Radar System Engineering

418 R-F COMPONENTS [SEC. 11.9
above the 0.5 mw needed to produce the standard 0.5 ma of crystal current.
The great excess of power permits loose capacitive coupling
between local oscillator and crystal. By the reciprocity principle, since
there is poor transfer of power from the local-oscillator input to the signal
line, there will be poor transfer from the signal line to the local-oscillator
branch, thus satisfying the second condition stated above. 1 A screw
on a quarter-wave stub support adjusts the proximity of the capacitive
probe to the signal line and provides the method of setting the crystal
current. This is the sole adjustment.
The local-oscillator input fitting contains a resistor disk which terminates
the 50-ohm cable. Since the probe is half a wavelength away
from the disk, the load at the probe will be in parallel with the disk
(Sec. 11. 1); but since the probe does not differ much from an open circuit,
the load for the local oscillator is still a good match. The voltage at the
probe is the same as that at the disk. It is necessary to have the probe
spaced a quarter wavelength on the signal line from the effective plane
of the coupling loop. With a loaded Q of 350 the TR tube is considerably
off-tune at the local-oscillator frequency, which is usually 30 Me/see
away from the signal frequency. Like all parallel resonant circuits, the
TR cavity presents a very low impedance, practically a short circuit, at
frequencies off resonance. If the probe were half a wavelength from the
coupling loop, it would be effectively at a short-circuit point for localoscillator
frequency and no power could be transferred. At a quarterwave
point the short circuit at the coupling loop is reflected as an open
circuit and the piece of line is, to the local-oscillator frequency, a stub
support.
In mixers for the 3-cm region, the crystal goes directly across the
center of the guide. Coupling between local-oscillator waveguide and
crystal guide is by means of adjustable windows in the short face of the
guide. At 10 cm coaxial cables carry the local-oscillator power to separate
radar and AFC mixers, but at 3 cm double mixers are used in which
the two crystals are on opposite sides of the local oscillator. Figure 11.29
shows a waveguide mixer and other r-f components for the 3-cm band
with AFC and beacon features.
11.9. Automatic Frequency Control.-Radar automatic frequency
control (AFC) is a scheme in which the difference in frequerwy between
the magnetron and the local oscillator is compared in a discriminator cir-
I In an exact calculation,accountmustbe takenof the differencein the impedance
of the various branches at signal and local-oscillatorfrequencies. As is mentioned
in the next paragraph,this is considerablefor the highly resonantTR cavity. QUalitatively,
however,the reciprocity y argumentis valid. The s~called ‘{magic T” provides
a method of decoupling the signal and local-oscillator inputs that does not
depend on an excess of local-oscillator power. See MicrmuaueMizsrs, Vol. 16,
Chap, 6.