Radar System Engineering

SEC. 11.12] ILLUSTRATIVE EXAMPLES OF R-F HEADS 425
A single remote meter with a selector switch, or selector relays in the
r-f head, can perform these minimum monitoring functions at the operator’s
position. If the set has beacon AFC, the radar-beacon switching
relay should transfer the remote meter to the beacon cavity crystal.
Then a steady meter reading indicates not only that the beacon AFC is
locked, but that it is locked at the correct frequency, since the crystal
energy has come through the reference cavity.
Of the test points usually provided for cover-off checking, the most
essential is a jack to measure the crystal current at every crystal, in order
to permit adjustment of the Iocal-oscillat or coupling. On crystals
where the same current is metered remotely as well, a cut-in jack is used.
Another current worth metering is the average diode current in the second
detector, for over-all sensitivity y tests and checking the receiver. A jack
for measuring the keep-alive current in the TR tube should be included
to provide verification that the TR switch is performing normally.
In radar sets where the r-f head is accessible to an operator during
operation, all of the above metering can be done on a single meter and
selector
switch.
The most important internal signal test point is the AFC video output.
On an oscilloscope the discriminator output is seen, as well as any spurious
signals. Sometimes this is made a phone jack also, so that with a pair
of headphones aural indication of crossover may be compared with the
frequency of best tune.
11.12. Illustrative Examples of R-f Heads. Airborne Search set.
Figure 11.27 shows an external view of the r-f head used on a 3-cm search
and navigation set of recent design. A broad flat container was dictated
by the space available in certain aircraft. A lightweight sheet-metal
pressure vessel of this form factor must of necessity have a dom~d top and
bottom. The pressure seal comes near the top of a short central cylindrical
section 18 in. in diameter.
The radar set is designed to work on three pulse lengths: (1) 5 ~sec,
200 pps, for search; (2) 2.5 psec, 400 pps, for beacon interrogation; and
(3) 0.5 psec, 800 pps for high definition. The long pulse, as shown in
Sec. 2.9, gives greater range, particularly in mapping land-water boundaries.
It must be longer than the upper discrimination limit of ground
beacons. The beacon pulse is well over the minimum length of 2 psec
necessary to trigger beacons.
The pulse is transmitted from the modulat or on a 50-ohm cable, which
enters the r-f head via a pressurized pulse connector fitting. The
shielded cable then goes to the quadrant-shaped pulse compartment.
Into this compartment project the pulse transformer bushings and the
magnetron cathode bushing. The interior detail is shown in Fig. 1I .28.
Ordinarily the quadrant cover is on and no high voltages are exposed