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

yes, this is a common problem. try checking out the arrl's radio direction
finding book for more specific techniques. but a couple simple ones are
1. a small loop, use the sharp nulls off the ends of the loop instead of the
big wide lobes.
2. a doppler system that electronically rotates antennas to give direction.
there is a very simple one on my web site at:
http://www.k1ttt.net/technote/doppler.html
3. a pair of phased verticals set up to give a cardioid pattern, again use
the sharp null not the fat lobe.

"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

Jim wrote:
"What kind of antenna design could I switch to when I get to close range
that would have a narrower beam so I could pinpoint my target?"

J. Roy Smith, W6YA described what he called "the simplest of DF loops,
with no ambiguity of direction" in CQ magazine`s 1963 "Antenna Roundup".
Roy also called ot the "Nobaloop".

Roy said it was designed for locating signals on the 10-meter band but
has been used with reasonable success on the lower frequency bands. Roy
used the loop on 80 transmitter hunts winning first place 15 times when
many of his competitors were using identical loops.

The loop is about a 1-meter length of 1/4-in. dia.copper tubing (gas
line) bent into a neat circle. The two ends are flattened for about half
an inch in a plane perpendicular to the radius. Number 28 holes are
drilled through the flattened tubing about 1/4-inch from the ends. The
loop is attached to a coax receptacle such as Amphenol 83-IR or military
type SO 239. One loop end is attached to the outer conductor by a 6-32
screw through one of the four holes. The other loop end is placed over
the receptacle`s center conductor lug and soldered well.

The small loop gives a figure-8 response in the plane of the loop. The
attached coax distorts this pattern, adding its vertical response, like
a sense antenna. The resulting pattern is a cardiod.

Nearby vertical objects may need detuning to avoid interference with the
vertical loop.

The maximum lobe is about 30-degrees wide.

The null is about 5-degrees wide, according to the author.

I have not built it but it seems simple, small, and easy to duplicate.

Best regards, Richard Harrison, KB5WZI

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



Jim,

Assuming your transmitters are NOT super well filtered I would build a
yagi for 3 times the frequency (498-501MHz) and listen on that frequency
when very close. Very few transmitters will be so clean as to not be
able to hear the 3rd harmonic. I DF and do running ARDF very often and
if you don't mind the small second antenna and you have a receiver that
can tune to the 3rd harmonic this will get you both the needed
attenuation for being close and the ability to pinpoint the source.

73,
Larry, W0QE

Believe me, the transmiters are not well filtered at all. In close range I
easily get 2nd, 3rd, 5th, etc. harmonics.

I'd try the 3rd harmonic antenna, but my receiver doesn't go beyond 173 mhz
or so, so I can't receive the 3rd harmonic.

I thought I'd be smart and have Yagicad design an antenna for the 2nd
harmonic and then use it with my current receiver. I suppose you can guess
the results from that failure...there is almost no null or beam width at all
(360 degrees I guess). Oh, well.

Thanks
Jim



"Larry Benko" wrote in message
...
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



Jim,

Assuming your transmitters are NOT super well filtered I would build a
yagi for 3 times the frequency (498-501MHz) and listen on that frequency
when very close. Very few transmitters will be so clean as to not be
able to hear the 3rd harmonic. I DF and do running ARDF very often and
if you don't mind the small second antenna and you have a receiver that
can tune to the 3rd harmonic this will get you both the needed
attenuation for being close and the ability to pinpoint the source.

73,
Larry, W0QE

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


You already got two good suggestions from others. Here's a third.
Build a MOXON square, less gain than yagi but if built right the MOXON
has a very distinct readward null, you exploit the null. Another
small advantage is it's smaller than a 2 element yagi for the same
band.

Allison

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

...
You already got two good suggestions from others. Here's a third.
Build a MOXON square, less gain than yagi but if built right the MOXON
has a very distinct readward null, you exploit the null. Another
small advantage is it's smaller than a 2 element yagi for the same
band.

Allison
Look up the "Tape Measure Yagi" It is designed to have a null off the back,
but you'll have to adjust the center freq.
73, Steve, K,9.D.C'I

On Wed, 12 Oct 2005 15:11:29 -0500, "Steve Nosko"
wrote:


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

...
You already got two good suggestions from others. Here's a third.
Build a MOXON square, less gain than yagi but if built right the MOXON
has a very distinct readward null, you exploit the null. Another
small advantage is it's smaller than a 2 element yagi for the same
band.

Allison
Look up the "Tape Measure Yagi" It is designed to have a null off the back,
but you'll have to adjust the center freq.
73, Steve, K,9.D.C'I

I've built both. The Tape measure yagi is not nearly as good off the
back and a bit larger. Moxon well done is an easy 35+ db null off
the back and sharper.



Allison

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.

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.