Radar System Engineering

258 It Al)AIt BEACONS [S,,c, S3
Similar considerations apply to the reply link. If we assume the
beacon to reinterrogated for all angular positions of theantmma of the
interrogator and the sensitivity of the receiver to be kept constant,
the angular width of the beacon arc on the PPI tube \rillvaryas did the
sector of interrogation in the previous case. Overa large distance the
reply arc will be of the order of magnitude of the half-po\rer bcam\ridth of
the receiving antenna. For close-in beacons, extensions attributable to
side lobes appear, and forvcry close beacons acomplete circle is obtained,
The complete circle or side-lobe pattern can be reduced to the narro\r arc
by suitable reduction of the gain of the radar receiver, again \vith th(,
loss of more distant beacons. Here, ho~rever, it is possible to usc a
FIG. S.S,—Interrogation of a beacon by side lobes. Interrogation of a sbipborne
10-cmbeacon by a radar set on \ft. Cadillac on Mt. Desert Island, }Ie, (a) shows the
extended pattern at about 4 o’clock in the picture; (b) showshow it ran he red”red to the
reply in the main beam by manipulationof the gain control of t’~c rereiver of the beacon
signals.
sensitivity-time-control (STC) circuit that gi~~esautomatic variation of
the sensitivity of thereceiver asafunction of the elapsed time after emission
of the interrogation pulse. The gain is thus automatically adjusted
to be correct for beacons at all ranges with the result that they all appear
as approximate half-power bearnwidth arcs. This circuit must be carefully
designed if it is to give good results with ground radars interrogating
beacons on aircraft flying in the maximum of the pattern of the receiving
antenna and still not unduly attenuate weaker replies from beacons in
low-flying aircraft.
It is clear from the foregoing that the width of the reply arc is always
limited in either one link or the other, whichever is the narrower, and that
it can be controlled by adjusting either transmitted power or receiver
gain. Where manual adjustment for good reception of a particular
beacon is to be used, cutting down the transmitted power would be