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Radio Direction Finding

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Radio Direction Finding

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Radio direction finding l.jpg

Radio Direction Finding

Scott Honaker


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Why do we need these skills?

  • Locating Harmful Interference

    • Jammers

    • Stuck transmitters

    • Local noise sources

  • Search and Rescue

    • ELT/EPIRBs

    • FRS/ham radios

    • Wildlife location

  • Navigation

Scott Honaker - N7SS

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Equipment Needed

  • Radio receiver and antenna that are directionally sensitive and can receive the signal

  • A way to determine your direction

  • A map and compass would be nice

Scott Honaker - N7SS

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Scott Honaker - N7SS

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Wire Antenna

  • Simple

  • Signal strength only

  • Directivity via body blocking

  • Works better with stronger signal and well shielded receiver

  • High skill level required

Scott Honaker - N7SS

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  • Very accurate bearings

  • Requires cleanpattern and goodfront-to-back ratio

  • Easily overloaded by strong signals

  • Tune to null (sharper than gain)

  • Can use front to help find weaker signals but with reduced bearing accuracy

Scott Honaker - N7SS

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  • More common on HF frequencies

  • Very compact

  • Tune to the null

  • Bearings not very precise

  • Has 180 degree ambiguity

Scott Honaker - N7SS

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Advanced Equipment

Scott Honaker - N7SS

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Adcock Device – Handi Finder

  • Based on phase difference

  • Antenna array MUST be symmetric

  • Can’t identify front/back without a third sense antenna

  • Tune to no tone (signals in phase)

  • Weak signals can be a problem

  • Strong signals may cause too much multipath

Scott Honaker - N7SS

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“Doppler” DF – DF Jr.

  • Not real Doppler – uses a pair of Adcock antennas

  • Gives real-time bearings

  • Only accurate to~30 degrees

  • Antenna size and spacing are frequency dependant

Scott Honaker - N7SS

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Locating the Fox (transmitter)

Scott Honaker - N7SS

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  • Doing the Math

    • α, β and distance AB are already known

    • C can be calculated by using the distance RC or MC:

    • RC: Position of C can be calculated using the Law of Sines

    • RC or MC can be calculated using the Law of Cosines and the Pythagorean theorem

Scott Honaker - N7SS

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  • Graphical Approch

    • 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

Scott Honaker - N7SS

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Advanced Tools

  • Excel + geofunc

  • Mapping Tool – Streets and Tips, MapPoint, Street Atlas

  • GPS System

Scott Honaker - N7SS

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Sources of Error

  • Operator Error

  • Antenna Gain Pattern

  • Propagation

    • Multi Path

    • Reflection

Scott Honaker – N7SS

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  • Containment Area

    • Large (county) vs. small (city) or local (park)

    • Drive times become significant

  • Starting location

    • Together vs. distributed

  • Radios/antennas

    • HTs vs. mobiles

      • HTs offer portability but mobile meters are often more accurate

    • Omni vs. beam

      • Even 2m beams are unwieldy, omnis offer some info while driving

Scott Honaker - N7SS

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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

Scott Honaker - N7SS

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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 3rd harmonic on 70cm)

    • Be aware of local multipath sources

Scott Honaker - N7SS

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  • Homing In Site

  • Tape Measure Yagi

  • Adjustable Attenuator

  • Arrow Antenna

  • Ultimate Automated Doppler DF System

Scott Honaker - N7SS