TWO ELEMENT ARRAY

 Introduction 

In general, antenna array is the radiating system in which several antennas are spaced properly so as to get greater field strength at a far distance from the radiating system by combining radiations at point from all the antennas in the system. The individual element is generally called element of an antenna array.As the antennas may be used in various configurations such as straight line, circle, rectangle etc..

Various Types of Antenna Arrays

1. Broadside Array 
2. End fire Array
3. Collinear Array
4. Parasitic Array

Broadside Array

          The broadside array is the array of antennas in which all the elements are placed parallel to each other and the direction of maximum radiation is always perpendicular to the plane consisting elements.

A typical arrangement of a Broadside array is as shown in the Figure.

A broadside array consist number of identical antennas placed parallel to each other along a straight line. This straight line is perpendicular to the axis of individual antenna. It is known as axis of antenna array. Thus each element is perpendicular to the axis of antenna array. All the individual antennas are spaced equally along the axis of the antenna array. The spacing between any two elements is denoted by‗d‘. All the elements are fed with currents with equal magnitude and same phase. As the maximum point sources with equal amplitude and phase radiation is directed in broadside direction i.e. perpendicular to the line of axis of array, the radiation pattern for the broadside array is bidirectional.

End Fire Array

 The end fire array is very much similar to the broadside array from the point of view of arrangement. But the main difference is in the direction of maximum radiation. 

Collinear Array

As the name indicates, in the collinear array, the antennas are arranged co-axially i.e. the antennas are arranged end to end along a single line as shown in the Fig. 4.2.4 (a) and (b)

Two Point Sources with Currents Equal in Magnitude and Phase

Consider two point sources Al and A2 separated by distance d as shown in the Figure of two element array. Consider that both the point sources are supplied with currents equal in magnitude 

and phase.

Consider point P far away from the array. Let the distance between point P and point sources Al

and A2 be r1 and r2 respectively. As these radial distances are extremely large as compared with 

the distance of separation between two point sources i.e. d, we can assume,The radiation from the point source A2 will reach earlier at point P than that from 

point source Al because of the path difference. The extra distance is travelled by the radiated wave from point source Al than that by the wave radiated from point 

source A2.

Hence path difference is given by,

From the optics the phase angle is 2π times the path difference. Hence the phase angle is given 

by 

Phase angle = ψ =2π (Path difference)

Above equation represents total field intensity at point P, due to two point 

sources having currents of same amplitude and phase. The total amplitude of the 

field at point P is 2E0 while the phase shift is βdcos@/2.

The array factor is the ratio of the magnitude of the resultant field to the magnitude of the maximum field.

The array factor represents the relative value of the field as a function of ϕ. It defines the radiation 

pattern in a plane containing the line of the array.

Dipoles with Parasitic Elements

Let I1 be current in the driven element D. Similarly 12 be the current induced in the parasitic 

element P. The relation between voltages and currents can be written on the basis of circuit 

theory as,

V1 = Z11 I1 + Z12 I2

0 = Z21 I1 + Z22 I2

 that as parasitic element P is not excited, the applied voltage V2 is written zero. V1 is 

the applied voltage to the driven element D. The impedances Z11 and are the self-impedances 

of the driven element D and the parasitic element P. The impedances Z12 and Z21 is the 

mutual impedance between the two elements such that,

Z12 = Z21 =ZM

Array in free space with one parasitic element and one driven λ/2 dipole element.

The current 

The resistance is given by

The gain in field intensity as a function of θ is

When the λ/2 parasitic element is larger than its resonant length, it is inductive in nature. 

Then such parasitic element acts as reflector.

Similarly when the λ/2 parasitic element is shorter than its resonant length, it is capacitive in 

nature. Then such parasitic element acts as director.