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Hidden Transmitter Hunts
Also known as
Fox Hunts
Bunny Hunts
T Hunts
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.
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!
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.
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
Beam Antennas
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
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
Loop Antennas
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
Loop antennas continued
• 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
Time Difference Of Arrival
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
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
Doppler Systems
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
WinDopp Demo
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
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 Methods
• 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
• Remove antenna – Radio without antenna makes a good “we’re here!”
indicator
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
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
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
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.
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.
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
Happy HuntersA brief photo montage
KC2ANT finds the ‘Fox’
Questions?