uuccio(at)gmail.com
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 Posted: Wed Apr 09, 2014 7:47 am Post subject: noise problem on radio |
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I'm back trying to look for a solution to my noise problem on the radio. The original thread was started in Feb so for those who missed it, the installation is a Rotax 912UL on a Kitfox with a Trio Avionics Propilot Autopilot, a GNS430 and a GMA240 audio panel. The mag leads are unshielded. I think is a NEW problem, as the first few hours of flying this installation did not seem to present this problem. I say I think because I'm not 100% sure here because I don't think I turned the a/p on during the first few flights (it was a year ago).
The noise appears in the form of an RX signal in the GNS430, and is correlated to two things:
- the a/p box being on (not necessarily the servos though)
- the engine RPM being above 3800 RPM
Specifically, when the a/p is off, I never hear any noise. And when the a/p is on and the RPM is below 3500 (e.g. in a descent) the noise can be squelched off by the audio panel (though the radio still shows an RX signal). But if I'm in a climb and have the a/p on then the only way I can get rid of the noise is by turning the volume of the audio panel right down (the volume of the GNS430 radio does not affect the loudness of the noise).
I have tried to turn up the squelch on the GNS430 but I'm not sure I'm doing it right. I press the volume button once and then turn it, but it doesn't appear that the pressing the button makes any difference. Am I doing something wrong there?
Below are the answers to some of Bob N's previous suggestions.
| Quote: | On 27 Feb 2014, at 15:15, "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com> wrote:
No, I haven't. Is it worth getting my hands on an SWR meter?
If your shield ground is loose at the radio, it can open a
pathway for conducting otherwise insignificant
signals into the receiver.
If you don't have ready access to one, hold off acquiring
it. But check the mechanical integrity of your coax
connections at the back of the radio. How well does the
comm transceiver work? Have you detected any shortfall
in performance?
The coax connections look ok mechanically. And the com transceiver appears to work fine, although I haven't tried it over longer distances.
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| Quote: |
http://www.trioavionics.com/Pro%20Pilot%20Manual%203.8.pdf
Okay . . . that's a busy little box. Refresh my
memory, is this noise a new thing or has it always
been present. Also, when receiving a weak signal
(tune in an ATIS and fly away from airport until
signal starts to get noisy . . . then turn a/p
on/off and judge how much effect the a/p noise
has on reception of weak signals).
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I'll try that next time I can get an ATIS (I am 60 miles from the nearest ATIS and not in theory allowed to fly more than 500 feet above the nearest obstacle in a 3 mile radius, so it's quite difficult to get reception).
| Quote: |
I sniffed around yesterday with the handheld and its regular antenna. What
I noticed was that as soon as I switch on the Avionics and the handheld
antenna is close to them, I hear a bunch of noise, but I guess that's
normal. When I switch on the AP and move the handheld antenna close to the
servo cables (that are shielded), I can hear a stronger kind of buzzing
noise. I really have to be almost touching those shielded cables though in
order to pick anything up. I didn't try putting the antenna close to the
radio's COM coax connector though.
You wont see any noise coming OUT of this junction,
but it is a potential point of ingress for noises
that tend to pile up behind the panel.
Do you have any pictures of how to build these? Is this
http://www.emcesd.com/tt120100.htm the right idea?
Yes! good find. I got a little ahead of myself
in sifting the simple ideas. Don't run off and build
one (or e-field probe) yet.
It's not clear to me yet as to operational significance
of the noise. DO-160 ALLOWS certain levels of noise
while putting potential victims ON NOTICE that such
noises may be present but normally insignificant.
My sense of the situation from your narrative
so far is that while the A/P is a noteworthy
contributor . . . it's not the sole potential
antagonist . . . we may discover that it is
within practical limits and you just need to
tighten the squelch on the receiver a tad.
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I tried (see above) but not sure I'm doing it right.
| Quote: |
I think I've related my experiences with the
symphony of noises that are often heard in
various systems while sitting on the ground
with engines off and wearing headphones . . .
noises that are completely insignificant
while in flight.
Let's size the task before we get out hammers-
n-saws.
Bob . . .
Sacha
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