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I want to take part in a direction finding exercise on 145.300 MHz FM. I have a 25-25 centre zero micro amp meter, which I intend to use to detect the direction of the FM carrier via an antenna. I am reasonably familiar with electronics but not clever enough to design a circuit, please can any one help with a circuit for the above?

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  • \$\begingroup\$ Do you have a radio that can receive this frequency? \$\endgroup\$ – W5VO Sep 3 '12 at 13:15
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    \$\begingroup\$ 1. Use a highly directional antenna. 2. Rotate antenna to maximize received signal. 3. ???? 4. Profit. \$\endgroup\$ – The Photon Sep 3 '12 at 17:35
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You might want to look in the ARRL Handbook for a variety of practical approaches, especially since you are talking about a signal within the 2m ham band.

Generally, there are three classes of techniques in use:

1) Very crude - "body fade" basically you use your body to block the antenna of a handheld in one direction, and see what that does to the received signal strengh

2) Specially designed antennas, either with a peak or a null in their response pattern. You sometimes see simple yagi-type beam antennas used with wildlife trackers, etc, but iirc nulling designs can actually be made more narrow than peaking ones. Signal-strength based methods tend to be inconclusive when very close to the transmitter.

3) Electrically rotate the antenna at a high rate in order to introduce a doppler shift phase modulation which can be recovered with an FM (or more properly, phase modulation) receiver as an audio tone, the relative phase of which is the bearing to the signal. This can be done with a circle of 4 or more antennas and electric commutation between them in turn with PIN diode switches, or it can even be done with just a pair of antennas, in which case the doppler modulation will null when the antennas are equidistant from the source - something you can detect by just listening to the audio.

Some types of nulling antennas, and also the two-antenna doppler method, are unable to tell the difference between the bearing to the transmitter and its reciprocal. To avoid walking in the opposite of the needed direction, you can move perpendicular to the bearing to take a second reading, or perhaps see if a unidirectional signal strength method (perhaps even the body fade trick) will tell you which of the two possibilities is correct.

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An antenna type usally used for this purpose (radio direction finding) is a HB9CV.

Of course you also need a receiver for the particular frequency range (2m).

Here are some example projects:

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