Radar System Engineering

SEC. 11 .8] THE MIXER 417
minimum loss of incoming signal into the local-oscillator input; (3) the
local oscillator must see a fairly good match, though the requirements are
not as strict as for a magnetron.
Figure 11.25 shows a coaxial-type mixer which is widely used for the
9-to 1l-cm band. It bolts directly on the TR-tube cavity and the output
FIG. 11.25.—Coaxial-type mixer; 10.cm band.
coupling loop is integral with the mixer. The crystal is mounted as a
part of the center conductor of a short coaxial line and makes a matched
termination with no special transformers. Since the i-f signal must be
extracted, the end of the r-f line cannot be a d-c short circuit. An effective
r-f short circuit, with no metallic contact between inner and outer
conductor, is provided by two concentric quarter-wave sections which
form a choke not unlike that in a rotary joint. The i-f output fitting
unscrews to permit replacement of crystals.
Local-oscillator power is introduced through a side arm. About 50
mw is available from reflex klystrons in the 10-cm region, which is 20 db