# Introduction to VHF Direction Finding - PowerPoint PPT Presentation

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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
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
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
• 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?