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Global TSCM Group, Inc.
12 W 32
nd
St, Suite 604, New York, NY 10001, USA
Tel.: +1 212 967 4030 +1 646 233 0383 Fax: +1 212 967 7560
THE STEALTH
The Confidential Catch
The Antenna Unit
1) Wideband Principles
a)
The
KN2C-2020T
antenna described here should be suitable for use across multiple octaves of
frequency range. The
KN2C 2020T
antenna unit is intended for use across the range of 100 MHz to 1
GHz, but operation outside these limits can reasonably be expected, and the exact limits can only be
determined experimentally. RF sensitivity of the antenna is another matter. Sensitivity will vary
significantly with large frequency changes, but this can be compensated to some degree by using
antenna elements of different length for specific bands. The information provided below is intended for
people who anticipate using the
KN2C-2020T
across a multi-octave frequency range.
b) The upper frequency limit of operation for a Doppler antenna is fairly well-defined, and is determined by
the separation distance between adjacent antenna elements, expressed in wavelengths. Distances
exceeding 1/3 wavelength will begin to degrade the RF sensitivity of the antenna, and distance of
wavelength will definitely cause trouble. The reason involves the amount of RF phase "jump" that occurs
when the DF switches from one antenna to the next. The receiver must "adjust" to the changes of (input)
RF phase, and the Doppler pulses are created in the receiver as a "side effect" of the re-adjustment
process.
c) For phase jumps exceeding 120 degrees, (= 1/3 wavelength distance) the amplitude of the RF signal
will suffer some decay while the receiver "re-adjusts" to the new RF phase angle. For phase jumps of
180 degrees (= wavelength distance) the RF signal will completely decay to zero amplitude, and then
"re-appear" in the receiver, (with reversed phase) resulting in a "noise" pulse instead of a Doppler pulse.
d) The lower frequency limit is more nebulous and ill-defined. Strictly speaking, there is no true lower
frequency "limit" for the operation of a Doppler. As the RF frequency descends, the size of the antenna
(expressed in wavelengths) grows progressively smaller, yielding Doppler pulses of progressively
smaller amplitude. The relationship is quite predictable and linear
cutting the RF frequency in half will
cut the Doppler pulses in half also. ( because the antenna dimensions have been cut in half, expressed
in wavelengths )
e) This can be compensated (to some degree) by increasing the audio level in the receiver, but there is a
limit to this method at some point, the audio level will be so great that serious distortion will result, either
in the Doppler input circuits or in the audio output stages of the receiver. Therefore, the factors that
determine the lower frequency limit are (1) The size of the antenna, expressed in wavelengths, (2) the
maximum audio level that will not cause distortion, and (3) the minimum detection level of the Doppler
DF.
2) The Antenna Unit
The Bottom of Antenna Base
Antenna Module
