Module 7 : Antenna
Lecture 51 : Antenna Arrays

Directions of Nulls

The nulls of the radiation patterns can be obtained by equating the array factor to zero.

   

The directions of the nulls, is given as

 
 
 
Which can be simplified as
 
 
   

For finding the directions of the nulls all possible values of have to be tested.

 

Directions of Side-lobes

Local maximum in the radiation pattern is called the side lobe.
 
There is one side lobe between two adjacent nulls except the main beam.
 

Whenever the numerator of the AF is maximum, there is a side-lobe in the radiation pattern.

 

Generally, the directions of the side-lobes are taken approximately half way between the two adjacent nulls.

 

The direction of a side lobe is given as

 
The amplitude of the side lobe is obtained by substituting the value of in the AF.
 

The amplitude of the side lobe is

For a large array and the side lobe amplitude is approximately
 

The first, second, third side lobe amplitudes are respectively.

 

The important thing to note: The side lobe amplitudes are independent of the array size and the direction of the main beam.

 
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