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
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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 Antenna
80 – 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
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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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