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Radio Direction Finding

Radio Direction Finding. Scott Honaker N7SS. 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. Equipment Needed.

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Radio Direction Finding

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  1. Radio Direction Finding Scott Honaker N7SS

  2. 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

  3. 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

  4. Equipment Antennas Scott Honaker - N7SS

  5. 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

  6. Beam/Yagi • 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

  7. Loops • More common on HF frequencies • Very compact • Tune to the null • Bearings not very precise • Has 180 degree ambiguity Scott Honaker - N7SS

  8. Equipment Advanced Equipment Scott Honaker - N7SS

  9. 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

  10. “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

  11. Theory Locating the Fox (transmitter) Scott Honaker - N7SS

  12. Triangulation • 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

  13. Triangulation • 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

  14. Advanced Tools • Excel + geofunchttp://www.afsc.noaa.gov/nmml/software/excelgeo.php • Mapping Tool – Streets and Tips, MapPoint, Street Atlas • GPS System Scott Honaker - N7SS

  15. Sources of Error • Operator Error • Antenna Gain Pattern • Propagation • Multi Path • Reflection Scott Honaker – N7SS

  16. Foxhunting • 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

  17. 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

  18. 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

  19. Resources • Homing In Site http://members.aol.com/homingin/ • Tape Measure Yagi http://home.att.net/~jleggio/projects/rdf/tape_bm.htm • Adjustable Attenuator http://home.att.net/~jleggio/projects/rdf/p_atten.htm • Arrow Antenna http://www.arrowantennas.com/ • Ultimate Automated Doppler DF System http://www.ahha-doppler.com/gpss/gpss.html Scott Honaker - N7SS

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