1. Introduction to VHF Direction Finding
Graham G0UUS

2. FDARC – Intro to VHF Direction Finding
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
Other reasons – locate someone who is “lost” (e.g., Light Aircraft)
Tracker – activated on a stolen car etc.
Security Services etc.
Bearing – The direction from the RX to the signal source – usually magnetic – degrees clockwise from magnetic north to direction of signal.
Range – Distance from RX to the source.
Plot on a map – draw line through the RX position in the direction of the source. Measure the range along the line – result is the signal source position.
Two or more bearings – plot the bearings from each RX position on a map – the lines should cross where the signal source is located.
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FDARC – Intro to VHF Direction Finding

3. FDARC – Intro to VHF Direction Finding
Bearing and Range
How can we measure range?
Could look at signal strength - Unfortunately that is extremely hard to calibrate. The relative signal strength from two readings could give an indication of whether we are closer or further away but we still wouldn’t really how much further we had to go!
RX/TX could cooperate. If the RX (or rather search) station transmitted a “poll” signal to which the TX (or target) station sent a response we can tell the distance by the elapsed time. We need to measure the elapsed time fairly accurately, especially for relatively short distances. The system also needs to be calibrated to account for the time taken by the target station to recognise the “poll” signal and to actually start transmitting – 1mS == 300m
Not really practical for amateur use but is used by Aircraft NavAids – specifically DME (UHF I believe)
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FDARC – Intro to VHF Direction Finding

4. Locating TX using multiple Bearings
RX1,2,3 could be separate fixed stations or mobile stations
Not limited to three – in theory 2 would be enough – IFF everything was perfect
The stations know their locations exactly (GPS – buts still +- 20m?)
Measured Bearing is correct – bearing read correctly, Antenna(s) are perfect and no multipath effects to mess things up.
In amateur usage we normally use a single mobile RX and take multiple bearings from different locations – Note if the TX moves between taking two successive bearings that messes things up - so we define a rule that doesn’t allow it move!
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FDARC – Intro to VHF Direction Finding

5. How do we measur the bearing
Simple directional antenna
Yagi or Dipole
Special DF system
Watson Watt - Adcock
Doppler
Pseudo Doppler
TDOA
Direction Antennas – we look for the maximum signal strength
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FDARC – Intro to VHF Direction Finding

6. Effect of bearing errors
The previous drawing showed that the three bearings we obtained crossed at a single point coincident with the TX.
As I said that is the ideal situation where the location of the RXs and the bearings are known and or measured exactly.
In real life neither are possible (you can make the errors very small with enough time and money but can never get to zero).
This drawing shows the case where a single mobile RX has a consistent error of about 4.5 degrees clockwise. The three lines no longer cross but form a triangle. In theory the Tx should be somewhere inside the triangle.
In actual fact it can still be outside but should be reasonably close! To show all errors the position of the RX stations should be a circle showing the possible actual locations given the inaccuracy of the position measurement and the bearings should be a set of pairs of lines showing the expected range of actual bearings given the measured bearing from each possible location of the RX. This significantly complicates the diagram (so I didn’t do it). In any case none of this takes into account propagation and multipath efffects.
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FDARC – Intro to VHF Direction Finding

7. 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.
May have antenna in a calibrated fixed location then we rely on some kind of mechanical linkage & indicator. (or possible electronic for some of the switched antennas we will see later)
Body effects – human body affects the response of the antenna. May be many degrees difference between holding antenna at arms length and holding it close. Other near objects may affect the signal as well
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FDARC – Intro to VHF Direction Finding

8. FDARC – 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!
We all know this!
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FDARC – Intro to VHF Direction Finding

9. Example Yagi Polar Diagram
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FDARC – Intro to VHF Direction Finding

10. 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.
Actually the figure of eight is only in a single plane – full 3D polar is a torus or doughnut.
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FDARC – Intro to VHF Direction Finding

11. FDARC – Intro to VHF Direction Finding
Dipole Polar Diagrams
3D torus is from a vertical dipole – shows that the horizontal pattern is a circle – receives equally from all directions.
If you take a vertical slice through the middle then you get the figure of eight shown in the other drawing.
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FDARC – Intro to VHF Direction Finding

12. FDARC – 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
Top band loops may be able to use MW broadcast ferrite aerials but may need to remove some turns to get best sensitivity.
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FDARC – Intro to VHF Direction Finding

13. FDARC – 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)
Based on literature from RDF Systems of Vancouver Washington USA.
Their literature seems to indicate that most other manufacturers use FM rather than AM systems. I though their system interesting enough to present.
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FDARC – Intro to VHF Direction Finding

14. FDARC – Intro to VHF Direction Finding
Two crossed dipoles
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FDARC – Intro to VHF Direction Finding

15. FDARC – Intro to VHF Direction Finding
Watson Watt DF
Consists of a directional antenna
A DF Receiver
A DF Bearing Processor
A DF Bearing Display
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FDARC – Intro to VHF Direction Finding

16. FDARC – Intro to VHF Direction Finding
WW-AD Func Diag
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FDARC – Intro to VHF Direction Finding

