design,” John Wiley & Sons,

New York, 1997.

[12]. H. Pues and A Van de Capelle, “Accurate transmission-line model for the

rectangular microstrip antenna,” Proc. IEE, vol. 131, pt. H, no. 6, pp. 334-340, Dec.

1984.

[13]. W. F. Richards, Y. T. Lo, and D. D. Harrison, “An improved theory of

microstrip antennas with applications,” IEEE Trans. Antennas and Propagation, vol.

AP-29, pp, 38-46, Jan. 1981.

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Websites

1. http:// www.ecs.umass.edu/ece/pozar/aperture.pdf

2. http:// www.abasabs.hardvard.edu/abs/2002lnphT.43.335c

3. http:// www.mitre.org/work/tech_papers

4. http:// www.wikipedia.com

5. http:// www.mentorg.com/seamless

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