1. Hidden Transmitter Hunts
Also known as
Fox Hunts
Bunny Hunts
T Hunts

2. What is a fox hunt? 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.

3. Why do it? Practice for real events – such as….
Finding downed aircraft via ELT
Finding lost or injured parties with radios
Finding jammers
Finding sources of interference
It’s FUN!

4. What do you use for 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.

5. How do you track the fox? Various forms of radiolocation are used
Signal strength plotting
Body fade (nulls)
Beam antennas with directivity (peaks)
Loop antennas with directivity (nulls)
Time Difference Of Arrival (TDOA)
Doppler systems

6. Beam Antennas

7. Using beam antennas 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

8. Beam antennas continued
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

11. Loop Antennas

12. Using loop antennas 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

13. Loop antennas continued
Fairly large amplitude change in null compared to beam peak db
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

15. Time Difference Of Arrival

16. Time Difference of Arrival (TDOA)
Based on doppler shift of signal between two antennas.
Requires use of FM receiver, even on AM signals (to detect frequency shift from doppler)
Simple to construct
Gives sharp null
Sensitive to reflections

17. TDOA continued
Can interfere with reception on other nearby receivers, TDOA’s, or dopplers – reradiates signal
Doesn’t resolve 180 degree bearing ambiguity
Wide bandwidth
Doesn’t require attenuation – gives good bearings in strong signal environment
Not as sensitive as other antennas

19. Doppler Systems

22. Doppler systems Most expensive and complicated Can be used in motion
Developed in Rochester by fox hunters
Wide bandwidth
Least bearing resolution
Sensitive to reflections and multipath
Relatively deaf compared to beam or whip
Easiest to use while mobile if good display

24. WinDopp Demo

25. Sensing amplitude changes
FM systems are limiting IF’s, lots of gain, not much linearity. A multimode radio is much better at sensing changes in level.
Need attenuation to bring signal into a range where amplitude can be judged. Knee of detector.
Listening to half quieting signal is sensitive
Get S meter to show partial scale – not linear

26. 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

27. Attenuation Methods Body fade technique Tune off frequency
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
Remove antenna
Radio without antenna makes a good “we’re here!” indicator

28. Attenuation continued
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
Passive attenuation – resistive pads
Much isolation needed = lots of shielding
Bulky, hard to build well
Direct radiation through case can defeat attenuation on antenna

30. Active attenuation
Depends on mixing to new receiver frequency – ie 4 mhz offset
Requires battery
Easy to build
Can provide over 100 db attenuation
Direct radiation through case doesn’t matter

33. Basic Techiques
Start out high – want direct signal path if at all possible
Use a map. Plot all data. Average bearings to sort good from bad
Move at right angles to starting bearings to get cross bearings to resolve ambiguity
Take bearings in the clear, away from buildings and topographic features
Attenuate signal as needed to keep linear

34. Techniques continued
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!
On a joint hunt, share information often.
Take terrain into account. Expect reflections.
Think about polarization – cross polarization attenuates the direct signal, and enhances response to reflections.

35. Spoofing Tecniques
Use a lot of power, saturate the hunters receivers! 
Directional antennas – beam the power in a specific direction.
Use topography to mask signal from hunters or direct it in another direction.
Add modulation to interfere with doppler and TDOA switching.

36. Spoofing continued
Vary power levels – confuse hunters as to whether they are getting close or not.
Don’t transmit continuously.
Use unexpected polarization or vary polarization

38. Happy Hunters
A brief photo montage

44. KC2ANT finds the ‘Fox’

45. Questions?