Radar System Engineering

600 EXAMPLES OF RADAR SYSTEM DESIGN [SEC. 15.8
adequate for a simple tracking or control problem, but complex problems
clearly called for higher scan rates even at the expense of coverage.
Foreseeing a wide range of applications for the new radar equipment,
its designers provided an antenna drive by means of which the scanning
rate could be adjusted over the range from 1 to 6 rpm. For one wartime
application that involved only simple tracking and control problems
but put a high premium on range performance, a scan rate of 2 rpm was
used exclusively. Other sets, which were used for more complex problems,
were commonly run at 4 rpm. Operation at 6 rpm was very limited
because of mechanical difficulty.
There is now good evidence that a scan rate of 6 rpm is desirable for
complex tracking and control, and even higher rates should be advanta-c
geous. Mechanical difficulties at high scan rates and requirements for
high azimuth accuracy will probably conspire to set an upper limit on
future scanning rates. In fact rates above 6 rpm may be entirely ruled
out by these limitations.
15.8. Choice of Beam Shape. Azimuth Beamwidth.—The merits of
a narrow beam are so widely understood that they will not be discussed
in detail here. All microwave development is recognition of the importance
of angular resolution. As wavelength is decreased, beamwidths
obtainable with antennas of practical size are decreased. With the
development of a microwave ground radar, the beamwidth for a longrange
air surveillance set was pushed down by an order of magnitude from
values previously used for this application, and results were spectacularly
successful. The set described here had an antenna aperture 25 ft wide
and operated in the 10.7-cm wavelength region, giving a beamwidth of
approximately 1.00.
“It will be of interest to see how far development toward increasingly
sharper beams might usefully be carried, and to establish as closely as
possible the optimum beamwidth for a long-range air-surveillance set.
It can readily be shown that a lower limit, and therefore an optimum
value, exists, entirely apart from questions of mechanical difficulty.
A scanning rate of 360/see gives an angular motion of 0.07° between
transmission of a pulse and return of the echo from a target at 200-mile
range. If the beamwidth were much less than 0.07°, the antenna gain
for reception of the signal would be greatly below normal. For a beamwidth
of about 0.15° or 0.2° this effect becomes negligible, and the limit
it sets on beamwidth is probably not of practical importance.
A similar and more important fundamental limitation on beamwidth
is determined by the requirement that at least one pulse hit the target
per scan, with the beam axis off in angle from the target by no more than
a small fraction of the beamwidth. At a scanning rate of 360/see
and a v, of 400, the angular separation between pulses is 0.09°, and for