Radar System Engineering

420 R-F COMPONENTS [SEC. 11.10
ing minimum items be mounted in a group and that the group be as near
the antenna as practical:
1. Pulse transformer.
2. Magnetron.
3. Duplexer, TR and ATR tubes.
4. Local oscillator.
5. Radar and AFC mixer.
6. AFC control circuits.
7. I-f amplifier (usually up to l-volt video level).
This set of components, mounted as a group and usually in a closed
container, is called the “R-f Head.” Alternative terms are “R-f Unit”
and “Transmit-receive Unit. ” Beacon local oscillator and beacon AFC
are included where beacon reception is required. In smaller radar sets,
the modulator network and switch tube and even the modulator power
supply may be put with the r-f unit. An extension of this trend leads to
an arrangement with all parts of the radar except the controls and the
indicator tube in a single container.
Adoarztaqes of an R-f Package.—On all but the largest radar sets the
group of components listed in the previous paragraph can be compactly
mounted in a single container that is not too heavy for one man, or at
most two men, to carry. There is considerable advantage in having the
vital parts of the transmitter and receiver centralized in a remountable
unit package. In the event of trouble, a spare r-f head can be substituted
and connected into the rest of the radar in a few minutes. Diagnosis of
trouble, repair, and readjustment, as well as assessment of performance,
can all be performed on a well-equipped test bench. This is particularly
helpful for airborne sets.
Good engineering practice favors pressurization of the r-f transmission
lines of microwave radar systems in order to keep out water and water
vapor. For high-altitude operation, pressurization may be absolutely
necessary to prevent breakdown of the line. The high-voltage cathode
circuit of the magnetron is a similar case. Pressurization assures dry
conditions and sea-level pressure, and thus makes unnecessary the huge
insulators and large air clearances that standard engineering practice
would otherwise prescribe. Other parts of the r-f head can be made
smaller and more compact if pressurized conditions are assumed. It is
but a slight extension to consider that the r-f head should go into a single
pressurized container, with the pressure common to the r-f line out to the
antenna. Such a plan solves two other problems: (1) the working parts
of the r-f head are protected against the most severe conditions of
moisture, salt spray, and dust, not only in use, but also during shipment
and storage; and (2) the shielding problem is greatly simplified, both for
keeping external disturbances out of the delicate parts of the receiver, and