Weird radio "problem"

[Michael 182]

The other day I when I was handed off to a center frequency I got a
continuous "rx", like a stuck mic, on my Garmin 430. I asked about changing
freq, he said he did not have another available. I explained the stuck mic,
and he queried other pilots on the freq, who all reported 5x5. I started
playing with my stack, and discovered that if I turned radio 2 (an old
King - probably original 1979 equipment) off 121.5, the interference went
away. The weird thing is that comm 2 was turned off on the audio panel
(Garmin 340). Never happened before or after - only on this center
frequency. What might have caused this?

Michael

[[email protected]]

On 20-Aug-2003, "Michael 182" > wrote:

> The other day I when I was handed off to a center frequency I got a
> continuous "rx", like a stuck mic, on my Garmin 430. I asked about
> changing freq, he said he did not have another available. I explained the
> stuck
> mic, and he queried other pilots on the freq, who all reported 5x5. I
> started
> playing with my stack, and discovered that if I turned radio 2 (an old
> King - probably original 1979 equipment) off 121.5, the interference went
> away. The weird thing is that comm 2 was turned off on the audio panel
> (Garmin 340). Never happened before or after - only on this center
> frequency. What might have caused this?


If I understand correctly, what happened is that the 430 comm receiver
continuously "unsquelched" on a particular center frequency when the older
King comm transceiver was tuned to 121.5. The symptom went away when the
King radio was tuned to a different frequency.

It's hard to identify a precise cause, but here is what is most likely going
on in general: Almost all radio receivers generate internal radio
frequencies used in the reception process. These internal frequencies are
usually close to, but outside of, the band on which the receiver operates.
The design of the receiver, particularly in avionics, should take into
account the possibility of interference between these internal frequencies
and frequencies likely to be used in other nearby radio systems. Apparently
in your case, despite these design intentions, there is some interference.
Most likely it is in the form of mixing between the King's internal
frequency (when tuned to 121.5) and some other internal frequency in your
stack that results in a signal on whatever center frequency the 430 was
tuned to. The fact that you were able to hear the center's transmissions
fine suggests that this interfering signal was quite weak.

The interference occurs at radio frequencies, and thus has nothing to do
with the switch settings on your audio panel, which deals only with audio
signals.

It is quite possible that this sort of interference will occur with
different combinations of frequencies tuned by the two radios. Since you
know at least one combination where it happens, an avionics shop may be able
to identify and correct the precise cause. For example, the internal signal
from the King may be getting onto the power bus. On the other hand, unless
you are really fussy about perfect radio operation, and if the radios are
otherwise functioning properly, it's probably not something that I would
lose sleep over.

-Elliott Drucker

[Michael 182]

> wrote in message
...
>
> If I understand correctly, what happened ... <excellent answer snipped>


Thanks, great explanation. I won't bother to fix it, has only happened once
in 100's of hours with this equipment, and now I know how to avoid it if it
happens again.

Thanks,

Michael