Some good suggestions.

One caution with "nulling" antennas. You say "small transmitters", so I
don't know how much of a factor the following is. Also, if you are using
the typical wildlife chirping transmitters, my #2 may not be completely
appropriate:
When close to the transmitter, RF can enter the receiver directly, via the
coax or just the receiver itself. When in the antenna null, this "Blow-by"
can exceed the level coming via the antenna and make the null antenna
useless. Loops are pretty simple, and the cardiod loop is pretty much an
optimum loop.
There are two relatively simple solutions. Any of the phase sensing
methods can solve this problem. (the Doppler is another solution, but it's
not simple).
You can also take two or more "nulling" readings at a comfortable distance
(terrain permitting) to triangulate the position. I even have a loop for 2M
and one for 440 (cute little 1" diameter).

One is an offset attenuator. Google on that & you'll see a simple but very
effective device except it won't narrow up your beam pattern.
A better, though a little more complex is what is commonly called the
"TDOA". This is a two antenna phase sensing system which, when well made,
gives an indication which allows you to walk right up to the transmitting
antennas until the target is actually be in between the two antennas. These
can find the keyed 5W. hand held in a shoulder-to-shoulder line-up of people
all holding one. The one I designed has an extra FET (not a bunch of ICs)
and a center reading meter which gives a left-right indication, nice but not
necessary. I don't remember if I have an electronic package on it though.

No "u's" in my address.

Good luck, & 73, Steve, K9DCI

"Jim" wrote in message
...
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