10_principle of antenna

7/28/2019 10_Principle of Antenna

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10PRINCIPLE OFANTENNA SYSTEM

Introduction

Antenna is usually a metalic device (as a rod or a wire) used for radiating or receivingelectromagnetic waves. The radio frequency power developed at the final stage of a

transmitter is delivered through cables/feeders, without themselves consuming anypower to the transmitting antenna. This travels in the free space in the form of radiowaves (electromagnetic waves). The receiving antenna picks up the radio waves anddelivers useful signal at the input of a receiver for reception of signals. The transmittingand receiving antennae are reciprocal in the sense, any characteristics of the antenna ingeneral applies equally to both.

Antenna Radiation Resistance

The input impedance Zin of an antenna is the ratio of voltage to current at itsinput terminals where the power is fed to the antenna.

Zin = Ra + jXa

Ra = Resistive part of impedance

Xa = Reactive part of impedance

Ra = Rr+Ri

Rr = Radiation resistance of the antenna

Ri = Ohmic loss resistance of the antenna.

It is through the mechanism of radiation resistance, power is transferred from the guidedwave at antenna input to the free space wave.

The reactive part of the input impedance is due to the storage of electric magnetic field(capacitive and inductive reactances) in the near field of the antenna. The net reactiveimpedance of the antenna can be matched with the conjugate impedance of the sourcedriving the antenna.


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Basic Course

Radiation Resistance is a fictitious term. It is equivalent of resistance which woulddissipate the same amount of power as being radiated by the antenna when fed with thesame amount of power.

Radiation Efficiency

The radiation efficiency determines the effective transfer of power from the input to freespace, and given by

Radiation Efficiency =ri

r

RR

R

+

Field Regions

a) Reactive near field (induction field) upto a distance

3

62.0D

b) Radiating near field (fresnel field) beyond near field upto a distance of

22D

.

c) Far field (Fraunhofer field) beyond fresnel field where 'D' is the largest dimensionof the antenna.

The measurements of a radiation pattern, gain etc. must be made only in the far fieldregion.

Isotropic Antenna

It is an imaginary (non-existent) point (dimensionless) antenna which radiates equallywith unity gain in all directions in three dimensional planes.

Power Gain of Antenna

Unlike the isotropic antenna, any practical antenna has physical dimension. The field atany point away from the antenna is the vectorial sum of the individual fields received atthat point from a large number of elementary portions of the whole antenna. Dependingupon the path length of these individual waves, they may reinforce or cancel at suchequidistance points around the practical antenna and thus contribute different levels offield in different directions, but at equal distances around the antenna. What howeveractually happens is that instead of laying equal field, field is accentuated in certaindirections and suppressed in other directions.

Extending this principle, very large power gain can be achieved in any plane by stackingin a particular way, a number of antenna elements in a perpendicular plane.

The power gain in a given direction is the ratio of the power to be fed to the isotropicantenna to actual power of the antenna in question to lay field at a given receiving pointin that direction.

STI(T) Publication 124 001/BC/2001


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Principle of Antenna System

However, in practice, the gain of the antenna is always with respect to the dipoleantenna. The dipole itself has a gain of 2.1 db (1.64 times) over the isotropic antenna.

Radiation Pattern (Polar Diagram)

Graphical representation of the directional radiation properties of the antenna as a

function of space coordinates in three dimensions is called the radiation pattern. Such arepresentation will be usually very complicated to interpret. It is usual practice torepresent the same in two dimensions for both horizontal and vertical planes. Thelength of vector from the centre or the reference point is proportional to the power gain inthat direction.

Half Power (3 dB) Beam Width

The angle between the two directions in which the radiation intensity is one half (3 dBbelow) the maximum value of the beam.

Bandwidth of Antenna

The range of frequencies within which the performance of the antenna with respect tocertain characteristic (such as input impedance, pattern, beam width, polarisation, sidelobe level, beam direction, gain)conforms to a specified standard. More commonly inbroadcasting the characteristics of importance are gain and input impedance.

Polarisation

The plane containing the electric vector in the electro magnetic wave describes thepolarisation of the radiated wave. Ideally maximum signal is coupled if the antennae(both transmitting and receiving) are oriented in the plane of polarisation of the electro-magnetic wave. A vertical radiator radiates/picks up vertically polarised wave,horizontal radiator radiates/picks up horizontally polarised wave.

There are number of well defined polarisations such as horizontal (HP), vertical (VP),slant (+ 45o (SP), circular (left or right) (LCP, RCP), dual (DP), mixed (MP), elliptical (leftor right) (LEP/REP) etc.

HP : The electric vector is in horizontal plane. TV broadcasting in India use horizontalpolarisation.

VP : The electric vector is in the vertical plane. The self-radiating MW masts of AIRradiate VP waves. Electric supply undertakings use vertical polarisation for their VHFcommunications.

CP: The electric vector in circular polarisation rotates in a circular motion. They maybe considered as the resultant of equal amplitude of vertical and horizontal polarisedcomponents combined in phase quadrature (90o).

The polarisation is said to be right or left circular polarised (RCP or LCP) depending onthe rotation of electric vector of the propagating wave clockwise or anti clockwise



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Basic Course

respectively, as seen from the transmitting point or by an observer with his back to thetransmitter.

INSAT down link signals are left hand circularly polarised.

Dual Polarisation

In the circular polarisation if there is no definite control on the phase relationshipbetween vertical and horizontal components, the polarisation is said to be dual. FMbroadcasts of AIR employ dual polarisation.

Elliptical Polarisation

In the elliptical polarisation, the two components horizontal and vertical are not equaland the phase relationship between them also is random.

The best example of elliptical polarisation is the shortwave signals arriving on the ground

after reflection from the ionosphere.

Mixed Polarisation

Collective term for slant, circular or dual polarisations.

Apreture of an Antenna 'A'

This term usually relates only to receiving antenna. Aperture (or effective area) of areceiving antenna is the ratio of power delivered to the load (connected to the Antenna)to the incident power density.

4

2GA =

where G is the gain with respect to the isotropic antenna.


10_principle of antenna

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