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 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

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 Hunters A brief photo montage

KC2ANT finds the ‘Fox’

Questions?