On Tue, 16 Aug 2011 09:51:53 -0700, Jim Lux
wrote:

On 8/16/2011 9:25 AM, Jeff Liebermann wrote:
Permit me to offer another idea. Instead of direction finding by the
peak signal, do it with a null.

Exactly.. you can get very sharp nulls pretty easily. For simple
antennas, there's usually an ambiguity, but you can resolve that by
other means.

With the cardioid pattern, there's no ambiguity. With a deep null,
and a non-reflective environment, it's easy to use. However,
reflections off buildings and hills are a problem. The antenna is
roughly an omnidirectional antenna, and reflections will appear to
fill in the deep null. Not much can be done about that except to move
and try a different location. My favored method is by using a map.
Find a position, determine a bearing, and draw a line of position on
the map. Then, find a different location, and do it again. Repeat as
often as practical generating as many LOP's as possible. Some of the
LOP's will be totally insane, but a majority should cross at one point
on the map. (This has been automated in software, but a map is good
enough).

In 1976, I helped design the AN/SRD-21 homer DF, which DOES have a 180
degree ambiguity.
http://802.11junk.com/jeffl/AN-SRD-21/
If the homing receiver works in the same manner as described in the
above manual (PIN antenna switch, synchronous demodulator, charge
pump, and zero center meter), resolving the ambiguity is easy. Just
rotate the homing antenna to the right. If the zero center meter
indicates that you should turn back to the left, then you have the
correct direction. If it indicates that you should continue to turn
to the right, then you have the wrong direction.

This has nothing to do with the topic at hand, but I thought it might
be of interest. Some of my ancient comments on Doppler direction
finding:
http://www.qsl.net/n9zia/doppler_notes1.txt
http://www.qsl.net/n9zia/doppler_notes2.txt

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