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.

'
The technical solutions, so far presented, are about as good as
you're going to get while still keeping the cost reasonable. However,
you ARE dealing with a consumer grade radio that was simply not designed
to any particular standard of interference rejection. And that you've
packaged this with a transmitter operating within the FM band also
offers the potential for overload artifacts on a broad spectrum of
frequencies. Sony Air 8, however, was known to experience this kind of
problem even off-airport. It would even pass 11 meter CB on a good day
if the signal was close, or large enough. As did some Sony FM broadcast
rigs of the period when an aircraft was reasonably nearby.

I lived 2 miles off the approach end of 30L/30R at Lambert, St Louis
for years, and I'd get aircraft to tower chatter on my Sony portable at
103.3, 102.5, even 93.7 everytime an aircraft went overhead. The front
end was simply not robust enough to withstand a close-in assault of any
magnitude. Nor was the chassis shielded well enough to prevent leakage
through the IF's. Your real option may to select a different receiver.
If you're doing this as a favor to the airport, you may get a
decommissioned rig culled from a wrecked aircraft, or one that's been
sitting on the shelf at one of the FBO's, donated to the cause. In that
case, robust interference rejection is assured for your purpose.

There is, however another, and potentially greater issue at play here.

Rebroadcast of non broadcast radio frequencies may be in violation of
several communications acts. Brush up on your regs, and be sure you're
in compliance. In today's climate, there is always some asshat who has
to make trouble for his jollies bringing the letter of the law down on
fairly innocuous activities that may actually benefit someone.
And with overlapping laws written the way they are, today, you may be in
compliance with one law, while in violation of another.

One of my colleagues, downstate, got a spanking for a similar type of
service involving Air/Water show comms for a small airport near his
home. Someone complained. Even though there was no interference was
measured, and all hardware was in compliance for license free
application. Unauthorized rebroadcast of comm channels was the issue.


So, make sure your ducks are in a row.