Radar System Engineering

SEC. 9.14] SCH WA RZSCHILD ANTENNA 297
arms rotate at 240 rpm, as do the four arms H which support the feed
horns. They remain in the angular relation shown. Between the two
sets of arms is a stationary ring with a 105° window through which
energy may pass into whichever feed horn is opposite the feeding edge
of the folded horn. When the horns and switch are rotated the effect
is to cause the beam of the antenna to scan a 10° azimuth sector each
FIG. 9.25.—Perspective of horn, reflector, and feed of Schwarzschildantenna used in
AN/MPG-l.
time a horn runs its W“ course. Since about 10° of the 90° are lust in
switching, we can think of the parallel-plate region as effecting an 8-to-l
optical reduction from the 80° scan of the broad-beamed primary horn
to the 10° scan of the very sharp antenna beam.
The transformation of the moving feed horn into the scanning beam
is made by means of two double-curved bends CC and DD in the parallel
plates (Fig. 9.26). Although one parabolic bend CC would serve to
collimate the primmy radiation and provide a sharp beam on axis, two
bends may be so designed as to correct for coma and thus give a good
beam off axis as well. The theory of two-mirror telescopes was used to
calculate the bends for the Schwarzschild antenna. The cylindrical