Radar System Engineering

SEC.11.10] REASONS FOR AN R-F PACKAGE 419
cuit with the standard intermediate frequency being used—for example,
30 Mc/sec. Anerror inthedifference frequency produces a voltage that
isapplied tothe reflector of the local oscillator in sucha sense as to bring
the local oscillator into correct tune. Because of the close connection
of the whole AFC problem with receiver design, consideration of this
subject is deferred to Sec. 12.7.
MOUNTING
THE R-F PARTS
Experience has demonstrated that good radar performance depends
not only on a well-designed set of microwave components but also on a
properly coordinated mounting of them, relative to each other and relative
to the other parts of the radar. Therefore it is necessary to add to
the treatment already given of the components themselves a consideration
of the engineering reasons for the accepted practice of grouping
certain vital components in a major unit called the “r-f head. ”
11.10. Reasons for an R-f Package. Preferred Grouping oj Components.—Considerations
of accessibility and convenience would often
argue for putting most of the components of a radar set near the indicator
rather than near the antenna, which, in order to get the proper view,
must usually be at a remote or isolated point. However, the long-line
eflect discussed in Sec. 11”1 makes a long transmission line from magnetron
to antenna something to be avoided if possible. In addition to the
fact that in a long line even a moderately low standing-wave ratio can
cause instabilityy in the magnetron, the extra junctions, bends, and elbows
needed for a long line are themselves likely to add to the mismatch.
Furthermore, as seen from Table 11”1, the r-f attenuation in a long line,
particularly at the higher microwave frequencies, results in the loss of
several decibels when two-way transmission is considered.
The shortest possible r-f line is realized by mounting the magnetron
on the back of the antenna reflector (“back-of-the-dish system”). This
ideal arrangement is often not practical because of a group of components
functionally associated with the magnetron which may be too large and
heavy for the antenna mount to carry. The pulse transformer must be
adjacent to the magnetron to avoid 1ong high-voltage leads, which would
have to be supported and properly insulated and would add unwanted
capacity. The duplexer T-junction and the TR tube must, of course,
go in the magnetron-antenna line. The extremely low-level echo signals
coming through the TR tube must be converted to the intermediate
frequency and amplified considerably before being transmitted to a
remote point. The local oscillator must be close, because the AFC is
based on a comparison of the frequency of the local oscillator and that of
the ruagnetron.
Electrical and functional considerations dictate, then, that the follow-