of cw carrier up to 780 mc.
Modulation of the jamming signal is,
of course, essential to achieve the maximum blanketing effect. Experience
has shown that random noise, such as may be obtained conveniently from
the space current of a gaseous vacuum tube, provides the most effective
modulation waveform. Noise modulation, received by the radar, has
the effect of multiplying enormously the normal noise level present in
the radar receiver.
Search Techniques
The technique of searching for enemy radar signals, as
a preliminary to jamming them, consists simply in tuning the search receiver
repeatedly over the radar spectrum. This is not only difficult technically,
but physically tiring. The technical difficulties reside in the great
width of the spectrum to be covered. One excellent example of how
the problem is solved is the AN/APR-4, which covers the range from 40 to
3000 mc, using four r-f heads. The tuning is motor driven over a
frequency sector which can be selected by the operator, thus relieving
him of a considerable physical burden. An automatic tape-recording
system is available to record the frequency at which signals are detected
as the spectrum is swept, thus further reducing the attention demanded.
The simplest method of observing the radar signals
is by an aural indication. Radar pulses are transmitted at repetition
rates which lie within the audible spectrum. Moreover, the pulse
represents, in effect, a high degree of overmodulation on a c-w carrier,
and this modulation
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can be recovered in a conventional
second detector, amplified at audio frequencies, and fed to headphones.
When a radar signal is intercepted a whine (repetition frequency plus harmonics)
is heard in the head-phones. The strength of the signal varies periodically
as the radar beam sweeps past the search plane. So long as this variation
continues, the radar is searching. But if the signal becomes steady,
at maximum volume, the chances are that the radar has detected the search
plane and is tracking it. Appropriate action is then taken to avoid
enemy gunfire and aircraft.
While aural or tape-recording methods
serve to identify the presence and carrier frequency of the enemy radar,
they give little indication of the pulse characteristics. A cathode-ray
pulse analyzer (oscilloscope) is available to determine the pulse repetition
rate, the pulse width, the pulse shape and relative
amplitude. Such an analyzer gives
important clues to the type of radar
under observation, since it reveals
the radar's maximum range, minimum range, and range accuracy.
Wideband Radiators
Implicit in the wide frequency ranges
covered by search and jamming equipment is the necessity for radiators
which will cover these ranges without excessive tuning adjustments.
The Radio Research Laboratory undertook to develop antennas which would
cover frequency ranges of several thousand megacycles without any adjustment
whatever. One of the most spectacular of these antennas is an approximately
cylindrical structure which covers the range from 950 |
|
to 2900 mc, a frequency ratio of 3-to-1, matching the
transmission line throughout this range. In general, the wideband
antennas make use of the principle that a thick, stubby radiator has low
stored energy and hence responds well over a wide band. Several of
the wide-band radiators are of the turnstile type, two dipoles at right
angles, extending through massive collars.
Closely allied with the wideband antennas
are suitable direction-finding structures. The direction-finding
problem is complicated by the fact that the enemy may choose vertical or
horizontal polarization at will. In the AN/APA-24, which operates
in the range from 100 to 450 mc, a four-element Adcock sys-tem is used
to receive vertical polarization, and a single horizontal dipole is used
for horizontally-polarized signals. The system operates on the null
of the pattern.
For higher frequencies (300 to 1000 mc), an automatic
direction finder was produced, using a continuously rotating radiator.
A cathode-ray oscilloscope, with polar sweep, indicates the strength of
the received signal and plots a polar |
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