Antenna Arrays

Submitted To

Sumita Shekhawat

Submitted By

Utkarsh Chinmay Paliwal

2011BTechECE024

Dinesh Kumar Goyal

2011BTechECE009

Contents

1.Antenna Arrays

2.Design Principles of Arrays

3.Types of Arrays

4. Isotropic Points

5. Radiation Pattern & Array Factor

6. Array pattern

7. Application of Arrays

Antenna Arrays

• An antenna array (often called a 'phased array') is a set of

2 or more antennas. The signals from the antennas are

combined or processed in order to achieve improved

performance over that of a single antenna.

• An antenna array is a cluster of antennas arranged in a

specific physical configuration (line, grid).

• Each individual antenna is called an element of the array.

• We initially assume that all array elements (individual

antennas) are identical.

• The excitation (both amplitude and phase) applied to each

individual element may differ.

• The far field radiation from the array in a linear medium

can be computed by the superposition of the EM fields

generated by the array elements.

Design Principles of Arrays

Array Design Variables:

1. General array shape (linear, circular, planar)

2. Element spacing.

3. Element excitation amplitude.

4. Element excitation phase.

5. Patterns of array elements.

Types of Arrays

• Linear array - antenna elements arranged along a straight line.

• Circular array - antenna elements arranged around a circular ring.

• Planar array - antenna elements arranged over some planar surface

(example - rectangular array).

• Conformal array - antenna elements arranged to conform two some

non-planar surface (such as an aircraft skin).

• Broadside - maximum radiation at right angles to main axis of antenna

• End-fire: maximum radiation along the main axis of antenna

• Phased: all elements connected to source

• Parasitic: some elements not connected to source

– They re-radiate power from other elements

Types of Array

Yagi-Uda Array

• Often called Yagi array

• Parasitic, end-fire, unidirectional

• One driven element: dipole or folded dipole

• One reflector behind driven element and slightly longer

• One or more directors in front of driven element and slightly shorter

Log-Periodic Dipole Array

• Multiple driven elements (dipoles) of varying lengths

• Phased array

• Unidirectional end-fire

• Noted for wide bandwidth

• Often used for TV antennas ends

Types of Array Monopole Array

• Vertical monopoles can be combined to achieve a variety of horizontal patterns

• Patterns can be changed by adjusting amplitude and phase of signal applied to each

element

• Not necessary to move elements

– Useful for AM broadcasting

Collinear Array

• All elements along same axis

• Used to provide an omnidirectional horizontal pattern from a

vertical antenna

• Concentrates radiation in horizontal plane

Broadside Array

• Bidirectional Array

• Uses Dipoles fed in phase and separated by 1/2 wavelength

End-Fire Array

• Similar to broadside array except dipoles are fed 180 degrees out of phase

• Radiation max. off the

Isotropic Points

The individual elements are characterized by

their element patterns F1(,).

At an arbitrary point P, taking into account the

phase difference due to physical separation

and difference in excitation, the total far zone

electric field is:

2 2

1 2( ) ( ) ( )j jE r E r e E r e

Field due to antenna 1 Field due to antenna 2

Here: coskd

The phase center is assumed at the array center. Since the elements are identical

2 2

1 1( ) 2 ( ) 2 ( )cos2 2

j je eE r E r E r

Relocating the phase center point only changes the phase of the result but not its

amplitude.

Radiation Pattern & Array Factor

• The radiation pattern can be written as a product of the radiation pattern of an

individual element and the radiation pattern of the array (array pattern):

1( , ) ( , ) ( , )aF F F

• The array factor is:

cos( , ) cos

2a

kdF

Here is the phase difference between two antennas. We notice that the array

factor depends on the array geometry and amplitude and phase of the excitation

of individual antennas.

Case 1: 2 isotropic point sources of same amplitude and phase

Case 1: 2 isotropic point sources of same amplitude and phase

Phase Difference =

Path Difference =

Case 1: 2 isotropic point sources of same amplitude and phase

The total field strength at a large distance r in the direction is :

Therefore:

Case 2: 2 isotropic point sources of same amplitude and opposite phase

Case 2: 2 isotropic point sources of same amplitude and opposite phase

Array pattern

Applications of Arrays

• To cancel co channel interferences : an array works on the

premise that the desired signal and unwanted co channel

interferences arrive from different directions.

• The beam pattern of the array is adjusted to suit the requirements

by combining signals from different antennas with appropriate

weighting.

• An array of antennas mounted on vehicles, ships, aircraft, satellites, and base stations is expected to play an important role

in fulfilling the increased demand of channel requirement for

these services

Application of Arrays

Use of an antenna array is to enhance the efficiency of

mobile communications systems. It presents an overview of

mobile communications as well as details of how an array may be used in various mobile communications systems,

including land-mobile, indoor-radio, and satellite-based

systems.

In a mobile communications system, highlights

improvements that are possible by using multiple antennas

compared to a single antenna in a system, and provides

details on the feasibility of antenna arrays for mobile

communications applications.

References

Wikipedia

CiteSeerX

E-Learning VTU

Antenna Theory