FOXHUNTING or 2 METRE DIRECTION FINDING
Many radio Clubs have within them groups who on a regular basis play Radio Hide and Seek. Basic ingredients for this are a radio (handset is most convenient), a directional beam (3 or 4 element yagi), map, compass and someone who will go and hide with a transmitter. In essence the “hounds” take bearings on the fox to locate its position and the first to find the fox is the winner.
There are variations on this theme where more than one transmitter is hidden but most Clubs have one fox, teams start in a car and invariably end up on foot in some woods for the final phase to find the fox hidden behind a bush. After this the “teams” retire to the Pub to compare bearings and provide excuses for poor performance.
The major hurdle new hunters find is that when they are within half a mile of the transmitter,
that the signals are so strong that the handset “S” meter is at maximum scale – even with the antenna
un-plugged. Screening (aluminium foil) helps to reduce this effect. Passive attenuators can be used to
good advantage and you may be able to get within 500 yards of the transmitter if the transmitter is
The “Active Attenuator” described here is a simple way to let you walk right up to the fox
without screening your radio or requiring any other special bits of kit.
ACTIVE ATTENUATORS - (TO GET CLOSE TO THE FOX.)
There is nothing novel about the circuit design, it follows the same technique as used many times previously. However, previously published designs have used simple transistor oscillators and a construction technique to enable “junk box” components to be used. They all required “tuning” and all had the tendency to drift off frequency, especially when the battery was less than brand new. Lastly, previously published designs required the Active Attenuator to be connected in series, between the antenna and the handset when the signals became too strong to resolve direction without them.
This circuit overcomes both of these problems:-
1) It uses an "off the shelf" Commercial 1 Mhz Xtal oscillator block.
2) It includes change-over switching so that it may be left connected permanently in line between the rig and the antenna.
A Printed Circuit Board has been designed on which ALL the components mount - apart from the battery. The completed board assy and small PP3 battery fit snugly inside a small RS (Electromail) die cast aluminium box. This ensures a reliable repeatable design.
A 4 pole change over push ON / Push OFF PCB mounted switch is employed. Two poles direct the RF between the two board mounted BNC connectors when the switch is in the out / off position.
When pressed IN, the switch routes the RF through the active attenuator circuit.
The second & third banks of this switch are connected in parallel and are used to connect the battery when the attenuator is switched ON. A LED illuminates at the same time.
The PP3 9 volts supply is regulated to 5 volts using a small voltage
regulator (REG1). The 1 Mhz Oscillator (X1) is free running and produces a
roughly square wave output of about 5 volts. The 3K9 resistor R2 feeds this supply
to the 1K Cermet potentiometer applying about 1 volt across the potentiometer.
Capacitor C2 slows the rise and fall times significantly.
The output of the potentiometer RV1 is routed to a diode switch comprising a
1N4148 diode (D1) that is forward biased when there is more than about 0.6 volt
applied to its anode. The DC current path being provided by choke L1.
RF in is routed from the switch by capacitor C1. As the waveform is very roughly
tri-angular the percentage of time that this diode is forward biased can be varied
from zero (no volts) to about 50% (full volts). Therefore, when the attentator is first
switched ON with the gain pot set at maximum there will instantly be a halving of the signal level applied to the radio as the diode is forward biased only half of the time.
As one closes into the fox the "gain" is wound anticlockwise to reduce the amount of
time the diode is forward biased.
Eventually, the signal is so strong that this is not enough attenuation. Now, wind the gain
up to maximum and tune your radio up (or down) 1 Mhz,. The unit now operates as a
diode mixer resulting in additional (and much smaller) RF signals that will the incoming
RF signal PLUS the 1Mhz oscillator and also the incoming RF signal MINUS the 1
Mhz oscillator. You will now be able to walk very close to the fox with your S meter
As the waveform applied to the potentiometer is far from sinusoidal the
waveform will be rich in harmonics. This can be used to further advantage by
the foxhunter. If the fox is running a LOT of power then tune your rig several
Mhz up (or down) for even more attenuation where the signal passed into the
handset will be the incoming RF plus or minus a harmonic of the 1Mhz
IMPORTANT NOTE – DO NOT PUSH THE PTT OF YOUR HANDSET
WITH THE ATTENUATOR TURNED ON.
This may cost you a new diode and RF choke!.
Component used are obtained from Electromail (RS Components to the trade).
© G4ODM July 2002
ACTIVE ATTENUATOR ASSEMBLY INSTRUCTIONS
2) Drill the PCB.
Fit the LED onto the PCB leaving it standing off the PCB by about 12mm.
The longest LED lead connects to the pad marked “+”
When fitting components, bend component leads flat against the solder
side of the PCB and cut off level with the side of the relevant solder pad
before soldering in position. Any component leads that extend excessively
beyond the back of the PCB may touch the aluminium box when assembled
so care is necessary. The leads of the switch will need a “haircut” after
soldering. Press the small button onto the switch.
Solder the red and black battery connector leads to the PCB as shown.
4) Locate the LED into small hole in box.
5) Fit the knob to the potentiometer shaft.
6) Ensure that the ON/OFF button is in the OUT (off) position, connect a PP3 battery (not supplied). Press the button IN and confirm that the LED illuminates.
7) Fit a small strip of adhesive backed foam inside the box lid such that it clamps the battery in position. Fit the box lid with the 4 fixing screws.
8) Stick the label onto box with adhesive and protect with clear adhesive film.
Position the label such that the word GAIN is in line with the potentiometer shaft.
9) Connect a small antenna (rubber duck for example) to the BNC connector
Use a small BNC – Rig patch lead (not supplied) to connect the connector marked RADIO to the antenna connector of a 2 metre handset.
10) Using another nearby 2 metre radio and antenna, select a free frequency and transmit.
11) Tune the handset to this same frequency.
Confirm that with the button in the OUT/OFF position that the received signal is at maximum on the S meter.
12) Tune the handset 1 Mhz away from the TX frequency.
Confirm that the received signal disappears and that there is no reading on the handset S meter.
13) Press IN the button on the attenuator, confirm that the GAIN potentiometer can be used to adjust the handset S meter reading between zero signal and maximum signal.
This completes the commissioning of the circuit,
but read the instructions below to enable your attenuator to be
more tolerant to mechanical knocks:-
14) Drill two 1 mm diameter holes each side of the switch
in line with the row of pads nearest the operating end of the switch
as picture below. Make a loop of copper wire and pass it through
both holes from the component side of the board clamping the
switch to the board.Solder one end of the wire to the nearest
switch pad by the edge of the PCB.
Using pliers, pull the other wire end tight taking care that the
wire loop is NOT in the small aperture in the switch associated
with its latching mechanism but tight against the shoulder on the
switch body.Fold this second end towards the other switch
solder pad near the edge of the board and solder in position.
Cut off surplus wire.
This modification will give the switch improved tolerance to sideways knocks.
(C) G4ODM July 2002
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