Radar System Engineering

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EXAMPLES OF RADAR SYSTEM DESIGN [SEC. 15.11
Positive identification. Aircraft control is frequently complicated
by the large number of aircraft operating at the same time within
range of a single radar set. Departure of aircraft from planned
fllght schedules makes reliance on movements information unsatisfactory
for identification. A beacon can afford positive identification
of the aircraft carrying it.
Freedom from ground clutter. By having the beacon reply at a
frequency different from that used by the radar for interrogation,
the beacon replies can be displayed without any confusing radar
echoes.
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FIQ. 15.S.—Beacon receiving antennamounted above lower-beamradar antenna.
Airborne beacons have been provided, almost without exception, with
vertically polarized antennas. They will thus work with the lower beam
of the set just described, but not with the upper beam since the latter is
horizontally polarized. The beacon is arranged to be triggered by the
pulses from the radar, but it replies on a different frequency, to receive
which an entirely separate beacon receiving antenna and receiver system
are provided. The beacon antenna, mounted on top of the lower-beam
radar antenna, is shown in Fig. 15.8. It is a horn-fed paraboloid cut to
8 ft by 4 ft. Despite the greater beamwidth of the beacon antenna than
that of the radar antenna, the beacon signals are almost as narrow in