lec 1terms and definitionss

7/29/2019 Lec 1Terms and Definitionss

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Antenna Parameters



• An antenna or aerial is a device for radiating

or receiving electromagnetic waves.• There is little fundamental difference between

transmitting and receiving antennas since

very often same antenna is used for bothpurpose as in radar.

• Antennas are also defined as “a metallic

device” (as a rod or wire for radiating or receiving radio waves)

Antenna



• Since antennas play a very important part in a

comm systems, it is essential to have thorough

understanding of the principles on which they

perform their task efficiently.

• In the past ants have been treatedsomewhat differently from the sys in which

they are used.

• But in recent years ants are designed alongsidethe system. This is because it is essential to

know the system performance as a whole rather

than that of the antenna alone.

Antenna



Electromagnetic Waves

• em waves

Em waves

• How radio waves are producedradio waves

• Impedance matching impedance matching

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Radiation Pattern

An ant radiation pattern or antenna pattern is

defined as a mathematical function or a

graphical representation of the radiation

properties of the antennas as function of space

coordinates.

In most of the cases determined in the Far Field

Region.

Radiation properties include power flux density,

radiation intensity, field strength, directivity

phase or polarization.



Radiation Pattern

The radiation property of most concern is the

two or three dimensional spatial distribution of

radiated energy as a function of the observer’s

position along a path or surface of constant

radius.

Received power at a constant radius is called

power pattern & E/H field as field pattern.

In simple words it is also defined as angular

variation of radiation around the antenna.



2-D Radiation Patterns

Linear Power Power (dB)



3-D Radiation Pattern



Radiation Pattern Lobes

Various parts of radiation pattern are referred to

as lobes which may be sub classified into major or main, minor, side and back lobes.

A radiation lobe is a portion of the radiation

pattern bounded by regions of relatively weakradiation intensity.

A major lobe (also called main learn) is defined

as “the radiation lobe containing the direction of

maximum radiation”. A minor lobe is any lobe except a major lobe. All

the lobes with the exception of the major lobe

can be classified as minor lobes.




A side lobe is “a radiation lobe in any direction

other than the intended lobe”. Usually a side

lobe is adjacent to the main lobe and occupies

the hemisphere in the direction of main beam.

A back lobe is radiation lobe whose axis makes

on angle of approximately 180° wrt beam of an

antenna. Usually it refers to a minor lobe that occupies

the hemisphere in a direction opposite to that of

the major lobe.




Level of minor lobe db (that lobe/main

lobe)

Minor lobes < -20 db not desirable

If not minimized in radars – false target

indication

Minor lobes usually represent radiation inundesired directions and they should be

minimized.



Radiation Lobes



On Linear Scale



Field RegionsThe space surrounding an antenna is

usually subdivided into three regions:- –Reactive Near Field

–Radiating Near Field (Fresnel)

–Far Field (Fraunhofer)

These regions are so designated to

identify the field structure in each.

Although no abrupt changes in the field

configurations



Field Regions

Although no abrupt changes in the field

configurations are noted as the

boundaries are crossed, there are distinct

differences amongs them.The boundaries separating these regions

are not unique although various criteriahave been established and are commonly

used to identify regions.



Reactive Near Field

• It is defined as that portion of the near field region

immediately surrounding the antenna wherein the

reactive field predominates:-

• For most antennas the outer boundary of this region is

commonly taken to exist at a distance

from the ant where D is the largest dimension of the

antenna and is the ware length.

• For a very short dipole it is taken as

3

62.0 D R

2



Radiating Near Field

• Defined as that region of the field of an antenna between

the reactive near-field region and the far-field whereinthe radiation fields predominate and wherein the angular

field distribution is dependent upon the distance from the

antenna.

• For an antenna focused at infinity, the radiating near fieldregion is sometimes reerredf to as Fresnel region.

• If an antenna has a max overall dimension which is very

small compared to the wavelength this field region may

not exist.• This region is b/w and

22 D R

3

62.0 D R



Far Field (Frounhofer)

• That region of the field of an antenna where the

angular field distribution is essentially independent

of the distance from antenna.

