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?