Radio Direction Finding
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Radio Direction Finding

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Radio Direction Finding

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1. Radio Direction Finding Scott Honaker N7SS

2. 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 Everyone has heard jammers or jerks on local repeaters or on the air. This provides a mechanism to report them to the FCC (who will rarely spend the time to track them down. They often stop once they?ve been discovered. Stuck transmitters can be a problem, particularly on repeaters, emergency frequencies and the APRS channel. Local noise sources are always an issue and in addition to standard diagnostics (cutting power), RDF can help. SAR applications require that some RDF capability be available. Aircraft ELTs on 121.5MHz, ham radios or FRS radios can all be used as a signal source for RDF Everyone has heard jammers or jerks on local repeaters or on the air. This provides a mechanism to report them to the FCC (who will rarely spend the time to track them down. They often stop once they?ve been discovered. Stuck transmitters can be a problem, particularly on repeaters, emergency frequencies and the APRS channel. Local noise sources are always an issue and in addition to standard diagnostics (cutting power), RDF can help. SAR applications require that some RDF capability be available. Aircraft ELTs on 121.5MHz, ham radios or FRS radios can all be used as a signal source for RDF

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 3

4. Antennas Equipment Scott Honaker - N7SS 4

5. 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

6. Scott Honaker - N7SS 6 Beam/Yagi Very accurate bearings Requires clean pattern and good front-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

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

8. Advanced Equipment Equipment Scott Honaker - N7SS 8

9. 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

10. 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 The value of real-time bearings cannot be underestimated. It makes it far simpler to identify multipath signals as they come and go. Simply seeing occasional false readings rather than relying on one where you might stop and get a bearing, can make a huge difference in time spent locating a signal source.The value of real-time bearings cannot be underestimated. It makes it far simpler to identify multipath signals as they come and go. Simply seeing occasional false readings rather than relying on one where you might stop and get a bearing, can make a huge difference in time spent locating a signal source.

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

12. Triangulation Doing the Math a, ? 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 12

13. 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 The better the geometry of these three points, the more likely to locate the contact inside. Terrain also plays a part in locating these signals because multipath can be very deceiving. When possible take bearings away from obstructions. Fields, parks or large parking lots are best. Watching the signal strength meter while moving is an excellent simple RDF technique. It does assume some knowledge of the terrain or transmitter.The better the geometry of these three points, the more likely to locate the contact inside. Terrain also plays a part in locating these signals because multipath can be very deceiving. When possible take bearings away from obstructions. Fields, parks or large parking lots are best. Watching the signal strength meter while moving is an excellent simple RDF technique. It does assume some knowledge of the terrain or transmitter.

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

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

16. 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 IARU rules for championship hunts call for five low-power (0.25 to 1.5 watt) foxes, automatically transmitting on the same frequency for exactly one minute each in numbered sequence. Fox #1 sends "MOE MOE MOE" continuously in slow-speed CW, fox #2 sends "MOI MOI MOI" and so forth. (The number of dits in the third letter discloses the fox number.) Timing should be such that just as fox #1 goes off the air, fox #2 comes on, and so forth. As fox #5 goes off, fox #1 comes on again, the cycle repeating continuously. Of course there must also be callsign identification at the end of each transmission. Local preferences govern the number of foxes in practice hunts. For instance, hams in Sweden like to hunt seven foxes scattered in a dense forest, usually at night! If there is snow on the ground, some of them hunt on skis. When the venue is very large or when good maps are not available, the course-setters place an additional transmitter on another frequency at the finish line, sending "MO" continuously to help hunters find their way home. IARU rules for championship hunts call for five low-power (0.25 to 1.5 watt) foxes, automatically transmitting on the same frequency for exactly one minute each in numbered sequence. Fox #1 sends "MOE MOE MOE" continuously in slow-speed CW, fox #2 sends "MOI MOI MOI" and so forth. (The number of dits in the third letter discloses the fox number.) Timing should be such that just as fox #1 goes off the air, fox #2 comes on, and so forth. As fox #5 goes off, fox #1 comes on again, the cycle repeating continuously. Of course there must also be callsign identification at the end of each transmission. Local preferences govern the number of foxes in practice hunts. For instance, hams in Sweden like to hunt seven foxes scattered in a dense forest, usually at night! If there is snow on the ground, some of them hunt on skis. When the venue is very large or when good maps are not available, the course-setters place an additional transmitter on another frequency at the finish line, sending "MO" continuously to help hunters find their way home.

17. 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

18. 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

19. 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


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