RADIO DIRECTION FINDING FUNDAMENTALS�

Excerpts from Ron Milione, Ph.D. W2TAP

And others

Why do we need these skills?

• Locating Harmful Interference
• Jammers
• Stuck transmitters
• Local noise sources
• Interference
• Search and Rescue
• ELT/EPIRBs
• FRS/ham radios
• Wildlife location

Ron Milione - W2TAP

2

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

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.

Triangulation

• Theory
• Take bearings from three points
• 90% of contacts will be inside intersecting triangle
• Don’t neglect signal strength info
• Signal strength will depend on terrain

Ron Milione - W2TAP

8

RDF Techniques

• Mapping
• Time vs. accuracy
• Accurate bearing plotting is time consuming but often valuable
• Terrain
• Multipath issues – particularly in urban areas
• Reflections can mislead and can be accentuated if the target uses a directional antenna

Ron Milione - W2TAP

9

RDF Techniques

• Going the last mile
• Attenuators for non-Adcock antennas
• Removing the antenna when within a couple blocks
• Body blocking
• Detuning decreases receiver sensitivity
• Tune a harmonic (2m 3^rd harmonic on 70cm)
• Be aware of local multipath sources

Ron Milione - W2TAP

10

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

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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

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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

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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

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

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