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Introduction to VHF Direction Finding

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Introduction to VHF Direction Finding Graham G0UUS FDARC Intro to VHF Direction Finding FDARC Intro to VHF Direction Finding FDARC Intro to VHF Direction ... – PowerPoint PPT presentation

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Title: 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 (Dont forget our hunt 14th July)
  • To locate a source of interference
  • Two basic ways
  • Bearing and Range
  • Two or more bearings

3
Bearing and Range
4
Locating TX using multiple Bearings
5
How do we measur the bearing
  • Simple directional antenna
  • Yagi or Dipole
  • Special DF system
  • Watson Watt - Adcock
  • Doppler
  • Pseudo Doppler
  • TDOA

6
Effect of bearing errors
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.

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!

9
Example Yagi Polar Diagram
10
A Simple Dipole DF antenna
  • Has a figureofeight 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.

11
Dipole Polar Diagrams
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

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)

14
Two crossed dipoles
15
Watson Watt DF
  • Consists of a directional antenna
  • A DF Receiver
  • A DF Bearing Processor
  • A DF Bearing Display

16
WW-AD Func Diag
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(!)

18
Adcock DF Antenna
19
Dual Band Adcock DF Antenna80 520 MHz
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 -gt unreliable

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

22
Whistling Dipoles DF
  • Uses a single pair of dipoles
  • Doesnt require a groundplane
  • Useable as handheld system
  • Works with unmodified 2m Handheld
  • Switches the two dipoles onto common feeder at
    audio frequency (1kHz)

23
Simple TDOA
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

25
TDOA 2 Schematic
26
(No Transcript)
27
Questions?
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