Radar System Engineering

180 THE GATHERING AND PRESENTATION OF RADAR DATA [%C. 69
were large gaps in its vertical coverage (Figs. 6.14 and 615). The range
on low-flying aircraft was poor. The system was supplemented in 1940
by 200-Mc/sec equipment, the so-called “ CHL” (Chain, Home, Low)
system. It was often very difficult to find sites that were flat enough
for stations. Since the station at Dover was on a 400-ft cliff and Ventnor
was on a cliff 1000 ft above the sea, it was im~ossible for these stations to
measure height at all. The processes involved in deducing the plan position
and. the height of an aircraft were complicated and not very reliable.
The maximum traffic-handling capacity was low (a good operator could
pass about 6 plots per minute), and the system could not keep track of
large numbers of independently routed aircraft. Nevertheless, formations
of aircraft were plotted reliably, and the number of aircraft in h
formation could be estimated by an experienced operator. With all its
imperfections, this s’ystem was the basis of the British radar defense for
the greater part of the war.
!!’he CXAM.—While the British Home Chain was being designed and
installed, the Army and Navy development agencies in the United States
were independently developing pulse radar equipment. The earliest
service equipment to be commercially manufactured was the CXAM
radar, designed at the Naval Research Laboratory. A laboratory-built
prototype of this set tvas tested at sea during battle maneuvers in early
1939, aboard the U.S.S. New York, and its performance was so promising
that a contract was let in October 1939 for the manufacture of six sets of
similar equipment.
The CXAM was operationally quite different from the CH. Instead
of using separate, fixed, broad-beam transmitting and receiving arrays,
it employed a common antenna for transmitting and receiving. To produce
as narrow as beam as possible, it operated at the then ultrahigh
frequency of 195 Me/see, and employed a “mattress” array of dipoles
with reflectors, giving a gain of 40 and a beam 14° wide in azimuth by
about 70° in elevation. The antenna could be rotated in azimuth at a
speed of 5 rpm, or manually trained to follow a particular target. The
peak pulse power was 15 kw, the pulsewidth 3 psec, and the repetition
rate 1640 pps. Range against bombers was about 70 miles, against
fighters about 50 miles.
The display was an A-scope in which the trace was lengthened by
causing the sweep to take place from left to right across the tube, then
drop down and return from right to lem Range was estimated with the
help of marks on the face of the tube; bearing was determined as
the direction of antenna-pointing which yielded maximum signal. The
height of targets could be estimated with the help of nulls in the vertical
antenna pattern (Sec. 611).
Despite its early design and its lack of adequate coverage against low-