Sorry I'm too tired to look up a better example, but
http://members.aol.com/BmgEngInc/Adcock.html
is still pretty good. It's essentially about like the
"phased verticals" the other fellow mentioned, in that
they both result in a "cardioid" pattern, i.e. a sharp
*notch* in the pattern, broadside to the array in this
case. That notch will be far sharper than any beamwidth
in gain that a Yagi can possibly have, and, a notch is
what you want, close-in, rather than overloading your
receiver. The one I used was built by another ham, and
IIRC the twin-lead between the two vertical elements was
some weird, 50 or 75 ohm twinlead, which is going to be
hard to find, and it was crossed, as hinted at in the lower
diagram in the URL above, but I think it probably just
connected to the coax to the rig at that point, rather than
any sort of "Combiner Box" like that shown. We used them
to DF a guy who was jamming a 2M repeater net every week.
We had cheap compasses, like you'd have on a boat,
fixed to the broomstick "boom" of the array, and it was
suprisingly accurate at to where all the lines intersected
over a period of time. Don't know what ever happened;
it was coming from inside a secure telco property, so we
couldn't do much "up close and personal" and FCC did
not show up as they were allegedly scheduled to do the
night I was involved.

Just surf around for "Adcock" and DF, antenna, whatever,
and I'm sure you'll find what you need. The Doppler antenna
the other fellow mentioned are spiffy, but I don't know how
inexpensively they can be done. Last time I saw one, it
was a dedicated, factory built unit... maybe these days
it's possible to "power" the logic with a laptop or something..
It'd be very simple to scale a 2m (144-148 MHz) antenna to
your band...

Good luck..

HTH,
73,
Sluggo

On Tue, 11 Oct 2005 01:03:34 -0400, "Jim" wrote:

This isn't strictly a Ham question, but I hope you all can help me anyway.

I am using small transmitters in the 166-167 mhz range in some Box Turtle
research I am doing. My RDF antenna is a 3 element Yagi designed via
Yagicad 4.1 which works pretty well. It has 48db front/back and about 90
degrees beamwidth in the H pattern.

This works well for initial locating......usually starting 1500 to 2000 feet
from my transmitter, but the closer I get, the more inaccurate it becomes.

What kind of antenna design could I switch to when I get to close range that
would have a narrower beam so I could pinpint my target? It would be nice
to have something smaller than my 35" x 21" yagi for close in work, but the
beam width is the primary concern.

Yagicad doesn't let me design solely on beam width (at least I haven't
figured out how) so is there another way to go on this??

Thanks
Jim