On Thu, 18 Aug 2011 17:49:21 -0700, (Dave Platt)
wrote:

Likely a Doppler direction-finder, possibly a Ramsey DDF-1 or one of
the others of that sort.

Agreed. There are various commerical implimentations. Basically, the
diameter of the "circle" of antennas controls the deviation of the
doppler signal heard on the receiver. The bigger the diameter, that
better the signal to noise ratio. Typical is about 1/2 wave diameter.
Smaller diameters will work, but require narrower digital filters
which slow down aquisition.

They aren't perfect... definitely subject to errors due to multipath,
and they works best when looking for a relatively clean, non-modulated
carrier.

Plenty more wrong with doppler DF. They don't like horizontally
polarized signals. Reflections cause false readings. Forward
alignment (0 degree heading) varies somewhat with frequency. Switching
PIN diodes in the antenna cause interpolation errors every 90 degrees.
Multiple transmitters on the same frequency result in erronious
readings. Antenna tilt causes big errors. Reflections off building
and in particular, the ground ahead of the antenna, causes errors.
There are fixes and work around for most of these, but I haven't seen
any on ham quality dopplers. Some of my ancient comments on the
topic:
http://www.qsl.net/n9zia/doppler_notes1.txt
http://www.qsl.net/n9zia/doppler_notes2.txt

Fun, though... I've used mine on several occasions to locate
sources of QRM or jamming.

Ummm.... How did you get it to produce a usable bearing if there were
two transmitters on the air at the same time? I can see if it you
waited until the jammer was the only carrier, but few jammers allow
that to happen. Mostly we use doppler for the traditional stuck
transmitter.

Mini-rant: Why do almost ALL commerical radios have tx timeout
timers, while almost ALL ham radios don't have this feature?

I've seen similar 4-vertical arrays on the roofs of some police cars
in my area. I infer that they're used to track down cars which have
had their Lojack locators triggered, after a car theft.

That's LoJack. I rode around in test vehicle with one trying to make
sense of the indication. In an urban jungle, it's very difficult to
get a decent bearing due to all the building reflections. I suggested
we try an elevated freeway, and the bearings improved. Works much
better in the suburbs.

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