Radar System Engineering

424 R-F COMPONENTS [SEC.11.11
container of reasonable size would be above the limit set in Sec. 11.11.
Under these circumstances, it is customary to mount the magnetron
anode and the pulse transformer case outside the pressurized shell, with
an external blower to cool them. The cathode connection to the magnetron
is made in a pressurized inner compartment, into which gasketed
bushings bring the pulse transformer output and the magnetron cathode
leads. The waveguide r-f line is pressurized from the magnetron output
to the pressurized compartment which houses the duplexer, as well as on
the antenna side. In this arrangement, high voltages appear only inside
pressurized spaces, while most of the heat is liberated in the open, where
it can be carried away by the air stream from the external blower.
Metering and Test Points. —Good radar performance depends on
ob.jective tests of the functioning of each part. The vital radar components
are all centralized in the r-f head, and the providing of necessary
test points there is a part of the design job. Testing should not require
removal of any covers. A directional coupler, used to introduce the test
signal, will be located on the antenna line either just outside the r-f head
or just inside with cable feed to a test jack on the outside wall. At the
higher microwave frequencies, cables and cable fittings introduce a variable
element and the external location would be preferred. The video
output jack required can be the normal output to the indicator, in which
case an interchange of cable is required. If the video output tube is
capable of handling parallel loads, a second jack for test purposes on]y
would be added. If the test set requires a trigger signal at the time of
the magnetron pulse, this can be obtained by inserting a few ohms in the
bypass-to-ground circuit at the low end of the pulse transformer. The
signal would go in a shielded cable to an appropriate external jack.
The other checks on the operation of the r-f head divide themselves
into (1) minimum inf orma.tion continuous y available to a remote operator
and (2) tests to be performed with the cover removed.
The average magnetron current is the most important single item in
monitoring a radar set. If it has the correct value, the magnetron is
operating at the recommended point on its performance chart and is
almost certainly putting r-f power into the antenna line. Sparking
of the magnetron or irregular modulator operation can be observed.
This meter reading is an over-all check on the operation of the radar
transmitter.
The second most important item for the operator to monitor is the
radar crystal current. If this maintains its original value, it is known
that the local oscillator is functioning and the crystal has not changed its
properties. A decrease may indicate a burned-out or damaged crystal.
A rhythmical variation indicates that the AFC is searching but not
locking.