Radar System Engineering

724 RADAR RELAY [SEC. 17.14
TaLE 17.1.—MAxIMuM RANGE OF RELAY SYSTEMS
Frequency,
Me/see
Power,
watts
Beamwidth,
9-ft
paraboloid
degrees
3-ft
paraboloid
Free-space range, miles
9-ft
paraboloid
3-ft
paraboloid
300
1,000
3,000
10, OOO
50
25
05
01
= 27
8
2.7
0.8
= 80
24
8
2.4
196
460
195
292
22
,51
22
32.5
When one considers the Dower involved, the decrease of man-made
interference with increased frequency, and the privacy and protection
from interference provided by narrow beams, microwave frequencies
appear to be the most desirable. The decision between 3000 and 10,000
Me/see was based largely upon the fact that in the latter case waveguide
of a convenient size could be used, an extremely desirable design feature.
Since a 32-mile range is adequate for the uses intended, and since too
great sharpness of beam might lead to alignment difficulties, 3-ft paraboloids
were used at both stations.
Frequency modulation was chosen, partly because of its advantages
when signal-to-noise and signal-to-interference ratios are high, but mostly
because it simplified the oscillator design. Large deviations of the oscillator
are easily accomplished, and there are no problems of r-f bandwidth.
Since the required video bandwidth was about 1.5 Me/see, a total
deviation of 6 Jfc/sec was chosen. The total frequency spectrum
involved is then a little more than 9 hlc/sec. In order to minimize the
required i-f bandwidth in the receiver, the discriminator was set on one
side of the pass band of the receiver. A value of 11 31c/sec was then
chosen for the i-f bandwidth to provide a margin to take care of improper
tuning.
The equipment is shown schematically in Fig. 17”19. The oscillator, a
2K39 reflex klystron, is stabilized against a cavity by means of a microwave
discriminator. 1 The output of this device is a d-c signal whose ~’oltage is
proportional to the deviation of the oscillator frequency from the frequency
for which a resonant cavity, used as comparison standard, is set.
This error signal is amplified by a push-pull d-c amplifier and used to
control the reflector voltage of the klystron in such a way that its frequency
is forced into agreement with the resonant frequency of the cavity.
l~ideo signals and pulses are applied directly to the reflector to produce
the desired frequency modulation. Rapid response is purposely avoided
I R. V. Pound, A Microwave Frequency Discriminator, RL Report No. 662,
Aug. 4, 1945; Reu: Sci. InSt. 17, 490 (1946).