FARS - T-Hunt Information
DIRECTION FINDING (T HUNTING)
Direction finding in amateur radio is like most other facets of the
hobby,
you need to learn all you can about it and then practice. The great
part
about transmitter hunting is that the practice is fun, like going on a
treasure hunt or playing hide and seek. Direction finding also has its
practical aspects: one is finding jammers or other pests, and another
is
helping in search and rescue.
Anyone who has ever aimed a TV antenna or swung a beam has
done radio
direction finding. Most of the things you had to contend with for your
TV antenna are present in VHF and UHF transmitter hunting. The three
things
that affect radio signals the most are topography, obstructions and
reflections.
UHF and VHF signals don't like going through hills. They don't even
like
trees and other plants much. Buildings (particularly those made of
steel
and concrete) obstruct and reflect signals. When transmitter hunting,
always
consider your surroundings. If a signal gets weaker as you approach a
hill,
the transmitter is most likely on the far side or behind the hill. If
the
direction of a signal suddenly points to a large building, it is most
likely
a reflection.
Next I will discuss a series of systems and techniques useful
for T
hunting.
A simple technique for radio direction finding is "body fade."
Just
like trees the human body attenuates radio signals. If a signal goes
through
you to get to your hand held radio it will be weaker than if it does
not
go through your body first.
Using a beam usually gives you even better accuracy than body
fade.
However, mounting a beam to your car can be a challenge, both from wind
load and from the mechanics of rotating it. When using a beam you need
an attenuator. Most S meters on FM radios saturate at about 20 to 100
microvolts.
As you get closer to the hidden transmitter, you have to reduce the
signal
to your HT to get a change on the S meter.
The Adcock or interferometer is a very accurate direction
finding antenna
system. An interferometer can get a bearing to about one degree. It is
made by placing two antennas out of phase so that when both antennas
are
equal distance from the transmitter, the signal nulls. They can be made
with 2 simple dipoles or with 2 20 element beams.
A time of arrival direction finder works similar to the Adcock
antenna
system. It switches between the two antennas at an audio rate,
typically
between 300 to 800 Hz. The switching causes an effective phase
modulation
in your FM radio. If the antennas are equal distance from the
transmitter,
there will be no phase change when the antennas are switched and no
tone
will be heard. This system also has a sharp null.
The Doppler system gives you a direct read-out of the
direction to the
transmitter. It works by switching 4 antennas (3 or more can be used).
As the antennas are switched, the FM radio detects the phase change
from
the antennas and generates a tone in the radio at the switching rate.
The
phase of the generated tone is compared to the signal switching the
antennas
to give you the direction to the transmitter.
Besides radio equipment there some other things a well
equipped T hunter
brings along. You will need a good continuous map of the area. A book
of
maps like the Thomas guide will not work. Covering your map with
plastic
or clear shelf paper will allow you to mark bearing on it as you hunt.
Most T hunts start from high locations, bearings from high location are
usually more accurate than ones from low locations. Mark your initial
bearing
on the map and FOLLOW IT! As you get closer to the transmitter mark the
bearings you get along the way. Do not follow just one signal that does
not match all the other bearings you have obtained as you
progress.
If you have a HT you are not using for the hunt while you are
moving
put it on the dash board. When you hear the transmitter in the HT you
are
very close.
Now that you are close to the transmitter the hunt gets more
interesting,
although some hidden transmitters are easy to find, because they are on
a ham's car. Some hidden transmitters are actually "hidden." When you
are
very close to a hidden transmitter your HT will even pickup a strong
signal
without an antenna connected. To reduce the signal into your HT you can
wrap it in aluminum foil or slip it into a metal tube. You also could
use
a field strength meter. Be careful to find the correct signal, because
you might find the local FM station. To eliminate this problem you
could
use a frequency counter. When using a counter you may need an
attenuator
to reduce the signal to a just detectable level.
Standard T hunts are scored by the shortest distance traveled
from the
start to the hidden transmitter. This does not allow the use of
triangulation
because you are always going directly toward the transmitter.
Team hunts are a situation where triangulation can be very
effective.
With these hunts you can place teams on two or three high locations.
This
can pinpoint a transmitter to within about 2000 feet.
-- Rich Harrington, KN6FW
Copyright © 2001 by FARS
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