Duncan wrote:
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



The doppler shift method mentioned by Dave has worked very well for me
but it only gives you a chioce between 2 directions. If you know the
kind of area your turtles are likely to be then it should work
otherwise you can use iyt in conjunction with you yagi antenna to get a
accurate direction. The doppler shift method (unlike directional
antennas) is not affected by signal strength. This is because it works
by using two small antennas and determining which one the radio wave
hits first (this type of device usually generates a tone which
dissapears when the wave hits both antennas at the same time). This
tells you with a good degree of accuracy that the transmitter is in
front of or behind you.


Let's not forget that the antenna on the turtle will not polarized in
any particular fashion so antennas such as dopplers may not show nulls
and may show peaks incorrectly. I would take a yagi that I can rotate
to match the polarization to the source any day over TDOA or doppler
type antennas. Also I would always favor hunting the peak versus the
null especially if any reflections are present. If you are having
problems with a full sized yagi is it due to equipment limitations or to
physical constaints such as vegetation?

73,
Larry, W0QE