Radar System Engineering

428 RF COMPONENTS [SEC.1112
magnetrons of this power; thus the tube is better able to furnish the
long 5-psec pulse without sparking. Its cooling blower also provides the
general air circulation inside the r-f head.
The waveguide fittings inside the r-f head are not airtight; they communicate
the head pressure to the antenna waveguide run. However,
electrically the waveguide is kept sealed as tightly as possible in order to
prevent the general level of leakage power in the closed container being
high enough to interfere with AFC operation. The sealing is done by
woven metal gaskets at every junction. A short section of flexible
corrugated waveguide next to the magnetron absorbs the dimensional
tolerances and permits all parts to be bolted together tightly without
strain.
The duplexer-mixer portion of the r-f system follows closely the block
diagram of Fig. 12.12. The TR tube is a 1B24. Its r-f selectivity is high
enough (about 50 Me/see between the half-power points) so that when
tuned up for radar reception the loss in receiving beacon signals may be
as much as. 20 db. This loss is averted by a solenoid-actuated plunger
inserted in the wave guide on the low-power side of the TR tube at a
distance of half a guide-wavelength from its output window. By adjusting
the depth of insertion, the TR tube can be pulled into good tune for
the new frequency.
The outputs of the radar and AFC crystals go in double-shielded
cables to the receiver and AFC chassis respectively. The AFC control
circuit is essentially of the form described in Sec. 12.7. For best use of
the available space, the receiver chassis is in the form of a segment of a
circle. The normal chain of i-f amplifier stages, second detector, and
video out put stage, is arranged around the periphery. The bandwidth of
the i-f amplifier is 5 Me/see, but for long-pulse operation a relay switches
the bandwidth to 1.0 Me/see by changing the loading of one of the inter- *
stage timed circuits. This is necessary to realize the full gain in sensitivity
to be expected from the lo~g pulse. The bandwidth is still 5 times
the reciprocal pulse length, rather than the value of 1 to 2 times established
as optimum in Sec. 2.9. There are two reasons for this: (1) frequency
modulation of ~he magnetron owing to variations of current
during the long pulse can lead to a spectrum wider than theoretical; and
(2) it is difficult to make a practical AFC that will hold a set in tune to a
small fraction of a megacycle per second.
No d-c power supplies are provided in the r-f head, except for the TRtube
keep-alive supply. This is a small half-wave supply with resistancecapacity
filter, located under the receiver chassis. All other voltages for
the local oscillators, receiver, and AFC come in over wires in the large
cable connector from an external centralized supply.
The various subunits in the r-f head are designed with plug connec-