Discussion in 'The Projects Forum' started by bradstormer, Sep 19, 2010.

Aug 6, 2010
65
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hi all
i am planning on building a circuit which can scan for VHF radio signals (30-88 MHz) and give the user information on the signal strength in a given direction.
i am not interested in demodulating the signal or finding any other information than its strength in a given direction.
my plan is to build two of these devices and use them in triangulating the position of a transmitter.
any information would be usefull, circuits, suggestions, anything! thanks for the help!!

2. ### beenthere Retired Moderator

Apr 20, 2004
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There was a lot of effort put into this in WWII - http://en.wikipedia.org/wiki/Radio_direction_finder

I have long since lost the source, but a pair of loops at right angles can give direction by X & Y deflection on a CRT, while a whip can give signal strength. The o'scope display is a line showing the relative bearing to the signal, and whose length corresponds to the signal strength. Somewhat expensive, as you need 3 receivers to make this work.

Aug 6, 2010
65
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i was thinking of using a directional antenna with a superhetrodyne reciever cct, once the signal is at the IF stage, feed it to a signal strength circuit. anyone have a signal strength circuit?

4. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
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There are entire clubs who make this a hobby, hiding transmitters then going on field hunts to find them.

The secret is often more in the antenna than the receiver circuit itself. That frequency is too low to use a cubicle quad array (preferred) or a Yagi so I'd suggest going through the ARRL handbook chapter on it.

5. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
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If you are going to try triangulation I would start off with a simple quad loop. At 30Mhz it will be about 2.5m square so it will be a big handfull. If you aim for a frequency in the middle of your band (50Mhz) It will be about 1.5m square. These sizes are if you want a 1/4wave antenna. As suggested by Marshall look up the ARRL handbook and you will get some gret ideas for loaded antennas. You could also try that other great handbook called Google. Lots of antenna designs out there for doing exactly what you want.