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

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

True, I haven't thought of everything, but I have done this:

The yagicad design, which others have used very successfully for foxhunting
antennas, should work o.k. at 3 or 4 feet above the ground, right? My
transmitter antennas are horizontal trailing insulated wire. I have a
fairly long cable so am able to hold it at arm's length from my body. It
does have a very good null and I use both front and null in my RDFing.

My radio might not be the best as far as sheilding.....it is plastic cased.
So I made a fiderglass holster for it with aluminum screen embedded in the
fiberglass resin all the way around...kind of like a Faraday cage. I don't
really know how this helps. It is not grounded, and I haven't really tested
it out all that much. At a fair range I can receive my transmitters by
connecting a rubber ducky antenna, and kill them when it is removed.
SOMETIMES using my body to block the rubber ducky gives me a good
null....but not good enough to RDF with.

Yes, the Box Turtle (several species in North America) is the one with the
hinge on it's plasteron ( the bottom "shell") which allows it to pull
entirely in its shell and close up. Box Turtle numbers are decreasing
thoughout thier ranges and will probably be rare over the next 50 to 100
years or so in many areas.



Thanks
Jim





"Bob Bob" wrote in message
...
Hi Jim

Hams will quite often get involved in hidden transmitter hunts during
field day games. You might be surprised at the expertise out there.

Some notes/thoughts for you;

- I'd be surprised that the yagi would exhibit 48dB F/B and if it did it
would only be for a very narrow frequency range. This however isnt a
huge problem. Even 20dB is more than ample for hidden TX finding.

- Ensure that you arent cross polarized with the turtle antenna. It is
possible for polarisation to change as the signal reflects off objects
so reflected signal maybe stronger than the direct. This only gets to be
a huge issue at cross polarised (say) plus or minus 10 degrees or so so
just make sure you are in the ballpark.

- Something that you modeled probably didnt allow for a ground so close
and your body so near. What I am trying to get at is you may not be
getting anything like the performance you expected because of these
local detuning effects.

- I'd suspect that the problem might actually be more in the receiver
than antenna. As the signal gets stronger/closer the radio might be
running out of signal reporting range,. ie everything is just over a
certain value so it is treated as being at maximum. It is quite common
under these conditions to switch in an amount of attenuation to reduce
the signal to a more reportable figure. This usaully goes in the antenna
feedline.

- In addition to the above your receiver may not be very well shielded
such that signal actually bypasses the antenna/coax path and thus gives
you less peaks and nulls to go off. You can check this by placing a
dummy load (usually 50 ohms) in the antenna cable socket and checking
for the nearby signal.

- The human body is excellent for using as a directional attenuator. If
you had a simple omni antenna attached and only a few inches in front of
your body you'll get maximum attenuation (lowest signal) when you are
facing away from the target turtle.

- If you antenna design isnt symetrical and there is an imbalance of
currents in the coax you may get to receive a signal directly to the
coax as well as the antenna. A few ferrite beads at the antenna
feedpoint around the coax, coiling the coax in a 4" dia 5 turn loop or
using some other form of balun (coaxial or otherwise might help.
Obviously you can test this theory by playing with the coax orientation
as well as the antennas.

- Considering also using the null or side of the antenna. ie turn it at
right angles to the turtle and where you get a very sharp loss of signal
thats the direction (or 180 degrees out) the TX is.

- Trying to narrow the beamwidth probably isnt a useful exercise with
the size limitations you have. You stated 90 degrees (3db down)
beamwidth. With (say) a 12 element yagi, 20ft long the beamwidth is
around 38 degrees. Better but probably not useful for you.

Hope this is useful to you.

Oh and a question. Is the box turtle the one one that has the hinged
flap in front? I found one on a busy road and I picked it up and moved
it along in what I hope was the direction it was going. Had never seen
one before! (New to the US)

Cheers Bob W5/VK2YQA

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

I have a homebrew analog attenuator that has male and female BNC connectors
that I can connect when necessary. It is variable 0 to 10 dB, and this
sometimes helps a lot, but not always, and not when I get within 40 feet or
so. I know this is not a problem when foxhunting (or is it?), but when
there is a lot of brush and brambles and holes and such, I sure would like
to reduce my hands and knees search area.

I worked with a researcher once that was able to locate his Bog Turtles
under water within 6" to a foot via "pointing" the antenna (150 mhz). When
the antenna pointed into the mud, he could stick in his hand and pull out
the turtle......now that's good RDFing, and I don't expect anything like
that, although the next time I'm taking measurements of his antenna to
duplicate it if I can.

