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1994-02-22
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A GENERAL OVERVIEW OF ADVANCED RADIO DIRECTION FINDING TECHNIQUES
This article deals with the advanced systems used in radio signal
locating and general methodologies involved in the use of this equipment.
The systems discussed will include doppler types, phase shift detection
devices, parasitic arrays, phased arrays, triangulation methods, circles of
signal strength, and how to apply the data collected.
We must first understand that radio waves produce a wavefront which
emanates from the signal source in a sequence of expanding concentric rings,
much as a stone produces ripples in a pond. We can use the information
contained in the wavefront to determine the point of origin of the wave;
ie the radio signal source.
PARASITIC ARRAYS are systems which have a driven element and a number of
parasites. These devices are what are commonly referred to as beams. They
function by focusing the radiated energy with respect to the driven element
in much the same way that glass lenses focus a light beam to project or
receive an image. A properly designed and tuned array will give you a
direction or bearing, usually forming a line along the axis of the array,
that when followed, will lead to the signal source.
PHASED ARRAYS are similar to the parasitic array except that all of the
elements in the array are driven; ie connected directly to the receiver. By
using delay lines on some of the elements the time that the various received
wavefronts arrive at a common point can be controlled. This allows the array
to be designed to add or subtract the signal components, dependant on the
phase angle received at a particular angle of attack with respect to the
wavefront. The array can be designed to produce a null or peak at a
particular angle of attack, with respect to the wavefront. If this angle is
known we will get a bearing that will lead us to the signal source.
PHASE SHIFTED ARRAYS are similar to phased arrays but usually only use
two antennas. The antennas are electronically alternated as the wavefront
arrives. The signals arriving at the antennas are compared for phase shift
angle. When the phase shift detected is none, the antennas are at an equal
distance from the signal source. This gives us a line perpendicular to the
line that the two elements are on. If we then rotate the array slightly this
will give us a direction to turn the array to restore the null phase shift
condition. If this direction is opposite to the direction that we are
offset from the original reading the source is ahead of the unit, if the
direction the unit gives is the same direction the source is behind the
unit. This information can be used to drive a meter that centers at the
zero phase condition and gives a direction to rotate that will correct the
meter to the center position. If the direction given causes the meter to go
farther off center the signal is behind the unit and the direction must be
rotated 180 degrees.
DOPPLER SYSTEMS use a series of antennas placed equidistant from a centre
point. The antennas are switched electronically in a continuously
incremental sequence. The wavefront data received is a close approximation
of the data a continuously rotating antenna would produce. The analysis of
this data will give us a doppler fluctuation that determines the direction
the wavefront travelled past the location of the unit. This will give us a
line back to the transmitter that we can follow. These devices usually
display direction with either a ring of LEDs or a digital bearing readout,
although some of the more sophisticated models display vectors on radar
like displays. or interface to computer systems or voice readouts.
TRIANGULATION is a method where several bearings are plotted on a map.
This plot will give us a location at which the transmitter can be found; ie
the lines will all cross through the transmitter. The information gathered
by any method, provided that it is accurate, can be used to triangulate a
signal. One of the advantages to triangulating is that, provided that the
signal source is stationary, the readings need not all have to be done at
the same time. An other advantage of triangulation is that we need only to
know the line to draw on the map and not the vector, as there is only one
possible solution that allows the plotted lines to cross. The other major
advantage of this system is that it can give you the location of the signal
in a very short time, especially if multiple stations are combining
collected data.
MULTIPLE CIRCLES OF SIGNAL STRENGTH is a system that uses a group of
stations of known calibration to draw a series of circles on a map. By using
signal strength and attenuation calculations the rings can be plotted and a
common intersection point found. The signal source would be at this
intersection. The circle plotter uses the attenuation data to determine the
circle diameter. A reading with more attenuation requires a plotted circle
of smaller diameter than one with no attenuation. Approximately 6db of
attenuation (on most radios this equates to one S-unit) is equivalent to
half the diameter, as this follows an inverse squared relationship. The
receiving stations should all be using omnidirectional antenna systems
preferably in a clear area free from reflections. The systems can all be
calibrated with a standard signal such as a handheld with rubber ducky
using one watt of RF output at a good high clear location somewhere in the
center of the sampling stations. This gives a baseline that can be used to
calculate all other circle plots for the mapping system.
Clive Apps VE3NSE
If any of the readers are local to Toronto we have a good local Direction
Finding group running. Foxhunts run every Sunday after 1:00 with start times
varying from 1:30 pm to after dark. We have been using 146.535 simplex as
the fox frequency for several years now. Anyone wishing to join one of these
hunts is welcome to. You can find out more about when and where the hunts
are by talking to one of the following people on
VE3XUU 145.23 (-) OFFSET:
VE3CHH Clive VE3GTU Andy VE3NSE Clive VE3RNC Doug VE3RQX Matt
these people are also excellent sources of technical material related to
direction finding / fox hunting
on 2M SSB:
VE3GUN Carl VE3UET Mike try 144.200 area
on VE3TWR 145.41 (-) OFFSET:
VE3BEG Lyman
in Oshawa on VE3OSH 147.12 (+) OFFSET
VE3HMG Eric
There is also a large group in Peterboro, and Hamilton / Niagara that
should be easy to find by asking around on the local repeaters.
Also anyone wishing to have their name added to a database of people who
are interested in foxhunting / direction finding send a brief packet note
to VE3NSE @ VA3BBS giving interests, locations, available equipment, phone
number (optional) etc. and I will compile a list of names for distribution
to all who asked to be put on the list allowing for idea / information
exchange.