Radar System Engineering

262 RADAR BEACONS [SEC, 86
the beacon by having to search over a band 01 frequencies. Thus, adequate
stabilization of the beacon transmitter is required, and the receiver
for the signals must have automatic frequency control, or a wavemeter
must be provided, or it must be possible to set the frequency accurately
to preset values by having mechanical parts of sufficient precision. All
of these techniques have been worked out and are now available.
This use of a spot frequency for the reply does have a disadvantage,
however. Each interrogating radar will receive not only the responses
of the beacon to its interrogation pulses, but also the beacon responses to
interrogating pulses from other sets. The replies that a radar set receives
to its own pulses are synchronous and appear at the proper range in a
regular way. Replies to other radar sets are, in general, not synchronous.
This is discussed further in Sec. 8.9.
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Time
(a)
FIG.S.10.—Variationof frequency with time in a beacon. Tl”ithsweepsaain (a), equal
intervals between replies will be obtained for all frequencies. For (~) the distribution
dependson the frequency.
Sweeping Frequency. -In some cases it has not been feasible to use a
single spot frequency of reply either because the number of replies at the
frequency would be excessive or because it was not practical to modify a
group of radar sets to receive the chosen frequency. In these cases,
beacons have been used in which the common frequency of receiver and
transmitter was made to sweep periodically over the band of frequencies
of the interrogators. A given interrogator got replies when the beacon
came into tune with it. This system is useful only with interrogators
that are pointed continuously at the beacon. If the interrogator scans
in space, it will not in general be pointed at the beacon during the interval
that the beacon is tuned to its frequency band unless the scanning period
is made short compared with the interval during which the beacon replies.
Some such beacons have a single tube used both as a superregenerative
receiver and as a pulsed transmitting oscillator so that by mechanical
changes of the tuning of the resonant circuit both frequencies can be
changed together. The frequency for reception and that for transmission
are nearly the same; they are not identical because of the different
voltages and transit times involved at the two clifferen t levels of operation.
Figure 8.10 shows two readily realizable cycles of variation of frequency