Put a tee connector on the receive line coax a length of coax on it with
a short at the far end. It must be cut to be exactly 1/4 wavelength
(including connector spur.) This will appear t be an open circuit at
the resonant frequency, but will severely attenuate your nearby unwanted
signal.

Failing that, a series LC network across the receive line will do a
similar task. Try a 47pf trimmer cap in series with a coil made from 4
to 6 turns around a bic pen. (you can remove the pen after making the
coil... Tune your receiver to the unwanted signal and tune the
trimmer until you see the unwanted signal drop out. You may have to
experiment with the coil turns and spacing, but this design will also
help wipe out the unwanted station.

Fortunately your undesired signal is likely a constant-on transmission
so it's always there for you to play with. Secondly, you can make and
tinker with all this at home (provided you live within receiving range
of the airport AWOS) and bring the working product to the airport for
installation.


Good luck.


B.



Bob Chilcoat wrote:
Sorry for the crossposting, but I'm looking for more expertise than I have.
I usually hang out in the aviation groups, but am an engineer by training.
I have a radio problem:

I have just completed a special rig for our local airport, but it has a
problem. I took a rather old but serviceable Sony digital air band receiver
(Air 8), boxed it up in a waterproof enclosure and piped the audio out to an
FM microwatt transmitter. The idea of this is that visitors to our airport
who like to sit in the parking lot and watch the airplanes can listen on
their car radios on FM 88.1 to the radio traffic on our Common Traffic
Advisory Frequency (CTAF), which at our airport is 123.00 MHz (AM).
Unfortunately, while this setup worked perfectly at home well away from the
airport, we have an Automatic Weather Observation Station (AWOS)
transmitting continuously on 120.60 MHz only 50-60 feet from the place I
need to site the receiver. Even though this is only a 5 Watt transmitter,
it overloads the front end of the receiver. As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

I've tried quick fix by attenuating the input signal by trimming
(shortening) the antenna, but this doesn't really help. This was supposed
to be a quick and dirty (gratis) job for the airport, and I've already spent
more time and money on it than I wanted to. Any suggestions as to how I
might fix this problem? Cheaply? Obviously a better receiver would work
(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 can't get it any farther away for several reasons. Anyone have a 120 MHz
preselector they can give me? Any really steep (and cheap) 120.6 notch
filter designs?

Thanks for any help you can offer.