*** Radio Direction Finding ***
to locate downed aircraft, ships lost at sea, pilots in the sea from being shot down or aircraft failure, and on the battle field, military personel using radios to communicate can be located and artillery fire may be called in on their position after their location has been detected.
We use direction finding to locate interfering radio signals or maybe one of our operators has his transmitter operating for some reason and he is unaware of it. This happens sometimes when an HT or Microphone gets
stuck in the car seat or maybe a couch at home and the ptt button is pressed accidentally. This causes
interferance and causes the repeater to stay in the transmit mode. This not only prevents someone from using
the repeater but somethimes causes the xmitter to time out or may cause damage due to the equipment overheating due to lack of a cool off period between transmissions.
Usually several operators HOME IN on this offending transmitter and we can get a pretty good idea where the transmitter causing the problem is located. In the case of a repeater, we must listen to the input frequency of
that repeater to find the offender.
opreate it on a frequency decided on before the hunt and all the other radios are tuned to this frequency
and the "hunters" search for this transmitter until one of them locates the FOX.
This is a fun game for us but it is, like most of the other activities that we do, practice for the day when a dangerous or emergency situation requires quick and accurate location of a transmitter.
First. of course, you need a radio that receives on the frequency that you are trying to locate. This can be a base radio, mobile radio or even a handheld radio. This radio must have some sort of signal strength indicator
whether it is a bar graph or a regulat signal strength meter.
Second, you have to have some sort of directional antenna. A 2 or more element yagi or quad may be used.
If you are at home you can use the biggest beam that you have to locate distant transmitters.
Third you must have some method of controlling the signal into the radio so that it does not give you a full
scale meter reading. This is some form of attenuator and may be one of the video attenuators for cable tv and
they are cheap and can be purchased at radio shack or other electronics outlets. The purpose of this device is to
let you turn the signal down so as to not overload the receiver as this would not let you detect an increase or
decrease in the signal as you orient your antenna to the signal. You attenuate the signal just enough to prevent a full scale reading on the meter or bar graph of the receiver.
I will have pictures of everything that is required for transmitter hunting and also show you the signals and
what happens as you turn the antenna in the direction of the transmitter.
a. Set your receiver to the frequency of the transmitter. Remember is the "target" is locking your repeater in
the transmit mode, you must "look" for the input frequency.
b. rotate your beam antenna until you get the highest reading on the bar graph or s-meter. If the reading is
full scale, you must turn the attenuator until the signal level comes down enough so it isn't full scale. Keep turning the ant. until you have the highest signal possible. Again if it goes to full scale turn the attenuator
again to bring the signal down enough so you can see it without full deflection of meter or bar graph.
c. Now continue to rotate the antenna until the signal starts to drop off. At this point, make a note of the
position of your rotor indicator. Then turn the antenna in the opposite direction until it reaches maximum
and again starts to drop. Mark the position of your rotor indicator again as you did the first time.
d. At this point you might want to repeat the above procedures. (c). Try to rotate the antenna slowly so that
you can detect the point where the signal starts to drop and make your reference marks of the rotor position again. After you have both drop off points marked, find the center point between them and this should be the azimuth reading to the offending transmitter.
e. As this only gives you one line on a map and the transmitter should be on that line but you can't tell the
range or distance to the "target". To find the distance, you need one and better two other stations following
the same procedure.
f. Mark your position and the position of the other trackers on a map of your area.
g. From each location, draw a line on the map in the direction of the center mark you found in the
procedures above. Where these lines cross that point on your map should be very close to the target
transmitter.
h. If one of the lines is not near the cross over point of the other lines, have that person follow the above procedures again as he/she did not get an accurate indication the first time. If they can't come to a common
point with the others, there may be something at his location that prevents an accurate reading in that
direction. In that case, you need to find someone else with a better location and similar equipment to attempt
to "lay another line for you.
These procedures will work and locate that transmitter for you. But remember that you must have the
attenuator in line so you can keep the signal indication below full scale on your bar/meter. It must be just
enough below full scale so you can see if it does show an increase.
So turn the rotor slowly, mark the first indication of a drop in signal level, do the same in the other
direction, then find the center point and you should have a pretty good indication of the location of the
"target" transmitter.
I will try to post some pictures of several meter and bar graph readings to show what I am talking about
but I am having some difficulty taking the pictures without the flash covering up the indicator.
This operation may be done with much simpler antennas and from a mobile. But if you can get an indication
from several points using the method I have described above, it will make location a lot easier.
