On Sun, 14 Aug 2011 00:59:40 +0200, Helmut Wabnig [email protected] ---
-.dotat wrote:

My goal is to find out what makes a direction finding antenna
different from a gain optimized antenna.

That's easy (having helped design 2 commercial direction finders). Low
side lobes and good front to back. Gain is well down the list of
requirements. Side lobes will drive any direction finding scheme
towards insane results by generating false peaks in the direction. The
ability to recognize these is a function of how much time you want to
put into the software. Reflections will also cause problems, but more
so with side lobes.

To prove a point, I once threw together a rotating antenna direction
finder using a low gain directional antenna (but with low side lobes,
and no rear lobe). Due to the low gain, the -3dB beam width was a
rather wide 60 degrees. I rotated the antenna slowly, and the
computah made lots of signal level measurements. The result was that
I could determine the direction within about +/-2degrees. Of course a
higher gain antenna would resolve the signal direction much better,
and could probably be rotated faster, but all would be much larger.
However, there's a problem. High gain antennas tend to have boresight
errors. Small asymmetries, such as mountings, gamma matches, and
baluns, will have an effect.

F/B ratio is all that matters for ARDF, for example.

F/B could be considered a side lobe at 180 degrees. Like other side
lobes, it needs to be minimized.

Gain is secondary and side-lobes should be absent.

Absent is difficult. Minimized might be a better goal. With a log
amp driving the signal level measuring device, three orders of
magnitude should be sufficient.

I must get that NEC working by all means.

Slow down. Such things take time.

What manner of direction finder are you building and what frequency?
Doppler? Rotating antenna? Phasing? Time difference of arrival?
Homer? Adcock? Ouija board?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558