Radar System Engineering

734 RADAR RELAY [f+iEc.17.16
The video switch of Fig. 17.14 alternates the video signals (radar or
radar alternated with beacon) with the coded pulses, and passes the
results to the transmitter (Fig. 17.22), where they ultimately modulate
the grid of the r-f power amplifier. The pulses are additionally used to
modulate the plates of the same tubes; this arrangement provides a much
higher power in the pulses than would otherwise be available. The
transmitter power is approximately 50 watts average, 80 watts peak on
video signals, and 175 watts peak on pulses.
An r-f filter is provided in the antenna lead. The antenna consists
of a vertically polarized dipole mounted on the tail section of the aircraft.
At the receiving station, the energy is received by an antenna consisting
of two vertically stacked dipoles and passes through a resonant
cavity of loaded Q equal to 100 on its way to the receiver described in
Sec. 17.12.
The analyzer and scan converter operate as shown in Fig. 17.14,
separating the various signals and providing a simulated scanner motion.
The separation of video signals from pulses is aided by an amplitude
selector which takes advantage of the higher power in the pulses. This is
helpful, especially during the locking-in period. The additional parts
necessary to separate the radar and beacon data are shown in Fig. 17.22.
The cosine pulse is delayed 30 psec to form a trigger available on every
cycle, regardless of whether radar or beacon video signals are being
transmitted. The modulator pulse indicates those cycles on which radar
video is transmitted. A trigger occurring only on the beacon cycles
can be formed by an anticoincidence circuit operated by these two pulses.
The video switch separating the two types of video is controlled by a
flopover which is thrown to the radar position whenever the modulator
pulse occurs, and back to the beacon position by the next cosine pulse.
The switch then passes beacon signals on every cycle in which the modulator
pulse does not occur.
This equipment gave reasonably satisfactory results in its final form.
In spite of all the precautions taken, however, the considerable interference
from radar and communications equipment, together with the
presence of diffraction minima, limited the reliable operating range to
about 30 miles when the interference was severe, and to 50 miles or so
under reasonably favorable conditions. These figures would be somewhat
improved by the use of diversity antennas. but in the absence of
tests no figures can be given.
Extensive tests of the 100-Mc/sec frequency-modulated equipment
have been made under less severe interference conditions than those
faced in tests of the 300-Mc/sec equipment. At 100 Me/see, diffraction
minima occur only at short ranges where the signal strength is high, and
they are much less pronounced than those at higher frequencies, as