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Old March 17th 06, 12:41 AM posted to rec.radio.amateur.antenna
K7ITM
 
Posts: n/a
Default compact antenna for short range direction finding

Assuming that the signal you're trying to track isn't moving around too
quickly, and is "on" for relatively long times, I'd suggest a simple
Yagi antenna connected to a receiver with a signal strength indicator.
Rotate the antenna till the signal peaks; the antenna points in the
direction of the strongest signal (usually the direction of the
transmitter).

You can find Yagi design info on the web. Do a Google search for it.
I'd suggest something modest, like 3 to 5 elements. The elements will
be about half a wavelength long, which at 870MHz is about 1/6 of a
meter or about seven inches. The whole Yagi might be a foot long. You
can easily make it using PVC pipe and pipe fittings, and elements made
from heavy copper wire. Be careful about having the feedline decoupled
from the antenna so that the directionality of the antenna is not
messed up by the feed line picking up signals.

Another poster suggested what I suppose is a Doppler scanning antenna,
an array of elements switched with PIN diodes, but the complications of
building such a system probably outweigh its advantages for what you
want to do, unless you're going to be doing a LOT of direction-finding.
Besides the PIN-switched elements, you need control/display
electronics and an FM receiver.

Cheers,
Tom