I forgot about these. These are easier to make than the "cardioid loops".

K,9.D;C'I
"Dave" wrote in message
...
actually... for some reason i didn't put it on that web page, but you can
have a unidirectional null with that simple system. to do that you make
the
antennas 1/4 wavelength apart and make one feed line 1/4 wave longer than
the other. in this way you get a cardioid pattern since the only
direction
that won't have a phase shift is when the signal gets to the antenna with
the longer feedline first and the other one exactly 1/4 cycle later. just
be sure to take the coax velocity factor into account.

"Duncan" wrote in message
ups.com...

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.