Radar System Engineering

SEC.913] THE AN/APQ-7 (EAGLE) SCANNER 293
if the waveguide carries the energy from right to left, the scan extends
for 300 to the right of the center line of the aircraft. By feeding r-f power
into the ends of the array alternately, a total scan of 60” is realized.
This alternate-end technique is always used in practice with the Eagle
scanner; it is accomplished with the aid of a fast-acting r-f switch which
operates synchronously with the guide-squeezing mechanism.
The azimuth beam angle 8 is related to the guide width a, the dipole
spacing s and the free-space wavelength AOby the formula
Here 0 is positive in the end-fire direction.
No provision is made for tilting the beam in elevation. Fixed 16-f t
aluminum flaps (Fig. 9“19) above and below the array serve to shape the
beam in elevation to a roughly cosecant-squared distribution.
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FIQ. 9.20.—Back view of part of AN/APQ-7 scanner,showingtoggles.
The fixed aluminum channel, which measures about 2% in. by ~ in.
by 16 ft over all, is first extruded, after which the critical surfaces are
machined. The plate is similarly manufactured. The plate is linked
to the channel by a system of 10 pairs of toggles and a tie rod, as shown
in Fig. 9.20. A cam actuates the motion of the plate, while the linkage
holds the a dimension uniform along the variable waveguide. To one
of the toggles is attached a cam so cut as to drive a synchro data take-off
through an angle equal to 6.
The chokes shown in the channel and in the plate serve to prevent
leakage from the varying guide so that the loss due both to the finite
conductivity of the waveguide and to the leakage past the long choke
is kept down to about ~ db/m during most of the scan. Although the
wavelength in the guide is nearly that which would be indicated by the a
dimension, small corrections must be allowed for the chokes and for any
variation of the clearance between the channel and the plate at the
chokes. For this reason the b dimension is maintained to a close tolerance
by means of rollers constraining the movable plate (Fig. 921). It has
been found very profitable to adjust variations in the effective guide