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by E. King Stodola; in 'Radar Developments to 1945', Edited by Russell Burns, Published by Peter Peregrinus Ltd., London, United Kingdom, on behalf of the Institution of Electrical Engineers. 1988 Pages 478 - 492 Web Sections...
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Some examples of post World War II radar in
the USA
E. K. Stodola
37.1 PULSE JAMMING
Pulse jamming
both
by enemy jammers and friendly radars degraded the performance of the
SCR-270
and 271 sets which were the Army's primary search radars in the early
part
of World War II. A 'de-obfuscator' scheme which cleared oscilloscope
screens
of the interference was developed; its key element was a novel
frequency
discriminator, Fig.37.1.
This comprised a
narrow band filter and a wide band filter in the i.f. amplifier with
the
output of each filter fed to a detector for the pulses being received;
it was incorporated in 'Interference Reducer BC-1086'. The detected
pulses
were combined in opposition to each other to provide a frequency
disciminating
detector which gave a positive pulse output for 'on-frequency' signals
and a negative pulse output for signals outside the acceptance band.
This
output applied to the intensity grid of the display c.r.t. blanked out
the undesired pulse interference. The composite signal was also usable
for the deflection signal. The use of frequency discrimination rather
than
sharp cutoff filters avoided the shoulders on the response curve which
would allow very high intensity off-frequency interference to break
through.
The device was very effective for removing pulse interference from the
oscilloscope screens, but did little against other jamming formats.
37.2 RECEIVER ENHANCEMENT
Great emphasis
had
been put on increasing transmitted peak power to improve radar range.
About
1940, improved understanding of communication theory applied to radar
stimulated
greater efforts in improving signal processing and reducing receiver
noise
figures. In one case, a 20dB system improvement was obtained by
receiver
noise figure
improvement alone. Expedients, including the
utilisation of laminated 'flat plate' lines as resonant elements in low
noise r.f. amplifiers were necessary for rapid results. A long search
for
'the best' was not allowed to prevent quick use of the 'very good'.
The Norton/Omberg
1943 Radar Equation Report which gave the mathematical relationship of
the numerous factors which affect the range of a radar set emphasized
the
importance of the noise figure concept based on available source power
as opposed to widely used 'microvolt sensitivities' which were not
directly
comparable without taking into account many other items. This led to
numerous
field modification kits to provide low noise amplifiers, optimised i.f.
bandwidths, and various other improvements. Fig. 37.2. illustrates an
unusual
600MHz grounded-grid amplifier using a 'lighthouse tube' and flat plate
resonant circuit.
Click on image for large view.
Click on image for large view.
480 Some examples of post World War II radar in the USA
An urgent
operational
need for the detection of moving targets in clutter also had to be
quickly
met. Available components were applied to a theoretically sound and
practically
achievable modification of the 270/271 radar series to obtain
sub-clutter
ratios which gave effective moving target detection. The transmitter
pulse
was phase synchronized by injection of a pre-pulse crystal controlled
r.f.
pulse, and this, along with receiver control by the same crystal,
provided
a very stable coherent pulse arrangement. A special display system was
provided to allow operators to distinguish moving targets; a delay line
or other pulse cancellation scheme was not attempted because of the
extended
time deemed necessary to develop such a system for the SCR-270 radar.
Emergency
internal production of six field kits for radar reconfiguration was
accomplished
in time to meet a key potential military need.
The block diagram,
Fig.37.3. of the transmitter/receiver display shows the general
arrangement
and the features described above.
The test site, in
a mountainous area near Ellenville, New York, USA, was particularly
selected
to provide dense clutter signals, (see Figs. 37.4. and 37.5).
Fig.37.6. shows
the various displays for the Doppler signals. Fixed echoes combining
with
the fixed phase coherent reference signal produce unvarying patterns
from
sweep to sweep, while returns from radially moving targets are
continually
changing in phase with respect to the coherent reference and result in
displayed echo pulses continually changing in amplitude from sweep to
sweep.
This is evident in the 'expanded A scope' shown in the figure. The
'search
indicator' used a full range scale for horizontal deflection, and a
slow
linear vertical sawtooth sweep at a frequency lower than the highest
Doppler
frequency: the incoming pulse signals were applied to control the
c.r.t.
beam intensity.
Click on image for large view.
This 'A/B scope' display showed fixed targets
as vertical solid lines and moving targets as striated vertical lines;
operators found that moving targets were readily detected.
The display also
included a 'velocity indicator' in which the echo pulse signals were
used
for vertical deflection, and the intensity grid was driven by a range
selected
pulse to display only the desired range. The horizontal sweep was a
sawtooth
wave having a frequency set by an operator using a control calibrated
in
radial speed. When the control was set to display a single cycle of the
Doppler signal, the calibration indicated miles per hour radial
velocity.
The receiving
system
had a large dynamic range and could deliver an unsaturated video pulse
of about 70V although only about 0.5V was required for full deflection.
The 0.5V 'window' could be moved up and down through the 70V range by a
manual or automatic back-biasing arrangement so as to display the
moving
target without excessive loss in 'differential gain'.
Tests of the
modified
SCR-270 at Ellenville NY were successful and planes were detected in
very
dense clutter estimated as more than 60dB above MDS. On controlled
flights
the full tracking range of 240km was achieved at 3000m flight altitude,
with lesser ranges for lower altitudes. A plane flying at 900m along a
valley was followed as it passed alongside a 1200m peak. The set was
also
tested in a navy landing craft and performed well; it correctly
indicated
the ship speed as it tracked fixed targets on an island towards which
the
ship was travelling. Because of the termination of the war, the
equipment
was not used in combat.
Fig. 37.4 PPI patter observed at test site |
Fig. 37.5 Plot of fixed echo numerical levels on selected radial at test site |
482 Some examples of post World War II radar in the USA
Click on image for large view.
Fig. 37.6 Coherent pulse display panel
To view next section of this article....Section 2 - First Radar Detection of the Moon
This radar adaptation was intended to prepare for the invation of mainland Japan and the kamikazi, read a story based upon the above.
Page updated August 31,
2007
page created August 30, 2001
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