Radar System Engineering

SEC. 6.13] HOMING 197
ber of the pair of antennas in use, and the radar echoes are dkplayed on
an L-scope in which signals coIPing from the left antenna cause a displacement
of the trace to~-ard the left, and signals from the right antenna
displace the trace to the right (Fig. 6“4). The range sweep is linear,
upward from the bottom of the scope, and the range is estimated by an
engraved scale in fron~ of the tube.
On the search antennas, a given target shows either on the left antenna
or on the right, depending on its location with respect to the aircraft,
never on both. On the homing antennas, however, because of the overlapping
coverage of the two beams, a target will show a signal at the same
range both to the left and to the right of the center line. A comparison
of the strength of the two returns shows the radar observer in which
direction course must be altered to home on the target. Care must be
taken in antenna installation to make sure that the axis of equal signal
coincides with the direction of flight of the aircraft. “Squint,” which
Left antenna
Sightantenna
Left
antenna
Right
ntenna
(a)
FIQ. 629.-Beam patterns of ASV Mark II (SCR-521). (a) Search antenna pattern.
(b) Homing antennapattern.
results from improper installation or trimming of antennas, has the same
operational effect as crabbing of the aircraft in a cross-wind; in either
case, the operator notices that the relative signal strength of the left and
right echoes changes when the aircraft is steady on a compass course
initially chosen to give equal signals. A skillful operator can allow for
this effect and choose a course on which the signals, though not equal, do
not change relative to one another when the course is held. This, regardless
of squint or crabbing, is a true intemeption course.
The first radar beacons (Chap. 8) were designed for use with this
equipment. The type L display is just as well suited to homing on a
beacon as it is to homing on a radar target, and the navigational aid
provided by beacons was very useful in bringing aircraft home after long
sea patrols.
A very similar equipment operationally, but of greatly reduced total
weight, has been widely used by the IJ.S. Navy. Referred to as the
ASB, it operates on 515 Me/see, at which frequency a dipole becomes
small enough to allow the Yagi arrays used as antennas to be mounted
on a rotating mechanism that permits them to be pointed broadside for
search, or turned forward for homing, at the will of the operator (Fig.
(b)