BSA and EFA

7/25/2019 BSA and EFA

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BSA and EFA

D.ARUN KUMAR

DEPT OF ECE


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TAPERING OF THE ARRAYS

The techniques used for reduction of unwanted side lobes arecalled tapering.

As the amplitudes of currents in the linear array sources is non-

uniform, one way of reducing the side lobe level is to make the

center source radiate more energy than the end ones, i.e. Tapering

is done form the center to end as per some prescription.

In this prescription which derives the name of the array.

Eamples!

Binomial array - tapering follows coefficients of binomial series.

Dolph Tchebysceff arrays - tapering follows coefficients ofTchebysceff polynomial.


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Binomial array

In binomial array side lobes can be totally eliminated if

a" The spacing between the two consecutive elements

does #$T eceed half the wavelength and

b" The current amplitudes in the radiating sources areproportional to the coefficient of successive terms of

the binomial series.


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Binomial arrayDisa!"an#a$es

The width of the beam %&'()" increases, thus

decreasing the directivity.

*or designing a large si+e array %more number of

elements" larger amplitude rate of currents for theradiating sources is required, which is difficult to

achieve in practice.


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Dolph Tchebysceff arrays

These arrays produce the narrowest beam width for a

given side lobe level or vice-versa.

As for a specified side lobe level, the narrowest beam

width is achieved by this distribution, it is consideredto be optimum. %d less than r equal to lamba by "

(y this method it is possible to design a high gain

narrow beam antenna for side lobe levels of -db

in /&* and 0&* bands especially for radars.


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Dolph Tchebysceff arrays A!"an#a$es

It provides a minimum, rather than optimum beam

width for required side-lobe reduction.

It results in side lobes of same amplitude as against

progressively decreasing levels of uniformdistribution.

Tapering is not etreme, unlike in (inomial arrays.

i.e, the ratio of current between the centre element

and end elements is small providing easier feed

design.


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Pattern Multiplication

1et E be the total field of the array of non-isotropic sources,

Ei2%3, 4" the field pattern of individual source,

Ea2%3, 4" be that of the array of isotropic point sources,

Epi2%3, 4" be the phase pattern of individual source and

Epa2%3, 4" be the phase pattern of array of isotropic point sources,

then by definition

E2 5Epi2%3, 4" Ea%3, 4" 6 mulitiplied by5Epi2%3, 4" 7 Epa2%3, 4" 6 is true for

any number os similar sources


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Pattern Multiplication

The total phase pattern is the addition of the phase

patterns of the individual sources and that of the array

of isotropic point sources. This concept is known as

pattern multiplication.A!"an#a$es%

8ives a speedy method of sketching the pattern of

complicated arrays 9ust by inspection.

'roves to be a useful tool in designing antenna arrays.

This method is very accurate as point by point

multiplication of pattern is carried out.

BSA and EFA

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