Introduction to VHF Direction Finding

Graham G0UUS

Why Direction Finding?

• We want to locate a transmitter– For a fox hunt (Don’t forget our hunt 14th July)– To locate a source of interference

• Two basic ways– Bearing and Range– Two or more bearings

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Bearing and Range

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Locating TX using multiple Bearings

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How do we measur the bearing

• Simple directional antenna– Yagi or Dipole

• Special DF system – Watson Watt - Adcock– Doppler– Pseudo Doppler– TDOA

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Effect of bearing errors

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Sources of Bearing Error

• Identifying the bearing from the antenna direction (reading a compass – errors in the compass itself)

• “Body” effects – for a hand held antenna• Bias due to the antenna construction• Inherent uncertainty in the antenna design• Multipath effects – may cause the apparent

direction of the signal to be many degrees away from the actual direction.

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Yagi

• Yagi has a non uniform response to radio waves coming from different directions

• Strongest signal when antenna pointed directly at the transmitter– Not easy to identify the maximum signal because the

peak is usually relatively wide (especially for something you can walk around with)

• A minimum signal is generally easier to identify – but there are lots of them so not useful!

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Example Yagi Polar Diagram

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A Simple Dipole DF antenna

• Has a “figure–of–eight” polar diagram• As for a yagi the maximum signal is too broad

to be useful – Generally wider than a yagi as well!

• Minima can be used – but there are two of them 180° apart so we can identify a line but not which direction along that line.– Multiple bearings can disambiguate since they will

cross on the correct side.

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Dipole Polar Diagrams

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Loops

• For lower frequencies Loops can be used since they have similar figure-of-eight response.

• Ferrite loops can also be used for the lowest frequencies – e.g., topband

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A Professional System

• Uses the relative signal strength received by two antenna set at 90°

• Needs an additional ‘sense’ antenna to disambiguate between two possible opposite bearings.

• Simplest seems to be a pair of dipoles or loops which have similar polar diagrams (loops work for lower frequencies)

• Actually set of 4 monopoles turns out to be even simpler (for vert. polarisation anyway)

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Two crossed dipoles

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Watson Watt DF

• Consists of a directional antenna• A DF Receiver• A DF Bearing Processor• A DF Bearing Display

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WW-AD Func Diag

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Watson Watt DF System

• Uses either loop or Adcock DF antennas• Antenna produces separate signals for N-S &

E-W directions (plus sense)• DF RX – fairly normal AM RX but two channels– Output is separate E-W(x) and N-S(y) signals

• DF Processor computes the bearing• DF Bearing Display displays the bearing(!)

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Adcock DF Antenna

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Dual Band Adcock DF Antenna80 – 520 MHz

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Doppler (FM) DF

• Consider a vertical dipole on the end of a rotating arm.

• A Frequency Modulation will be impressed on any carrier received.

• Mechanically hard (rotating coax connections)• Achievable rotation freq too low to be useful• Moving parts -> unreliable

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Pseudo Doppler System

• Use a circular array of aerials• Electronically switch each aerial in turn to a

common feeder• No moving parts – Much higher “rotation” frequency possible– Much more reliable

• There are amateur implementations– These generally roof mount on cars

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Whistling Dipoles DF

• Uses a single pair of dipoles• Doesn’t require a groundplane• Useable as handheld system• Works with unmodified 2m Handheld• Switches the two dipoles onto common feeder

at audio frequency (~1kHz)

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Simple TDOA

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Indicating Version

• Adds a phase sensitive detector and indicator• The audio recovered by the RX is input to a

phase sensitive detector.• Output is a DC signal whose sign depends on

the relative phase of the audio and switching signal AND whose level is directly related to the audio level.

• DC Signal displayed on centre zero meter

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TDOA 2 Schematic

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Questions?

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