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

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?

"Bruce" wrote in message
...
...

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.... -Bruce

Two things here.

The "offset attenuator" gets you the equivalent to a shielded receiver
without the shielding and very simple construction.

There are designs on the web for "quieting meters" and I have even seen one
design where the receiver s-meter signal was summed with the quieting signal
resulting in one very large dynamic range signal strength indicator.
73, Steve, K,9.D;C'I