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Background

Fox hunt is a common name for hunting a hidden radio transmitter

(other names: Transmitter hunt, T-hunt, Amateur Radio Direction Finding (ARDF))

A transmitter is set up at a secret location
At the beginning of the hunt, the transmitter is activated
The hunters try to find the transmitter using various radiolocation techniques
Scoring can be by first arrival, by mileage traveled, number of transmissions requested, etc.

Regional and international hunts following IARU rules are on 80m and 2m

Local hunts add additional bands with rules by the sponsoring club (50, 223, 440, and 1200 MHz)

May be on-foot, mobile, bicycle, cross country skiing, etc.

Ham license is not required to participate in the hunt (hunters do not transmit)

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Why a Fox Hunt?

New way to use those VHF/UHF handhelds
Learn and use some new antenna types
Improve radio skills

Finding sources of RFI
Finding jammers
Search and rescue
Wildlife tracking

Takes practice and patience

Getting to know the equipment

Different benefits and quirks

Signals are usually not well-behaved

Can change in time, location, quality, etc.

Environment

E.g. open field vs. city with multi-path (reflections)

Fox hunts are a great way to build practice

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Basic radiolocation techniques concept

Use the directional properties of some antennas and triangulation to locate the hidden transmitter

Strong signal

Almost no signal

Weak signal

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Basic concept: Triangulation

Position A

Hunter finds the direction of max signal

(compass), corrects for declination and draws a cone on a map

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Basic concept: Triangulation

Position A

Hunter moves to another position finds the direction of max signal

(compass), corrects for declination and draws a second cone on a map

Position B

Area where the fox hides.

Repeating this from different positions the area becomes smaller and smaller

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Required tools for triangulation (kit will be provided)

Note there are 13.3º of declination: to draw on the map add 13.3º to what the compass say

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The Fox

The fox can be a manned or automated station
Manned stations typically only transmit on request, and the requesting station may be penalized time or miles, while everyone else gets to use the transmission to take their own bearings
Automated stations may have a continuous carrier or timer based.

We will use an automated station transmitting a message on 446.5MHz at 2 minutes intervals

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Antennas for Fox Hunting

Beams have a broad (30-60 degree) peak
Many have sharp nulls on sides
Practical sizes have limited gain and directivity: 5-7dB gain, 20 dB f/b
Limited gain and f/b ratio mean you have to be sensitive to small amplitude changes
Difficult to sense amplitude changes on an FM radio
Relatively easy to build yagis and quads
Narrow effective bandwidth
Clumsy to transport in car and deploy
Relatively clumsy to use
Good gain for distant transmitters
Require attenuation to use when closer to transmitters

Wide bandwidth
Give a sharp null, sharper than beam peak
Null is at right angles to the plane of loop
Doesn’t resolve 180 degree bearing ambiguity
Require more precision in construction
Fairly large amplitude change in null compared to beam peak - 30-40dB
Small size and convenient to use and transport
Relatively deaf (10-15 dB down from beam or whip)
Need attenuation when close to transmitter to prevent swamping

Beam Antennas

Loop Antennas

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We are going to use a HB9CVs variant

Typical HB9CV Horizontal pattern

Typical HB9CV Vertical pattern

HB9CV Measured horizontal Pattern

Main lobe

Null

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Attenuation

Why is it needed:

Strong signal swamps gain differences in beams and loops
Move receiver response out of limiting into a more linear range
Move signal level to below full quieting to allow signal level judgment by ear
Help reduce responses to reflections

Attenuation techniques:

Body fade technique

Hold the HT close to your chest, rotate to find signal null off your back side

Tune off frequency

Tuning the radio off the fox frequency in 5khz steps will reduce the signal by the attenuation on the slope of the IF filter

Use an Attenuator

Passive
Active

Remove antenna

Radio without antenna makes a good “we’re here!” indicator

Mailing tube attenuator

A cylinder covered with aluminum foil
Lower the HT down into the tube for increasing attenuation
Tube acts as waveguide High Pass Filter

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Offset Attenuator

Mixes the received signal with a 4MHz square wave
By doing so spurious harmonics of the received signal (green) are generated at +/- 4MHz and multiples
Harmonics progressively attenuated by the order and by the amount of mixed signal that is passed to the receiver

How to use it:

If received signal is to strong and no direction can be identified switch on the attenuator, tune the radio down 4 MHz and/or rotate the knob
If the signal is still too strong go down another 4 MHz and so on

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Basic Techniques

Start out high – want direct signal path if at all possible
Use the map. Plot all data. Average bearings to sort good from bad
Plan a round course (no zigzag)
Take bearings in the clear, away from buildings and topographic features
Take terrain into account. Expect reflections.
If there are hills, always choose a route on the top of a ridge. Careful with reflections in valleys
Take frequent bearings. The more you can average, the better idea you get of the real ones
Don’t make early assumptions about the location. Individual bearings can be misleading!
Attenuate signal as needed to keep linear
Think about polarization – cross polarization attenuates the direct signal, and enhances response to reflections.
Do not trust other fox hunters
Have fun!

Hints, hints, hints…..

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Step by step

Open your map and align it so it is orientated the same way as real life
When the transmission starts move your beam round for the peak reading
Get a partner to take a beam heading using the compass.
Note the beam heading in degrees from north adding declination
Place your Protractor on the map with it’s center where you are and mark a dot on the map at the appropriate heading
Join the dots with a ruler and extend the line as far as you can go
Then draw a 30º cone around it
Repeat for additional bearings

So you are in the vicinity of the fox…

Can you get any sort of bearing?
Make sure your attenuator is in line
Take your antenna off, or use other attenuating tricks. Can you still hear him?
Look carefully at your map for any obvious places your prey may be.

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