Radar System Engineering

304 ANTENNAS, SCANNERS, AND STABILIZATION [SEC, 916
to make the wavelength less in the line than in free space. This shaft is
displaced from the axis of the outer conductor and supported by bearings
at 10-in. intervals so that it can rotate. As the inner conductor turns,
the wavelength in the line varies, causing the beam to scan 6.5°. Larger
scans by this method are quite feasible.
Another rolled parallel-plate scanning antenna, similar to the type
described in Sec. 9“15 in the respect of having a single rotary waveguide
feed, and also capable of rapid scan, has been developed. The equiva-
~Moving
waveguide horn
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Cyhnder
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Wave fronts
(a)
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bl
(b)
FIG. 9.34.—Reflector roll scanner,
lent simple antenna is a polystyrene lens placed between parallel trapezoidal
conducting plates and fed by a waveguide at one edge of the plates
(Fig. 934a). Moving the guide tilts the beam. A conducting reflector
strip is placed between the plates in such a way that the waveguide feeds
the lens from an adjacent edge of the plates (Fig. 9.34b). It is now
possible to roll up plates and reflector strip in such a way that the feed
path is annular. This type of roll is ordinarily simpler to build than the
one described in Sec. 9“15.
In general it can be said that antennas of the array type (Eagle and
LRASV) are compact but that the beam position depends on the frequency,
while antennas of the parallel-plate type (for instance, Schwarzschiid,
SCI) have a beam position independent of frequent y but have
more bulk and weight. Thus, the array has found use in airborne work
and mobile equipment, while the others tend to be used in ground and
ship equipments.
THE STABILIZATION PROBLEM
Airplanes and ocean vessels are unsteady bases from which to make
observations by radar or by any other means. It has been necessary to