I am not familiar with a "cavity filter". I'll do some Googling on that.
Can you fill me in on it's use?

Thanks
JIm



"Bruce" wrote in message
...
Bob Bob wrote in
:

Hi Jim

Hams will quite often get involved in hidden transmitter hunts during
field day games. You might be surprised at the expertise out there.

Some notes/thoughts for you;


- I'd suspect that the problem might actually be more in the receiver
than antenna. As the signal gets stronger/closer the radio might be
running out of signal reporting range,. ie everything is just over a
certain value so it is treated as being at maximum. It is quite common
under these conditions to switch in an amount of attenuation to reduce
the signal to a more reportable figure. This usaully goes in the
antenna feedline.

- In addition to the above your receiver may not be very well shielded
such that signal actually bypasses the antenna/coax path and thus
gives you less peaks and nulls to go off. You can check this by
placing a dummy load (usually 50 ohms) in the antenna cable socket and
checking for the nearby signal.

Ahhh, you beat me to this.

Now, if the receiver IS well shielded enough, if you can insert a
switchable attenuator in the coax, you can reduce the signal strength
significantly...and, FM receivers get very non-linear below about 20dB
quieting, they quiet faster than the actual input. In otherwords, you
could get 3-4 dB quieting change for 1dB of singal change. What this will
do is make you antenna pattern SEEM narrower.

What I used to do was to change to an RF field strengtth meter with a
small cavity filter tuned to the frequency I was hunting when I got
really close.


- The human body is excellent for using as a directional attenuator.
If you had a simple omni antenna attached and only a few inches in
front of your body you'll get maximum attenuation (lowest signal) when
you are facing away from the target turtle.

YES! But, you need to keep you body completely symetrical, stick an elbow
out and you will distort the pattern. Also, when the signal gets large
enough, remove the antenna.


-Bruce

Since Bruce hasnt responded

A cavity filter is a very narrow band filter usually of silvered copper
pipe construction. At 160MHz its going to be about 18" long and maybe 3"
dia. You might be able to get away with a helical filter instead. Thats
a copper/silverplated coil inside a silvered/copper box. I'd suggest
three coupled together if you were serious. They could then be in a
(say) 4" x 2" x 2" box.

In this case the use is as a tuned TRF receiver connected direct to a RF
measuring device. When you are close you simply watch the meter and walk
towards where it gets stronger and away from where it is weaker!

I'll comment on your other post in a moment.

Cheers Bob

Jim wrote:

I am not familiar with a "cavity filter". I'll do some Googling on that.
Can you fill me in on it's use?

Hi again Jim

I'd suspect you need an attenuator of maybe 60dB in 6dB steps! I cant
see the 10dB variable one as being useful. Of course at 50dB or more
coax leakage might be an issue!

The other researcher you mentioned most likely dropped to signal
strength only mode when they got close. Antenna directivity up close in
my view isnt a viable way of doing it. As a rough guide you get a 6dB
change in signal power everytime you double or halve the distance. This
means that if you maximum signal measured distance is maybe 6ft and it
must work out to 2000 yards then you have to allow for 60dB of
measurement range. This is further complicated by the turtle antenna
being at ground level, in fact in the mud and dirt. I dont know what
that loss represents but I'd add 30dB to be sure. Your whole system then
must have some method of measuring over that range of 90 odd dB and have
useful directional nulls etc doing so. This is where the big step
attenuator and antenna changing/removal help.

Should the antenna work okay? I guess the question is did you include
the ground in the modelling? Obviously you'd get some major sky
direction lobes/response from ground reflection but the worry is how it
affects overall directivity. My gut feel is that the F/B would be
markedly different and nowhere near the 48dB. At 1M above ground there
may be sufficient detuning of elements to widen the frontal lobe but I
doubt it would be a huge excursion. Do you find the null is much sharper
than the frontal lobe? The bottom line though is that in your
application it isnt as critical as someone (like amateurs) trying for
maximum performance. This means I should stop complaining!

Okay on the screening. Well as I mentioned the test is to plug a 50 ohm
load in the antenna socket and see if it responds at a distance of say
5ft. If it doesnt then dont worry about screening. If it does it depends
on how much and then whether things like coax leakage have to be
factored in.

I'll stop ranting/waffling on now..!

Cheers Bob

Jim wrote:
True, I haven't thought of everything, but I have done this:

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

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

Duncan, This is commonly called the TDOA to distinguish it from the 4-6
antenna Doppler systems.
73, K9DCI, Steve


"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.

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.