Radar System Engineering

SEC. 15.13] GENERAL DESIGN OF THE AN/APS-10 615
although it was not then known, the absorption of 1.25-cm radiation in
water vapor is so strong as to remove almost entirely the usefulness of
this wavelength for navigational purposes.
Next, a decision had to be taken regarding the pulse power. High
pulse power is very costly in the design of radar for aircraft, both in
terms of the primary power requirement and in terms of total weight. 1
A comparison with the performance of the AN/APQ-13 served as a guide
in determining the pulse power of the AN/APS-10. The AN/APQ-13,
using about 50-kw pulse power and a high-altitude cosecant-squared
antenna of gain 950, showed a range of about 45 miles for general ground
painting and 50 to 100 miles on cities. Its receiver had a sensitivity of
about 5 X 10–13 watts
for a signal equal to noise.
The 45-mile ground-painting range of the AN/APQ-13, as compared
with the 25 miles required of the ANT/APS- 10, allowed a 10-db reduction
of transmitter power for the latter, if the antenna gain and the receiver
sensitivity were kept the same. However, it was felt that a 30-in.
antenna was too large to be tolerated for the AN/APS-10. Experiments
showed that an 18-in. cosecant-squared antenna designed for an altitude
of 7000 ft would be useful in the usual altitude range from 1000 to 10,000 ft
without unduly overilluminating short-range targets at low altitude or
causing too serious a hole in the center of the pattern at altitudes higher
than 7000 ft. This antenna proved to have a gain of 700; this meant
that the reduction of antenna gain would require about 3 db of the
calculated 10-db leeway between the AhT/APQ-13 and the AhT/APS-lO.
The remaining 7 db could be and were used to permit a reduction
in pulse power. The lightweight, low-voltage 2J42 magnetron (see
Fig. 10.4b) was used in the AN/APS-10, giving an output pulse power of
10 kw. The lower voltage and lower power required by this tube enabled
the design of an extremely compact and simple pulser, shown in Fig. 10.47.
Since the whole of the 10-db differential between the two sets had
been used up in the smaller antenna and the lower pulse power of the
AN/APS-10, it was necessary to make sure that the receiver sensitivity
of the latter set did not fall below that of the AN/APQ-13. Further,
special pains had to be taken to provide good range performance against
microwave beacons. In the AN/APQ-13, the TR-switch tuning was not
changed from search to beacon operation, and a narrow-band ATR was
used. The loss of beacon signal in the TR and ATR alone could be as
much as 15 or 20 db. In addition, no automatic frequency control of the
beacon local oscillator was provided. Better facilities for working with
beacons were desired in the AN/APS-10.
1W. L. Myers, “ Weight Analysis of Airborne Radar Sets,” RL Rsport No. 450,
Jan. 1, 1945.