"Bob" wrote in message
...
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.


While this approach looks good on paper, it often fails badly when the
desired frequency is so close in to the notch frequency. I just put a
quarter wave stub on our VNA and found that while it does diminish the 123
signal -33dB, it also attenuates the 120.6 signal by a whopping -22dB.
There is also an enormous VSWR upset -120:1 or so- this is perhaps not
important in your receive only application.
Each year we build hundreds of filters for this exact application-
AWOS/UNICOM separation. Typical insertion loss is under 1dB while the notch
is -40dB. The filter is about the size of a cigarette pack exclusive of the
N connectors.

W4OP