introduction to vhf direction finding graham g0uus
TRANSCRIPT
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
19/04/23 2FDARC – Intro to VHF Direction Finding
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
19/04/23 5FDARC – Intro to VHF Direction Finding
Effect of bearing errors
19/04/23 6FDARC – Intro to VHF Direction Finding
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.
19/04/23 7FDARC – Intro to VHF Direction Finding
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!
19/04/23 8FDARC – Intro to VHF Direction Finding
Example Yagi Polar Diagram
19/04/23 9FDARC – Intro to VHF Direction Finding
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.
19/04/23 10FDARC – Intro to VHF Direction Finding
Dipole Polar Diagrams
19/04/23 11FDARC – Intro to VHF Direction Finding
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
19/04/23 12FDARC – Intro to VHF Direction Finding
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)
19/04/23 13FDARC – Intro to VHF Direction Finding
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
19/04/23 15FDARC – Intro to VHF Direction Finding
WW-AD Func Diag
19/04/23 16FDARC – Intro to VHF Direction Finding
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(!)
19/04/23 17FDARC – Intro to VHF Direction Finding
Adcock DF Antenna
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Dual Band Adcock DF Antenna80 – 520 MHz
19/04/23 19FDARC – Intro to VHF Direction Finding
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
19/04/23 20FDARC – Intro to VHF Direction Finding
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
19/04/23 21FDARC – Intro to VHF Direction Finding
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)
19/04/23 22FDARC – Intro to VHF Direction Finding
Simple TDOA
19/04/23 23FDARC – Intro to VHF Direction Finding
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
19/04/23 24FDARC – Intro to VHF Direction Finding
TDOA 2 Schematic
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19/04/23 26FDARC – Intro to VHF Direction Finding
Questions?
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