HAWGPLEXER
WHAT IS A HAWGPLEXER?
Forget everything you knew about radio, you are entering a new paradigm!
Two diodes in a funny piece of waveguide, a Gunnplexer, form the front
end of a full duplex microwave radio. The common homodyne type Gunnplexer
may be used for microwave communications with the addition of a 30 MHz
wideband receiver or scanner and some miscellaneous circuitry to provide
power, frequency control and modulation.
These miscellaneous functions are all provided by the HAWGPLEXER kit. If
you have a gunnplexer and a receiver, this simple kit will convert them into
a full duplex microwave radio. The kit is composed totally of Radio Shack
parts. The HAWGPLEXER continues the fine tradition of the HAWG FENCE
antenna by W4KIP, a $3 ATV antenna with 11 db gain made of hawg fence wire.
The HAWG tradition provides almost-commercial-performance radio equipment
which can be cobbled together from cheap household items.
The common homodyne Gunnplexer is found in automatic door openers, motion
sensors, and speed guns. While not designed for FM communications, a little
ingenuity overcomes this deficiency.
THEORY OF OPERATION
The Gunnplexer contains a Gunn diode, which is a negative resistance
oscillator. The oscillator runs at a frequency determined by the resonant
waveguide cavity in which it is mounted. A mixer diode is mounted outside
the cavity, which mixes the outgoing and incoming microwaves. Two
Gunnplexers in communication are tuned 30 MHz apart, each mixer detects a
30 MHz product modulated by both its own and the other Gunn oscillator.
A 30 MHz receiver amplifies and demodulates the signal.
The homodyne Gunnplexer is normally tuned by mechanical means only, but
drifts with temperature, VSWR and voltage. By slightly varying the Gunn
supply and adding some amplified audio to it, we achieve some tuning and
FM modulation in the homodyne type units.
Variable DC supply voltage to the Gunn diode is regulated by the LM317T
and associated components. Main tuning potentiometer controls the voltage
from around 4v up to around 11v. Electrolytic capacitors filter and
stabilize the regulator. The audio amp module amplifies the mike signal
to sufficient level for modulation. Amplified audio is injected into the
Gunn supply downstream of a 10 ohm resistor which isolates the audio from
the bypass capacitors and regulator.
CONSTRUCTION
1. Build the audio module kit as directed, EXCEPT put C4 in backwards.
2. Use discarded leads to add "pins" to the module board.
3. Arrange the audio module and LM317T conveniently on the perfboard, pay
attention to keeping ground leads short.
4. Wire the perf board components per the schematic. Leave flying leads
for power, inputs, outputs, and controls.
5. Install perfboard, connectors and controls in a chassis box and complete
the chassis wiring.
6. Cut DIN cord in half, install DIN pigtails on Gunnplexer(s).
7. Without Gunnplexer connected, apply power, check no smoke. Check 12v
on the audio module and regulator input. Vary the pot through its range
while looking for a smoothly varying voltage at the gunn diode output.
PROCEED NO FURTHER UNTIL CHECKS ARE GOOD.
8. Connect Gunnplexer, power up and check for Gunn current and mixer
current. To read Gunn diode current in mA, measure the DC voltage drop
across the 10 ohm resistor, and multiply by 100. Typical Gunn operating
point might be 8 v at 210 mA. (2.1v drop) Apply a milliammeter from the
mixer jack to ground to measure mixer current, normally a few mA for a
healthy Gunnplexer. PROCEED NO FURTHER UNTIL CHECKS ARE GOOD.
9. Connect receiver, microphone, power up and test against a known good
gunnplexer station or test equipment. Tune to center frequency with
nominal Gunn voltage, and set deviation.
10. Have fun on 10250.0 MHz !
!! CAUTION !!
NEVER look into powered waveguides or loaded gun barrels !!
Use a meter to determine the point where the rated voltage of your
Gunnplexer occurs, proceed further AT YOUR OWN RISK. I have never lost
a diode operating as much as 1 volt above its rating. But it can happen.
It is safer to tune to the strongest signal BELOW the rated Gunn voltage.
73 es enjoy de n4ywk
will.e.payne at lmco.com