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The frequency of rotation is:
fr = 500 x 1879.8/146 x 13.39
A rotation frequency of 480 Hz translates to 480 x 60 = 28,800 or almost
30,000 r/min, which pretty much rules out any ideas of mechanically rotating
the antenna! Fortunately, Terrence Rogers, WA4BVY, proposed a clever
method of electrically spinning the antenna that works very well. Roger's pro-
ject, the DoppleScAnt, uses eight 1/4-
vertical whips arranged in a circular
pattern. Only one antenna at a time is electrically selected. By controlling the
order in which the antennas are selected, the DoppleScAnt emulates a sin-
gle 1/4 –
whip antenna moving in a circle. A clever feature in Roger's de-
sign is the use of a digital audio filter to extract the Doppler tone from voice,
PL tones and noise.
The DDF1 design offers slightly improved audio filtering, 74HC-series logic
circuits capable of driving the LED display directly, a wideband VHF/UHF an-
tenna switcher and the four 1/4- mag-mount antennas. Total project cost is
about one third the cost of purchasing a commercial RDF unit - and building
the project is a lot more educational.
HOW IT WORKS
To understand the operation of the Doppler RDF circuit, see the block dia-
gram of Figure 2. An 8 kHz clock oscillator drives a binary counter. The out-
put of the counter performs three synchronized functions: "spin" the antenna,
drive the LED display and run the digital filter. The counter output drives a 1
of 4 multiplexer that spins the antennas by sequentially selecting (turning on)
one at a time in the order A,B,C,D,A, etc., at 500 times per second. The
counter output also drives a 1 of 6 multiplexer used to drive the LED display
in sync with the spinning antenna. The RF signal received from the spinning
antenna is connected to the antenna input of a VHF or UHF FM receiver.
The spinning antenna imposes a 500 Hz frequency deviation on a 146 MHz
received signal. A 146 MHz FM receiver connected to the spinning an-
tenna's RF output demodulates the 500 Hz frequency deviation and sounds
like a 500 Hz tone with loudness set by the 500 Hz frequency deviation. The
receiver audio, including 500 Hz Doppler tone, is processed by a series of
audio filters. A high pass filter rejects PL tones and audio frequencies below
the 500 Hz Doppler tone. A low-pass filter rejects all audio frequencies
above the 500 Hz Doppler tone, and a very narrow bandwidth digital filter ex-
tracts only 500 HZ Doppler tone.
The output of the digital filter represents the actual Doppler frequency shift
DDF1 • 6
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