Radar System Engineering

SEC. 6.13] HOMING 201
application, since the target speed is much higher (and much more nearly
that of the intercepting aircraft), and since homing must be carried out in
two dimensions instead of one.
An early AI equipment was designed by the British as an extension
to AI of the design principles of ASV Mark II. It was designated AI
lMark IV by the British; a similar system made for the U.S. Army
by an American manufacturer was called SCR-540. In this set, which
operated near 200 Me/see, a single transmitting antenna sent a broad
lobe in the forw-ard direction from the aircraft. Two pairs of receiving
antennas were provided; one pair produced overlapping lobes like the
“homing” lobes of the ASV Mark II, to give an indication (also identical
with that of the ASV Mark II) of the homing error in azimuth. The
other pair of lobes was used to measure homing error in elevation angle;
they overlapped exactly when viewed from above the airplane, but one
was displaced slightly above the center line of the airplane and the other
slightly below the center line. Elevation horning error was shown on a
second indicator exactly like that of the ASV Mark II, turned through
90° so that the range sweep occurred horizontally from left to right.
Signals from the upper lobe displaced the trace upward; those from the
lower lobe displaced it downward. The receiver was rapidly switched
to each of the four receiving antennas in turn, and the display corresponding
y switched to the proper deflection plate of the appropriate indicator
tube. Range could be read either on the azimuth tube or on the elevation
tube. The arrangement of the receiving antenna patterns and of
the indications is shown in Fig. 633.
The principal operational limitation of this equipment was a result
of the fact that very broad beams were produced by the single-dipole-andreflector
antennas used. These broad beams gave strong reflections
from the ground beneath the aircraft, restricting the maximum range at
which aircraft echoes could be seen to less than the altitude at which the
AI-equipped fighter was flying. In the case of ASV operating on 200
Me/see, similar returns from the sea were experienced, but the returns
from ship targets were so strong that signals were sought and tracked at
ranges beyond the sea return. Aircraft echoes were many times weaker,
so that aircraft could be seen only at ranges shorter than the ground
return.
It was clear that the best hope of escaping thk limitation was to make
use of a sharper beam in AI equipment, and since the maximum antenna
size was limited by the necessity of aircraft installation, considerable
effort was exerted to develop a microwave AI. The American equipment
which resulted is the SCR-720. In this equipment, a 29-in. paraboloid
reflect or rotates continuous y in azimuth and is S1OW1 y tilted in elevation.
The helical scan thus produced covers an elevation interval of 25°. The