On Mon, 19 Sep 2005 18:07:25 -0400, "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),

What is intermediate frequency (IF) of the receiver ? Some offending
signal (or mixing product) could fall on the image frequency and get
through that way.

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

Is the interface present when this transmitter is turned off and the
signal is monitored through the speaker ? If the transmitter cables
are disconnected, does this change anything ?

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.

One thing to try is to rotate the antenna, so that the receiver
antenna is in opposite polarisation than the transmitter position, one
in vertical polarisation, the other horizontal polarisation. This will
attenuate the offending signal by 10-20 dB. The null might be quite
narrow, so you would have to slowly rotate your receiver antenna to
find it.

As soon as anyone keys on
123.00 and the automatic squelch is triggered, all you hear is the AWOS
recording.

If you defeat the squelch, do you constantly hear the AWOS
transmission ? Does it matter if the 88.1 MHz FM transmitter is on or
not ?

Any other VHF transmitters on site (VOR?) that might take part in the
mixing process ? Do you hear any other background noises than the AWOS
recording ?

Paul OH3LWR