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.

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:

"Bob Bob" wrote in message
...
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!


When you see the numbers for the Offset attenuator you won't believe them,
though true.



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.

Good test.

73, Steve, K,9.D;C'I

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

... My
transmitter antennas are horizontal trailing insulated wire.

I think talk about polarization is relatively meaningless with this.



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.

With a good rear nul, your presence in the rear should give minimal
effect.


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,

Ground is sort of meaningless at this point. Look up the "Offset
Attenuator".

73, Steve, K,9.D;C'I