Introduction to VHF Direction Finding

1. Introduction to VHF Direction Finding Graham G0UUS

2. 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 FDARC – Intro to VHF Direction Finding

3. Bearing and Range FDARC – Intro to VHF Direction Finding

4. Locating TX using multiple Bearings 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 FDARC – Intro to VHF Direction Finding

6. Effect of bearing errors 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. FDARC – Intro to VHF Direction Finding

8. 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! FDARC – Intro to VHF Direction Finding

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

11. Dipole Polar Diagrams FDARC – Intro to VHF Direction Finding

12. 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 FDARC – Intro to VHF Direction Finding

13. 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) FDARC – Intro to VHF Direction Finding

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

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

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

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

18. Adcock DF Antenna FDARC – Intro to VHF Direction Finding

19. Dual Band Adcock DF Antenna80 – 520 MHz FDARC – Intro to VHF Direction Finding

20. 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 FDARC – Intro to VHF Direction Finding

21. 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 FDARC – Intro to VHF Direction Finding

22. 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) FDARC – Intro to VHF Direction Finding

23. Simple TDOA FDARC – Intro to VHF Direction Finding

24. 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 FDARC – Intro to VHF Direction Finding

25. TDOA 2 Schematic FDARC – Intro to VHF Direction Finding

26. FDARC – Intro to VHF Direction Finding

27. Questions? FDARC – Intro to VHF Direction Finding