17. FDARC – 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(!)
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FDARC – Intro to VHF Direction Finding

18. FDARC – Intro to VHF Direction Finding
Adcock DF Antenna
Consists of 4 individual aerials in two pairs. N-S & E-W
Figure of 8 produced by subtracting the individual antenna N-S and E-W pairs
Sense signal produced by summing all 4 antennas
N-S, E-W signals ultimately become the Y & X axis signals for the display – Sense needed to resolve the 180 ambiguity
Sensitivity increased by increasing spacing between antennas BUT not without limit – the lobes become elongated. Typical designs use something between 0.1 & 0.35 wavelength spacing. Lower limit set by sensitivity, Upper limit by the directional distortion.
Could use more pairs – e.g., 8 pairs can be spaced at a full wavelength BUT much more complicated system – 8 signal channels to be decoded etc.
For single frequency use ,use tuned aerials (monopoles or dipoles)
For multi frequency use need to ensure the spacing limits are maintained - limits practical range. Generally use very short whips,
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FDARC – Intro to VHF Direction Finding

19. Dual Band Adcock DF Antenna 80 – 520 MHz
In additional to the differencing and summing networks, the Antenna contains a pair of AM modulators that impress audio tones on the N-S & E-W signals. This means that the RX only needs a single channel – no need for closely matched channels.
Demodulator reproduces the pair of tones – relative levels are the x & y signals. Sum is related to overall signal strength.
This means that the display can distinguish between different strengths – relative range – as signal gets bigger we must be getting closer.
Some rather complicated circuitry in the box – Active Sum & Difference networks and the am modulators.
Also the separate channels need to be closely matched to avoid distorting the response.
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FDARC – Intro to VHF Direction Finding

20. FDARC – 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
When dipole is approaching the frequency will be higher, when receding lower.
By measuring the phase difference between the angle of rotation and the impressed FM we get the bearing
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FDARC – Intro to VHF Direction Finding

21. FDARC – 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
Can work from 4 upwards.
Seen amateur systems using 4, 9 and 16 ¼ wave whips.
Use discrete logic or microprocessors to demodulate the signal and identify the phase difference.
Not suitable for handheld operation because they need a large ground plane
~ 1 wavelength or more.
Can be used for car based fox hunts though – possibly switching to a hand held system for final approach!
Obviously the bearing is relative to “car-forwards” rather than to an absoulte reference such as North.
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FDARC – Intro to VHF Direction Finding

22. FDARC – 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)
If centre line of the dipoles is inline with the TX, there will be no phase difference between the two signals (assuming perfect construction!)
If centre line is NOT inline with the TX there WILL be a phase difference between the two signals. This will be impressed on the signal at the switching frequency. FM RX will demodulate this to give the audio signal.
The audio tone will be a minimum when the aerials are inline with the TX – nulls easier to detect than maxima.
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FDARC – Intro to VHF Direction Finding

23. FDARC – Intro to VHF Direction Finding
Simple TDOA
Similar to the one (mine has extra switching diodes – later drawing)
The 555 generates a squarewave at audio. One phase will forward bias D1 the other will forward bias D2. When one of the diodes is forward biased, the signal from the dipole is able to pass through to the common feeder.
RFC1 stops the 555 etc from loading the signal. C4 is a blocker to stop DC going into the front end.
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FDARC – Intro to VHF Direction Finding

24. FDARC – 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
When the antennas are pointing towards or away from the TX audio will be a minimum – meter in the middle
When antennas pointing LEFT of TX, signal will be positive – meter moves to right – towards the TX
When antennas pointing RIGHT of TX, signal will be negative – meter moves to left – towards the TX
In the above case the meter needle moves in the opposite direction to the antenna
If we are 180 deg out meter will move the SAME direction as the antenna.
This allows us to disambiguate the directions.
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FDARC – Intro to VHF Direction Finding

25. FDARC – Intro to VHF Direction Finding
TDOA 2 Schematic
Original site seems to have disappeared!
This one shows the antenna switching circuitry better.
U1 – 555 generates square wave at ~1kHz
Q1 buffers the audio from the RX
4066 forms a phase sensitive detector – Two gates are fed directly with the switching signal, the other two fed anti phase (Q2)
NOTE that you MUST calibrate the detector using a specific RX since there may be a 180 degree difference between the audio signals produced by different RXs – doesn’t matter for audio but will reverse the sense of the meter.
Two common mods – add an audio loop through socket to feed an earpeice.
Add a meter reversing switch to work with different RXs – best if this is internal to avoid accidental operation during a foxhunt.
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FDARC – Intro to VHF Direction Finding

26. FDARC – Intro to VHF Direction Finding
Diodes should have schottky characteristics.
If schottky diodes are not available a 1N4007 or 1N4005 should work ok.
Resistors needs to be low enough to ensure the diodes are forward biased adequately but not so low that they load the RF signal too much.
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FDARC – Intro to VHF Direction Finding

27. FDARC – Intro to VHF Direction Finding
Questions?
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FDARC – Intro to VHF Direction Finding