Radar System Engineering

244 RADAR BEACONS [SEC. 8
duration and frequency of the radar pulse, and is not appreciably delayed,
the beacon acts somewhat like an echo amplifier. The intensities of
received and transmitted signals are, to be sure, independent, rather than
in fixed proportion as in the case of a true amplifier. In general, however,
the frequency of the reply will be different from that of the triggering
radar set. In order to receive such replies, one needs a receiver tuned to
the frequency of the pulses sent out by the beacon instead of to the frequency
of the initial transmitted pulses. This may be either the receiver
of the radar set tuned to the new frequency, or a second independent
receiver tuned to the beacon. In either case, the reeeiver of the beacon
signals does not receive radar echoes since it is not tuned for them. Thus,
Receiving
Transmitting
antenna J L J antenna
) b
Transmitter +- Modulator
Coder
7( or
simple
triqer
Recewer + Decoder
(optional)
FIG. S.I.—Block diagram of a beacon.
the beacon signals are separated from radar reflections and can be displayed
without being swamped by heavy permanent echoes. Also, since
the pulse power of the beacon transmitter can be made as great as
desired, there is no limit to the strength of the reply. The range is
limited only by the power of the radar transmitter and the sensitivity of
the beacon receiver, which determine whether the beacon transmitter is
triggered or not. Figure 82a shows the radar echoes and Fig. 8.2b the
beacon replies on the indicator of the same 3-cm radar set. The pictures
were taken one immediately after the other on the same flight.
By its very nature, the radar-beacon combination involves two sendreceive
links as does any two-way communication system. The two
links are ordinarily connected automatically in a simple regular way and
are uninfluenced by human reactions. Since the channels exist, however,
they afford the basis for a communications system. In the past,