Direction Finding Part 1: Introduction, Classic Methods

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Direction FindingPart 1 Introduction, Classic
Methods
INA Academy Workshop Spectrum Management
Series Workshop 3 "Measurements and Techniques"

• Mario Makraduli
• University "Ss Cyril and Methodius"
• Skopje, Republic of Macedonia

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RADIO DIRECTION FINDING APPLICATIONS

• Searching for interference sources
• Localization of non authorized transmitter
• Identification of transmitters, known and unknown
• Dealing with spread spectrum techniques,
  specially in wireless communications
• Military and security services

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HISTORICAL DEVELOPMENTTRADITIONAL CONCEPT (1)

• The DF technique is as old as electromagnetic
  waves are known
• The first experiments of Heinrich Hertz in1888 in
  decimetric waves give clear indication for
  antenna directivity
• The first patent for DF was proposed by Scheller
  in1906
• In 1907 Bellini and Tosi proposed DF principle
  based on combination of two crossed directional
  antennas with a rotatable coil.

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HISTORICAL DEVELOPMENTTRADITIONAL CONCEPT (2)

• In 1917 Adcock presented invention based on
  vertical antennas rather then on antenna loops,
  in this way horizontally polarized interfering
  components are not picked up
• In 1925/26 Sir Watson-Watt introduced electronic
  visual direction finders, specially suitable in
  shortwave range
• In 1941 DF operating on the Doppler principle was
  built, later in 1943 this systems operate at 3000
  MHz

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DEFINITION OF EMITTER DIRECTION
Reference direction
Transmitter
a
e
DF ANTENA
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REFERENCE DIRECTION
Transmitter
North
True radio bearing
Dead ahead
DF Vehicle
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PROPAGATION OF SPACE WAVES
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DF USING DIRECTIONAL ANTENNA
BEARING INDICATOR
BEARING INDICATOR
RECEIVER
RECEIVER
IF WITHOUT AGC
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DF USING SUM-DIFFERENCE METHOD
U S
UD
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Watson-Watt DF PRINCIPLE

• This DF method is based on two directional
  antennas pairs and one omnidirectional antenna
• The response of the two pairs are proportional to
  sine and cosine of the arrival signal.
• Watson-Watt DF consists of three phase-matched
  receivers and displays angle of arrival in terms
  of sine and cosine functions utilizing third
  omnidirectional channel to solve ambiguity.
• Accuracy 1 -20
• Very short response time 1ms typically

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MAIN FEATURES

• This system has drawbacks because requires
  considerable amount of alignment.
• Usually the picked signals are very weak and so
  called integration time was inserted, realized
  with low pass filters attached on the meter
  outputs.
• This technique is suitable up to 600 MHz, because
  at higher frequencies other techniques are find
  to be superior
• Elevation calculation is not possible

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WATSON-WATT DF WITH CROSSED LOOP ANTENNA
Brightness
DF converter
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CALCULATION PRINCIPLE
BENEFITS UNDELAYED BEARING INDICATION AND
MONOPULSE CAPABILITY OVER THE FULL AZIMUTH
RANGE
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ADCOCK ANTENNA WITH WATSON-WATT EVALUATION
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WATSON-WATT DF MODERN CONCEPT OF EVALUATION
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DOPPLER DF CONCEPT
Typically four to eight antennas are arranged in
a circular array and are RF combined in a way
that simulates rotation.
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DOPPLER DIRECTION FINDER

• If antenna system rotates, the incoming signal
  with frequency w0 is modulated with the rotating
  frequency wr.
• From the instantaneous amplitude frequency is
  derived by differentiation

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BEARING CALCULATION
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INTERFEROMETER BASED DF

• This concept is based on phase measurement
  between a subset of possible pairs of antennas
  with minimum of two coherent receiver channels
• Phase measurement for a specific pair can be used
  for arrival angle determination, based on
  knowledge of relative antenna distance
• When more then one pair is used elevation
  calculation is possible
• Accuracy is about 10
• Response time 10 ms

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3- ELEMENT INTERFEROMETER
l
l
a
a
e
a e
A e
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AZIMUTH AND ELEVATION CALCULATION
Azimuth and antenna elevation determination is
possible if spacing between antennas is not
greater then half a wavelength. If three phases
are measured than the following calculation is
performed