"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

Sorry I'm too tired to look up a better example, but
http://members.aol.com/BmgEngInc/Adcock.html
is still pretty good. It's essentially about like the
"phased verticals" the other fellow mentioned, in that
they both result in a "cardioid" pattern, i.e. a sharp
*notch* in the pattern, broadside to the array in this
case. That notch will be far sharper than any beamwidth
in gain that a Yagi can possibly have, and, a notch is
what you want, close-in, rather than overloading your
receiver. The one I used was built by another ham, and
IIRC the twin-lead between the two vertical elements was
some weird, 50 or 75 ohm twinlead, which is going to be
hard to find, and it was crossed, as hinted at in the lower
diagram in the URL above, but I think it probably just
connected to the coax to the rig at that point, rather than
any sort of "Combiner Box" like that shown. We used them
to DF a guy who was jamming a 2M repeater net every week.
We had cheap compasses, like you'd have on a boat,
fixed to the broomstick "boom" of the array, and it was
suprisingly accurate at to where all the lines intersected
over a period of time. Don't know what ever happened;
it was coming from inside a secure telco property, so we
couldn't do much "up close and personal" and FCC did
not show up as they were allegedly scheduled to do the
night I was involved.

Just surf around for "Adcock" and DF, antenna, whatever,
and I'm sure you'll find what you need. The Doppler antenna
the other fellow mentioned are spiffy, but I don't know how
inexpensively they can be done. Last time I saw one, it
was a dedicated, factory built unit... maybe these days
it's possible to "power" the logic with a laptop or something..
It'd be very simple to scale a 2m (144-148 MHz) antenna to
your band...

Good luck..

HTH,
73,
Sluggo

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

Sorry I'm too tired to look up a better example, but
http://members.aol.com/BmgEngInc/Adcock.html
is still pretty good. It's essentially about like the
"phased verticals" the other fellow mentioned, in that
they both result in a "cardioid" pattern, i.e. a sharp
*notch* in the pattern, broadside to the array in this
case.

Thanks, Sluggo, but this is partly an oops!

The BMG pages are a very good resource (as are many RDF sites).

OOPS! The Adcock is just like a loop -- two nulls off both broadside faces.
The version that looks similar (previously mentioned) where you have two
vertical dipoles spaced 1/4 wave and a 3/4 wave connecting cross-feed line
gives the cardioid -- one null.

"Sluggo" wrote in message
news:h3dmk1p0od57ig3q5qpm1qgq5gkdtdq3lm


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

On Wed, 12 Oct 2005 15:37:17 -0500, "Steve Nosko"
wrote:


OOPS! The Adcock is just like a loop -- two nulls off both broadside faces.
The version that looks similar (previously mentioned) where you have two
vertical dipoles spaced 1/4 wave and a 3/4 wave connecting cross-feed line
gives the cardioid -- one null.

"Sluggo" wrote in message
news:h3dmk1p0od57ig3q5qpm1qgq5gkdtdq3lm


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

Thanks for the correction; I'm only in my 40's but I've been
hamming a long time, and the years are starting to blur...
Now that I think back on my "real world" experience, the boom
on what I thought was an Adcock could well have been a
wavelength at 2M, tho, as I said, it's kinda fuzzy now... so
it may be that the elements were NOT fed at the center.
Oh well, either way, a good notch is going to beat the
best yagi.

Man, those must be some tasty turtles, to go to all this
trouble to track 'em down...

73, tnx agn..

Sluggo

"Sluggo" wrote in message
...
...

Thanks for the correction; I'm only in my 40's but I've been
hamming a long time, and the years are starting to blur......
Sluggo


Damn young whipper snappers... (;-)

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

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