• If the antenna has a maximum overall dimension D,

the far field is commonly taken to exist at a distance

greater than from antenna.

• For an antenna focused at infinity, the far field

region is sometimes referred to as the Fraunhofer

region on the basis of analogy to optical

terminology.

2

2 D



Radian and Steradian

•Radian

– A measure of a plane angle is a radian.

– One radian is defined as” the plane angle with its

vertex at the centre of a circle of radius r that is

subtended by an are whose length is r.

– Since the circumference of a circle of radius r is

there are rad ( ) in a full circle.

r C 2

r 2r

r 2



Radian and Steradian

• Steradian

– The measure of a solid angle is a steradian.

– One steradian is defined as “ the solid angle with its vertex

at the centre of a sphere of radius r that is subtended by aspherical surface area equal to that of a square with each

side of length r.

– Since the area of a sphere of radius r is there are

( ) in a closed sphere. – The infinitesimal area dA on the surface of a sphere of

radius r is

– Therefore the element of solid angle of a sphere can

be written as

24 r A

2

24

r

r

d r dA sin2

2

m

d

sr d d d sin



A t G i



Antenna Gain

Another useful measure describing the performance of an

antenna is the gain. Although the gain of the antenna is closely

related to the directivit

It is a measures that takes into account the efficiency of the

antenna as well as its directional capabilities.

Absolute gain of an antenna (in a given direction) is defined as “

the ratio of the intensity in a given direction to the radiation

intensity that would be obtained if the power accepted by the

antenna were radiated isotropically.

Mathematically represented as:-

Gain = radiation intensity

total input (accepted) power

4

A t G i



Antenna Gain

An atternate way to define antenna gain is :-G = Power radiated by an ant

Power radiated by ref ant

The i/p power to both the antenna is the same and the reference

ant generally chosen is an isotrope.



Polarization Polarization is defined as “that property of the

electromagnetic wave describing the time varyingdirection and relative magnitude of the electric field

vector; specially the figure traced out as a function of

time by the extremity of the vector at a fixed location in

space and the sense in which it is traced as observed

along the direction of propagation.

Polarization is the curve traced out by the end point of

the arrow representing the instantaneous electric field.

The field must be observed along the direction of

propagation. Polarization can be classified as linear, circular or

elliptical. If the vector that describes the electric field at a

point in space as a function of time is always directed

along a line, the field is said to be linearly polarized.



Polarization (contd) In general however, the figure that the electric field

traces is an ellipse and the field is said to be ellipticallypolarized.

Linear and circular polarizations are special cases of

elliptical and they can be obtained when the ellipse

becomes a straight line or a circle respectively.



Polarization (rotation of wave)



Polarization Ellipse

polarization

Radiation Resistance

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Radiation Resistance

• An important property of a transmitting ant is its radiation

resistance which is associated with the power radiated by the

ant. If

I = rms ant current

= antenna radiation resistance

Then power radiated is I2 watts where is a fictitious

resistance which accounts for the radiated power somewhatlike a acct resistance which dissipates heat.

• The radiation resistance should be large as the greater is,

the greater the power radiated by ant.

• In contrast, for a receiving antenna its i/p impedance isimportant. The i/p impedance is defined as the ratio of voltage

to correct at its i/p and it should be matched to connecting lines

or cables.

• The i/p impedance may or may not equal to its radiation

resistance, though very often it does.

r R

r R r

R

r R



Effective Length

• An antenna with a non-uniform distribution of current

over its length l can be considered as having a shorter

effective length le over which the current is assumed to

be uniform and equal to its peak value. The relationship

b/w le and l is given by:-

= area under non – uniform current distribution

area under uniform peak current distributionl

l e



Effective Aperture

• The power received by an antenna can be associated with a

collecting area. Every antenna may be considered to have

such a collecting area which is called its effective aperture A.

• If is the power density at the antenna and is the received

power then.

• = A watts

or

For an antenna with power gain G, the effective aperture A atthe operating wavelength λ is given by

d P

r P

r P

d P

2m

P

P A

d

r

4

2G

A



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