8/10/2019 9. Antennas_2014.pdf

1/34

9. Radar Antennas

8/10/2019 9. Antennas_2014.pdf

2/34

Functions of an Antenna

The antenna is one of the most critical parts of a radar system. It performs thefollowing essential functions:

! It transfers transmitter energy to space with the required pattern and with highefficiency. This process is applied in an identical way on reception.

! It has to provide the required frequency of target position updates. In the case of amechanically scanned antenna this equates to the revolution rate. A high revolution

rate can be a significant mechanical problem given that a radar antenna in certainfrequency bands can have a reflector with immense dimensions and can weighseveral tons.

! It must measure the pointing direction with a high degree of accuracy.

The antenna structure must maintain constant operating characteristics under allenvironmental conditions.

Radomes are generally used where relatively severe environmental conditions are

experienced.

8/10/2019 9. Antennas_2014.pdf

3/34

Functions of an Antenna

8/10/2019 9. Antennas_2014.pdf

4/34

Radar Antennas

The name antenna has its seeds in the work of the Italian inventor GuglielmoMarconi.

During his experiments with the electromagnetic waves he used a woodentent pole along which was carried a radiating wire. The word for tent pole inItalian is l'antenna.

An antenna transmits or receives electromagnetic waves. It is a transducerto convert electromagnetic waves into high-frequency electrical currents andvice versa.

The size of the antenna will be a multiple of wavelength. The size determinesthe antenna directivity.

There are antennas with more than hundred meters length using HFfrequencies and antennas with a length of just a few millimeters for milli-meter ranges.

8/10/2019 9. Antennas_2014.pdf

5/34

Radiation from an Antenna

0

Z

X

Y

Mu

B =1

! u ! E

E = ! B ! u

The wave propagates with velocity

! =1

"

= # / k

The radiated field in spherical coordinates is: E (r ,! ," ) = exp(! jkr )

kre(! ," )

! =

"

8/10/2019 9. Antennas_2014.pdf

6/34

Radiating Fields

Consider a negatively charged particle sitting somewhere in space. All chargedparticles have an electric field associated with them.

A negative charge has an electric field that occurs everywhere in space.

The electric field is a vector quantity. It has magnitude and a direction.

The field strength reduces with increasing distance.

8/10/2019 9. Antennas_2014.pdf

7/34

Radiating Fields

Consider the charged particle to be disturbed such that the charge accelerates andtravels away at a constant velocity.

The resulting electric field disturbance will propagate away from the charge at thespeed of light - c = 3x10 8 meters/second.

Once the charge has accelerated away (or stopped), the fields re-align themselves.

The fields want to surround the charge exactly as they did before they weredisturbed. However, the fields can only respond to events at the speed of light.

If a point is very far away from the charge, it will take time for the disturbance (orchange in electric fields) to propagate to the point and will be diminished instrength.

8/10/2019 9. Antennas_2014.pdf

8/34

There are 3 regions. In the light blue (inner) region, the fields close to the chargehave readapted themselves and now line up. In the white region (outermost), thefields are still undisturbed and have the same magnitude and direction as they wouldif the charge had not moved. In the pink region, the fields are changing - from theirold magnitude and direction to their new magnitude and direction.

This is the fundamental reason for radiation - the fields change because charges areaccelerated. The fields always try to align themselves around charges. If we producea moving set of charges (i.e. a time varying electric current), then we will haveradiation.

Radiating Fields

8/10/2019 9. Antennas_2014.pdf

9/34

Far Field (Fraunhofer) RegionThe far field is the region far from the antenna, as you might suspect. In thisregion, the radiation pattern does not change shape with distance (althoughthe fields still die off as 1/R, the power density dies off as 1/R ^2). Also, thisregion is dominated by radiated fields, with the E- and H-fields orthogonal toeach other and the direction of propagation as with plane waves.

If the maximum linear dimension of an antenna is D , then the following mustbe satisfied to be in the far field region:

Near Field (Fresnel) RegionThe radiating near field or Fresnel region is the region between the near andfar fields. In this region the radiating fields begin to emerge. However, unlikethe Far Field region, here the shape of the radiation pattern may varyappreciably with distance.

Far and Near Fields

R =2 D 2

!

R