The receiver is already in a (steel) waterproof box. Tomorrow I will try
moving the receiver as far away as I can. If that doesn't fix the problem,
I'll try the stub antenna notch filter solution. I have a BNC Tee. Can
anyone point me to the 1/4 wave length formula for 120.6 MHz? Does 0.591
meters (23 5/16") sound right?

I'm pretty sure right now that the interference is coming in on the antenna.
It's not enough to trip the squelch, but as soon as someone keys on 123 and
trips the squelch, the AWOS is on the audio. OTOH, I guess that doesn't
prove anything...

I suppose the wiring to the 88.1 MHz transmitter or its wiring could be
picking up the 120.6, although all that wiring is shielded (one audio line
with shield terminated at only one end, and one 3v power line with its
shield as return). The transmitter itself is in an unshielded plastic box,
but that's mounted flat against the bottom of a 10" ground plane for the
transmitter antenna. The last possibility is the 6v power line going into
the receiver box. It's not shielded and starts at a wall wart
transformer/psu very near the AWOS xmitter. I could put a few turns through
a ferrite toroid just outside the box, I suppose. Couldn't hurt. Gotta
find a suitable torroid.

Allison, I'm not experienced enough at this stuff to visualize how to make a
bandpass section out of coax. I can follow the stub notch filter, but the
bandpass isn't there. Could you explain a bit more?

Thanks to you all for all the help.

--
Bob (Chief Pilot, White Knuckle Airways)


wrote in message
...
On Mon, 19 Sep 2005 18:07:25 -0400, "Bob Chilcoat"
wrote:


I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed

Bad thing to do and likely didn't reduce the signal as much as you'd
like. Try an attenuator, enough to kill the offending signal. Once
that is known the next step may be easier.

IF the attenuation needed is under 10DB and leving it in is acceptable
your done. Usually local signals are plenty strong enough.

If you need more than 10DB. Try a suckout stub tuned to the awos
at 120.6 as others have suggested. That should be enough as your
listening on 123.

(my Yaesu aviation handheld works perfectly at the same location), but I
have no other (free) receivers handy. I can move the receiver another 50
feet down the fence, which is my next option, but what if this doesn't
work?

I'd try that first, distance will always help the problem. Allowing
for a plastic case on the RX distance is more liklely helpful than
filtering. The problem is the case of the RX is plastic and there is
no shielding so any filter will be compromized by back door entry.
A metalic water proof box with filtered ins and outs for RX and fm TX
will be needed then.

It's possible to make a filter with a steep enough curve for that by
using a bandpass section for 123 and a notch section at 120.6 using
sections of UT141 (.141" copper jacket coax).

Allison
KB1GMX