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Microstrip Patch Antenna

Introduction

Microstrip patch antennas are widely used because of their many advantages, such

as low profile, light weight, and conformity.

Microstrip antenna in its simplest configuration consists of a radiating patch on

one side of a dielectric substrate (r 10), which has a ground plane on the other side.

The very thin (t

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The radiating patch may be square, Rectangular, Thin strip (dipole), Circular,

Elliptical, Triangle or any other configuration. These configurations are as shown in the

figure below. Square, Rectangular, Dipole and circular are the most common used

configuration because of easy analysis, fabrication and their attractive radiation

characteristics, especially low cross-polarization radiation.

Common Shapes of the Patch

Feed Techniques of Microstrip Antennas:

Microstrip patch antenna has various method of feeding techniques. These

feeding techniques are being put as different categories. They are,

Contact Feeding Technique. Non-Contact Feeding Technique.

Contact Feeding Technique:

Contact Feeding Technique is the one where the power is being fed directly to

radiating patch through the connecting element i.e. through the Microstrip line.

Contact Feeding Technique

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Non-Contact Feeding Technique:

Non-Contact Technique is the one where an electromagnetic coupling is done to

transfer the power between the Microstrip line and the radiating patch.

Non-Contact Feeding Technique.

A feed line is used to excite to radiate by direct or indirect contact. There are

many different methods of feeding and four most popular methods are,

1. Microstrip line feed2. Coaxial probe3. Aperture coupling4. Proximity couplingMicrostrip Line Feeding:

Microstrip line feed is one of the easier methods to fabricate as it is just a

conducting strip connecting to the patch and therefore can be considered as extension of

patch. It is simple to model and easy to match by controlling the inset position. However

the disadvantage of this method is that as substrate thickness increases, surface wave and

spurious feed radiation increases which limit the bandwidth.

Microstrip patch line connected directly to the edge of the patch.

Both elements located on the same substrate.

Microstrip feed line is also a conducting strip smaller in width compared to thepatch.

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Easy to fabricate.

Simple to match by controlling the inset position.

Found experimentally that cutting such an inset does not significantly affect theresonant frequency but it modifies the input impedances.

Substrate thickness increases surface waves which for practical designs limit theBand width.

Microstrip line feeding.Coaxial Feeding:

Coaxial feeding is feeding method in which the inner conductor of the coaxial is

attached to the radiation patch of the antenna while the outer conductor is connected to

the ground plane.

Coaxial Probe

Inner conductor of the coaxial is attached to the radiation patch.

Outer conductor of the coaxial is connected to the Ground plane.

Easy to fabricate and match.

It has low spurious radiation.

It has narrow bandwidth.

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Control line is parallel to the ground plane.

Center conductor extends across the dielectric substrate and is connected to thepatch.

Input impedances depended on the position of the feed.

It is designed independently.

Advantages:

Easy of fabrication

Easy to match

Low spurious radiation Disadvantages:

Narrow bandwidth

Difficult to model specially for thick substrate

Possess inherent asymmetries which generate higher order modes which producecross-polarization radiation.

Aperture Coupling:

Aperture coupling consist of two different substrate separated by a ground plane.

On the bottom side of lower substrate there is a microstrip feed line whose energy is

coupled to the patch through a slot on the ground plane separating two substrates. This

arrangement allows independent optimization of the feed mechanism and the radiating

element. Normally top substrate uses a thick low dielectric constant substrate while for

the bottom substrate; it is the high dielectric substrate. The ground plane, which is in themiddle, isolates the feed from radiation element and minimizes interference of spurious

radiation for pattern formation and polarization purity.

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The radiating patch and Microstrip line are being divided by the ground plane.

Main features wider bandwidth and the shielding of the radiating patch from theradiation gets from substrate.

Coupling between patch and feed line through the aperture in the ground plane.

Lower and upper substrate parameter are chosen separately to increase thebetween and for the optimization of the feed and radiation separately.

There is a feature of improving the polarization purity.

The aperture slot can be of any size shape.

Aperture coupling

Advantages:

Allows independent optimization of feed mechanism element.

Disadvantage:

Difficult to fabricate as this has got multiple layers, due to this thickness of theantenna increases.

Proximity Coupling:

Proximity coupling has the largest bandwidth, has low spurious radiation.

However fabrication is difficult. Length of feeding stub and width-to-length ratio of patch

is used to control the match.

It has largest bandwidth (13% as high).

Easy to model and has low spurious radiation.

It uses a two layer substrate.

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1. Microstrip lineLower layer.2. Patch Antenna upper layer.

This feed is better known as an electromagnetically coupled Microstrip feed. Coupling between the patch and the Microstrip is capacitive in nature. The open end of the Microstrip line can be terminated in a stub.

Stub parameter can be used to increase the bandwidth. The substrate parameters of the two layers can be selected to increase the

bandwidth of the patch and to reduce spurious radiation from the end of

the Microstrip. For this, lower layer should be thin. Radiating patch being placed on the double layer gives the layer bandwidth. Fabrication is slightly more difficult because of the requirement for

accurate alignment between the patch and feed line.

The length of the stub and the width-to-line of the patch can be used to control thematch.

Proximity Coupling

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Characteristics of the different feed techniques

Microstrip Coaxial Probe Aperture Proximity

CharacteristicsLine Feed

FeedCoupled Coupled Feed

Feed

Spurious feed

radiationMore Very Less Less Less

Poor due to

Reliability Bettersoldering

Good Good

Soldering and Alignment Alignment

Ease of fabrication Easy

drilling needed required required

Impedance matching Easy Easy Easy Easy

Bandwidth (achieved

with impedance2-5% 2-5% 13% 21 %

matching)

Rectangular Microstrip Antennas:

The Rectangular MSA is one of the simplest and widely used MSA

configurations. For a simple Microstrip line, the width is much smaller than wavelength.

However, for rectangular MSA, the width is comparable to the wavelength to enhance the

radiation field from edges.

For the fundamental TM10 mode, the length L should be slightly less than / 2 ,

where is the wavelength in the dielectric medium. Here, is equal to o/ e , where o

is free space wavelength and e is the effective dielectric constant of the patch. The

value of eis slightly less than r, because the fringing fields around the periphery patch

are not confirmed in the dielectric substrate but are also spread in the air.

The expression for calculating the value of e is given,

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r1 r112h1/ 2e 1

2 2 w

Radiating Patch

substrateFeed point

Ground Plane

Rectangular Microstrip Antenna

It may be observed from Fig that the vertical components of the electric field (E-

field) at the two edges along the width are in opposite directions and hence they cancel in

broad side direction, whereas the horizontal components are in same direction and hence

they add. Therefore, the edges along the width are termed as radiating edges. The fields

due to the sinusoidal distribution along the length get cancel in broadside direction, and

hence the edges along the length are known as non-radiating edges. The fringing fields

along the width are modeled as radiating slots as shown in the below figure.

Fringing Fields

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Radiation from Microstrip Antenna:

Radiation may be ascribed mostly to fringing fields at the open circuited edges of thepatch.

The fields at the end can be resolved into normal and tangential components withrespect to the ground plane.

The normal components are out of the patch line is /2 long, therefore the far field

produced by them cancel in broad side direction.

The tangential components (those parallel to the ground plane) are in phase, and theresulting fields combine to give maximum radiated field normal to the surface of the

structure; i.e. the broad side direction.

Therefore, the patch may be represented by two slots /2 apart excited in phase andradiating in the half space above the ground plane.