P. Venkman wrote:

This is somewhat off topic; I apologize, but it seems like the people
that hang out here might actually be able to help me with my problem.

I fly radio control gliders. I recently purchased a new transmitter
(a Royal Evo 9 with synth module, for those that might care). I'm in
the US so this broadcasts on 72MHz.

I have problems with this transmitter at one particular flying site
that's right next to a military base. The xmitter is fine at other
locations, and all the gliders respond just fine to my 'old' xmitter
at this site. I've done a bunch of things and it really is just the
new xmitter at this particular flying site.

It seems like there must be an interference problem with some signal
being broadcast from the military base. I've tried to shield the
transmitter without much improvement. That makes me think the
offending signal may be coming in through the antenna.

Being relatively naive electronically, it seems like I could simply
insert a filter between the antenna and the rest of the transmitter
that passes through the 72 MHz signal but blocks everything else.
However, I'm smart enough to know I'm not that smart. That's why I'm
here.

Is it as simple as finding a filter that passes 72 MHz along and
splicing it in to the wire going to the antenna? Is that likely to
cause other problems (transmitting on the wrong frequency, overheating
the RF module due to impedance mismatch, sudden death)? Assuming I
can't find a filter that passes along precisely the frequency I want,
is it OK to put filters in series (like a high-pass plus a low-pass)?
And finally, am I just totally missing the boat here with this idea?
I'm open to other suggestions.

First, you can really mess things up by playing the filter game, so
don't do it unless you need to.

Second, it may be that the transmitter has a weaker signal, or that it
is a bit off frequency. Since your average military base has about a
gazillion different transmitters it's likely that there is some spur
being generated at 72MHz, or a valid signal at your RX's image
frequency. If your transmitter is a bit weak than such a signal from
the base would have an easy time getting into your RX.

If you can get the receiver out of the plane (or if you can get to the
antenna) do a range check with the RX antenna rolled up but with the TX
antenna extended. I'd do this at some _other_ flying site. Check the
range with the suspect TX and with a known good one. If it's
significantly lower with the suspect TX then you can do all the
filtering in the world and it won't help you -- but having the new TX
fixed (or selling it and getting another brand) may help a lot.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com