RADAR ANTENNAE

Introduced by Dudley Roessler, Member, on 25 June 1999

Following a general introduction to the use and operation of radar systems which was given earlier, it seemed appropriate to consider the means by which radar pulses are direct towards likely targets.

It was first explained how radar pulses were generated and fed along waveguides to a non-reciprocal switch. This component is known as the T-R (transmit-receive) Switch. Pulses leaving this switch cannot leak into the receiver device which would simply burn out if that happened. They then continue with negligible loss towards the feed horn or array of horns (or other radiators). Whichever is used is called the Primary Feed.

The Primary Feed then radiates the energy towards the focusing surface, usually a parabolic reflector, which generates a narrow beam. Several different sorts of reflector were described in the talk: all caused the microwave energy to travel in a direction dictated by the shape and orientation of the antenna.

Pulses which encounter a target are scattered. That part of the scattered pulse energy which returns along the beam direction is intercepted by the antenna, routed back through the Primary Feed and into the T-R switch. The energy level reaching the receiver may be no higher than a fraction of a microwatt — all that remains of the megawatt of peak power commonly generated by the magnetron which produced the pulse in the first place.

A short explanation followed of the conflict between getting the narrowest possible radiation pattern, so as to achieve higher acuity, hence greater accuracy of target-indication, and the inevitable production of "side-lobes". These extraneous lobes of undesirable radiation may radiate up to 5% as much energy as the main beam. They pose the problem that a large close-in target will produce the same response in the receiver (but indicated in the wrong direction), as a more distant but vitally necessary one.

Finally, some techniques were explained for reducing side-lobes in radar antennae at the expense of slightly broadening the main beam, hence slightly reducing its gain and thus the detection range of the radar. By super-imposing transparencies of typical radiation patterns, some of these effects were demonstrated and quantified.

Dudley Roessler