On Tue, 20 Sep 2005 02:00:13 GMT, "Dale Parfitt"
wrote:


"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.
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

The 1/4 wave stub works because of huge impedence upset it
introduces. However having a short (or nearly so) on the coax for
transmitting would be deadly for transmitters.

The problem is sections of coax have only moderate Q as resonators
and they are also resonant at harmonics.

I'd approach the problem by pulling the RF amp out of the radio and
loosely coupling to the mixer. Even a RX with 15uV sensitivity is
adequate for miles around an airport.

Allison