Radar System Engineering

SEC. 920] INSTALLATION OF SURFACE-BASED SCANNERS 313
where they can search for and track enemy fighters approaching from the
rear or other anticipated directions, or are located on the turret guns.
An important question in mounting a scanner in the belly of an airplane
is ‘‘ How far shall the antenna protrude below the keel line ?“ Too
great a protrusion will add to the aerodynamic drag, whereas if the
antenna is retracted too far the vision is inadequate in nearly horizontal
dh-ections because of partial blocking of the field of radiation. The ray
diagram of the radiation field is not a sufficient guide in planning an
antenna installation. In prototyping each installation the minimum
protrusion allowable from the standpoint of the diffraction of the radiation
must be determined by measurements of the antenna pattern made
FIQ.9.37.—The ray diagram showsthat fan-beam antennamay be installed partly inside
the fuselage.
with the antenna installed in a mockup. 1 Such measurements have
shown that some circularly scanning fan-beam antennas may be so
installed that nearly half their height is above the keel line of the airplane,
and this conclusion has been confirmed in observation of the indicator in
flight. Figure 9.37 shows qualitatively why such an installation is possible
with the type of scanner shown in Fig. 9.8.
In attaching the scanner to the airframe it is not necessary or advisable
to use shock mounts unless electronic equipment is inseparably attached
to the scanner. It is frequently essential, however, to provide adjustments
for accurately orienting the scanner base. To allow this adjustment
it is customary that the short length of transmission line between
the scanner and the r-f unit be flexible: this flexibilityy also allows the r-f
unit to be shock-mounted.
9.20. Installation of Surface-based Scanners.-The installation of
scanners on shipboard presents a siting problem slightly simpler than in
the airborne case. It is necessary to avoid locations where the structure
of the ship will blank the radiation or produce spurious reflections.
Sometimes it becomes necessary to install two scanners, each of which
covers the region in which the other is blind. When a ship has more than
one radar on a given wavelen~h, one can t‘j am” the other, filling its
screen with spurious indications; this effect can be mitigated by proper
siting of the scanners and by blanking the receiver of each at the instant
1A recent installation is described by W. 11. Cady in RL Report NTO.848, “The
AN/APQ-13 (60-in.) Scafiner in B-29 Airplanes.” See also Figs. 9.7, 9.8, and 9.39.