Leccture--6

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Anten nna parameteers: 1.4.14 Impedance I e Bandwid dth

w w

.m

ac

ha

fo

re

ve

rb

lo

g.

w

dp

or

The symb bols in the abo ove equation are depicted in i Fig. 30 bellow.

re

Impedancce bandwidth is defined as

w

C of im mpedance band dwidth Fig. 30: Concept

hen  = -10 dB B, Note: Wh

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Hence thee impedence bandwidth b can n also be speccified by the ffrequency rannge within whhich VSWR  2.

1.4.15 Effective E Aperture A

re

c powerr from an inccident wave aand deliver it to its It is a meeasure of the ability of an antenna to collect terminals..

rb

lo

g.

w

or

dp

The effective aperture (area) of a reeceiving antenna looking ffrom a certainn direction (, ) is the raatio of the averag ge power PL delivered to a matched load to the maggnitude of thhe average poower density Pavi of the incideent electromaagnetic wave at the positiion of the anntenna multipplied by the normalized ppower pattern Pav(, ) of thaat antenna.

ha

fo

re

ve

The effecttive aperture is related to the t directivity y as:

w

w w

.m

ac

A maximu um effective area Aem can n be defined when w the anteenna is receivving in its maaximum-direcctivity direction. That is,

1.5 Co ommunication Lin nk and Friis F Tran nsmission n Equatioon

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Fig. 31: Transmitting T and a receiving antennas for derivation off Friis transmiission equatioon

re

The Friis transmission equation relaates the powerr received to the power traansmitted betw ween two anttennas o each other.. placed in the far-field of

rb

where et: efficiency of the transmittiing antenna.

g.

w

or

Pt 4R 2

lo

Wo  et

dp

mitting anten nna, then the iisotropic pow wer density att a distance R from If Pt is thee input power to the transm the antenn na can be giveen by:

re

Pt Dt 4R 2

fo

Wt  et

ve

mitting antenn na, this value changes to For a direectional transm

ha

where Dt: directivity off the transmittting antenna.

.m

ac

nsity is receiv ved by the recceiving antennna. In order tto find the vaalue of Not all off the transmittted power den that, we need n to consid der the effectiv ve aperture of the antenna and the absoorption losses in the atmospphere.

w w

Thus the received r pow wer can be exp pressed as:

w

Pr  AerWt But

 2 Aer  e r Dr   4

   2

So

 2  PD    Pr  er Dr    et t t2  er et Dr Dt   Pt R R 4  4  4     

This equation is known as the Friis Transmission Equation and it relates the power Pr (delivered to the receiver load) to the input power of the transmitting antenna Pt. The term (/4πR)2 is called the free-space loss factor, and it takes into account the losses due to the spherical spreading of the energy by the antenna. Friis transmission equation is the basis to find the RADAR range equation.

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At this point, we are in a position to define the term effective (or equivalent) isotropically radiated power, EIRP, a term frequently used in communication systems. It is defined as the power gain of a transmitting antenna in a given direction multiplied by the net power accepted by the antenna from the connected transmitter. Suppose an observer is located in the direction of maximum radiation from a transmitting antenna with input power Pt, then

w

w w

.m

ac

ha

fo

re

ve

rb

lo

g.

w

or

dp

re

EIRP = Pt Gt .