Radar System Engineering

CHAPTER 8
RADAR
BEACONS
BY L. A. TURNER
Introduction. —Radar waves are reflected by targets of different sizes
regardless of their importance to the user of a radar set. The echo from
an aircraft may be lost in much larger echoes from near-by mountains, or
it may become too weak to be followed to ranges as great as desired. The
echo from a friendly aircraft is like that from a hostile one. The exact
location of a place on the ground may be of importance to a radarequipped
aircraft even though there is no distinguishable radar target
at that point. In nearly all cases where it would be advantageous if an
echo could be made much stronger or more readily distinguishable from
other confusing ones, the use of a radar beacon is indicated. (An enemy
aircraft obviously constitutes one difficult exception. )
The usefulness of beacons was demonstrated with the early radar sets
that were operated at long wavelengths. A large proportion of the
beacons used in the war operated at frequencies about 200 Me/see.
These included the beacons used with ASV Mark II search radar, the
transponders used for identification, the portable Eureka beacons that
were part of the independent Rebecca-Eureka beacon system, and a
much-used system for precise bombing, the Oboe Mark I. Another
system for precise bombing, the Gee-H system, used beacons of even
lower frequency. In this chapter, more emphasis is put on the newer beacons
at higher frequencies since the trend in radar is toward microwaves.
The beacon is essentially a repeater of radar pulses. It has an
antenna and receiver that convert pulses of energy, received at high
frequency from a radar set or special interrogator, into triggering signals.
Each such signal triggers the transmitter in the beacon and causes it to
radiate one or more pulses cf radio energy that may have almost any
desired power, frequency, duration, number, and characteristic spacing.
Figure 8.1 gives a block diagram of a beacon. Since it takes time for the
beacon to react, the first reply pulse comes back to the radar set slightly
delayed and indicates a range slightly greater than the true one. In
man y applications this delay is negligible, in others it is made to have a
constant known value for which allowance can be made. The delay can
be kept down to a few tenths of a microsecond when necessary. In the
special case where the radiated pulse is single, has approximately the
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