Radio Direction Finding

Radio Direction Radio Direction FindingFinding

Scott HonakerScott Honaker

N7SSN7SS

Scott Honaker - N7SS 2

Why do we need these skills?Why do we need these skills?

► Locating Harmful Locating Harmful InterferenceInterference JammersJammers Stuck transmittersStuck transmitters Local noise sourcesLocal noise sources

► Search and RescueSearch and Rescue ELT/EPIRBsELT/EPIRBs FRS/ham radiosFRS/ham radios Wildlife locationWildlife location

►NavigationNavigation

Equipment NeededEquipment Needed

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

►A way to determine your directionA way to determine your direction►A map and compass would be niceA map and compass would be nice


ANTENNASANTENNASEquipment Equipment



Wire AntennaWire Antenna

►SimpleSimple►Signal strength onlySignal strength only►Directivity via body blockingDirectivity via body blocking►Works better with stronger signal and Works better with stronger signal and

well shielded receiverwell shielded receiver►High skill level requiredHigh skill level required


Beam/YagiBeam/Yagi

► Very accurate bearingsVery accurate bearings► Requires cleanRequires clean

pattern and goodpattern and goodfront-to-back ratiofront-to-back ratio

► Easily overloaded by Easily overloaded by strong signalsstrong signals

► Tune to null (sharper than gain)Tune to null (sharper than gain)► Can use front to help find weaker signals Can use front to help find weaker signals

but with reduced bearing accuracybut with reduced bearing accuracy


LoopsLoops

►More common on HF frequenciesMore common on HF frequencies►Very compactVery compact►Tune to the nullTune to the null►Bearings not very preciseBearings not very precise►Has 180 degree ambiguityHas 180 degree ambiguity

ADVANCED EQUIPMENTADVANCED EQUIPMENTEquipmentEquipment



Adcock Device – Handi FinderAdcock Device – Handi Finder

►Based on phase differenceBased on phase difference►Antenna array MUST be symmetricAntenna array MUST be symmetric►Can’t identify front/back without a Can’t identify front/back without a

third sense antennathird sense antenna►Tune to no tone (signals in phase)Tune to no tone (signals in phase)►Weak signals can be a problemWeak signals can be a problem►Strong signals may cause too much Strong signals may cause too much

multipathmultipath


““Doppler” DF – DF Jr.Doppler” DF – DF Jr.

►Not real Doppler – uses Not real Doppler – uses a pair of Adcock a pair of Adcock antennasantennas

►Gives real-time bearingsGives real-time bearings►Only accurate toOnly accurate to

~30 degrees~30 degrees►Antenna size and Antenna size and

spacing are spacing are frequency dependantfrequency dependant

LOCATING THE FOX LOCATING THE FOX (TRANSMITTER)(TRANSMITTER)

TheoryTheory


TriangulationTriangulation►Doing the Doing the

MathMath α, β and distance AB are α, β and distance AB are

already known already known C can be calculated by C can be calculated by

using the distance RC or using the distance RC or MC: MC:

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

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



TriangulationTriangulation

► Graphical ApprochGraphical Approch Take bearings from Take bearings from

three pointsthree points 90% of contacts will 90% of contacts will

be inside be inside intersecting triangleintersecting triangle

► Don’t neglect Don’t neglect signal strength infosignal strength info

► Signal strength will Signal strength will depend on terraindepend on terrain

Advanced ToolsAdvanced Tools

►Excel + geofunc Excel + geofunc http://www.afsc.noaa.gov/nmml/softwahttp://www.afsc.noaa.gov/nmml/software/excelgeo.phpre/excelgeo.php

►Mapping Tool – Streets and Tips, Mapping Tool – Streets and Tips, MapPoint, Street AtlasMapPoint, Street Atlas

►GPS SystemGPS System


Sources of ErrorSources of Error

►Operator ErrorOperator Error►Antenna Gain PatternAntenna Gain Pattern►PropagationPropagation

Multi PathMulti Path Reflection Reflection

Scott Honaker – N7SS 15


FoxhuntingFoxhunting► Containment AreaContainment Area

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

Drive times become significantDrive times become significant► Starting locationStarting location

Together vs. distributedTogether vs. distributed► Radios/antennasRadios/antennas

HTs vs. mobilesHTs vs. mobiles► HTs offer portability but mobile HTs offer portability but mobile

meters are often more accuratemeters are often more accurate Omni vs. beamOmni vs. beam

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


RDF TechniquesRDF Techniques

►MappingMapping Time vs. accuracyTime vs. accuracy Accurate bearing plotting is time consuming Accurate bearing plotting is time consuming

but often valuablebut often valuable

►TerrainTerrain Multipath issues – particularly in urban areasMultipath issues – particularly in urban areas Reflections can mislead and can be Reflections can mislead and can be

accentuated if the target uses a directional accentuated if the target uses a directional antennaantenna


RDF TechniquesRDF Techniques

►Going the last mileGoing the last mile Attenuators for non-Adcock antennasAttenuators for non-Adcock antennas Removing the antenna when within a Removing the antenna when within a

couple blockscouple blocks Body blockingBody blocking Detuning decreases receiver sensitivityDetuning decreases receiver sensitivity Tune a harmonic (2m 3Tune a harmonic (2m 3rdrd harmonic on harmonic on

70cm)70cm) Be aware of local multipath sourcesBe aware of local multipath sources


ResourcesResources

► Homing In SiteHoming In Sitehttp://members.aol.com/homingin/http://members.aol.com/homingin/

► Tape Measure YagiTape Measure Yagihttp://home.att.net/~jleggio/projects/rdf/tape_bm.htmhttp://home.att.net/~jleggio/projects/rdf/tape_bm.htm

► Adjustable AttenuatorAdjustable Attenuatorhttp://home.att.net/~jleggio/projects/rdf/p_atten.htmhttp://home.att.net/~jleggio/projects/rdf/p_atten.htm

► Arrow AntennaArrow Antennahttp://www.arrowantennas.com/http://www.arrowantennas.com/

► Ultimate Automated Doppler DF SystemUltimate Automated Doppler DF Systemhttp://www.ahha-doppler.com/gpss/gpss.htmlhttp://www.ahha-doppler.com/gpss/gpss.html

Radio Direction Finding

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