Help with CCS (chronic comm static) - Page 2

Rainier Lamers · #11 02-09-2012, 08:10 AM

Unfortunately they can.
In most cases you would hear a "modulated" signal - usually an aggressive sounding periodic noise. If a LCD display is the source, just changing the image (for example bring up a menu) can change the noise pattern.

Having said that we have had many cases over the years where interference is indistinguishable from static background noise - just at a higher level, sometimes much higher. We find the higher the digital performance (for lack of a better word) of a piece of equipment the more likely the noise starts sounding like static - simply due to the many individual noise sources, i.e. many digital signal edges at a multitude of different frequencies. The strongest spectral parts tend to be suppressed by design to meet DO-160 but leaving a lot of weaker signals that are all within the limits - but there are so many of them that the combined noise starts to sound "white".

We have seen a lot in this regard and are seeing even more since we started building radios - so all sorts of interesting things come out of the wood work.

The worst cases by far happen when conducted noise (say from an EFIS) is allowed to go right into the airframe turning the entire aircraft into a suprisingly efficient transmitting antenna. We had one such case on an Ultralight where we could pick up one of our older engine monitors with a hand-held radio from some 400 yards away ! Yes, the easy fix was to tell the owner NOT to use his metal airframe as conventient ground connection for his electrics !

Still, sometimes we get cases that I do not understand and once a fix is found don't understand the fix either...

Rainier
CEO MGL Avionics

Quote:

Originally Posted by rocketbob

EMI/RFI problems don't manifest themselves as static. Your problem is either a poor antenna connection or the receiver is out of alignment (poor SINAD - signal/noise and distortion). Connect a good handheld to the same connection at the radio to see if the problem persists.

rocketbob · #12 02-09-2012, 10:40 AM

Periodic noise is not static.

RV6-O540 · #13 02-09-2012, 04:30 PM

I was able to conduct a few of the suggested tests today on the comm
reception static. Heres some preliminary findings.All test done outdoors.

With engine running and all avionics on, squelch off:
#1 comm antenna coax removed then reconnected, better, slight smoothing of static when disconnected.
#2 comm antenna ditto.
Mag check 1 and 2 - no effect (will do full off tomorrow)
Pulled breakers on by one - no effect
EFIS power off - Slightly better

With engine off, all avionics on, handheld on comm #1 antenna, squelch off
Typical white noise, little less without engine static.
Pulled breakers. Electric turn coordinator shut off. much quieter.
EFIS screens off, Even quieter

Rebooting the screens make the most noise. When I push "accept" during the boot up it gets a little quieter, but not much. After its running more color on the screen makes more noise. One screen is quieter that both running.
Interestingly my Garmin MX-200 display dosent seem to affect the noise level.

During the trouble shooting I found the BNC connectors were very difficult to lock into position on the antennas. A backing plate inside the fuselage under the antennas held the connector up slightly. Also the male end of the antenna connection was coming through about a 1/2" hole so the female end on the coax couldnt go all the way down on the antenna. I took out the antenna and redrilled to 9/16". Now it goes on easily. Dont think it was a problem, but more testing tomorrow.

Thanks all for some great input.

Mike S · #14 02-09-2012, 05:09 PM

Here is my $.02 on this.

The statement that the problem only happens when the engine is running is a huge clue.

I suspect the noise is caused by either one of two possibilities, either being generated in one of the engine systems-----ignition, or alternator. You have already gotten good info on dealing with that.

Or, the vibration form the engine running is allowing another source to infiltrate into the radio system, which is where the loose coax or other wiring issue comes in.

Something I have done is the past is to use an orbital sander, and a rubber pad to generate vibrations---lay the rubber pad (to protect the paint) on the skin close to an antenna, and hit it with the sander-----no sandpaper just the pad on the sander.

Do this engine off obviously, and have a helper run the sander, while you listen for the problem.

You can test various parts of the plane this way, simulating the engine vibration.

Be creative

RV6-O540 · #15 02-09-2012, 08:09 PM

Thanks Mike..that's not a test I would have thought of. A great one too, since isolating a vibration problem with the engine running would be near impossible.
Everyone's had some great suggestions just when I was running out of ideas.
I'll be back at it tomorrow so I'll add this one to my test list.
Thanks a million!

Quote:

Originally Posted by Mike S

Here is my $.02 on this.

The statement that the problem only happens when the engine is running is a huge clue.

I suspect the noise is caused by either one of two possibilities, either being generated in one of the engine systems-----ignition, or alternator. You have already gotten good info on dealing with that.

Or, the vibration form the engine running is allowing another source to infiltrate into the radio system, which is where the loose coax or other wiring issue comes in.

Something I have done is the past is to use an orbital sander, and a rubber pad to generate vibrations---lay the rubber pad (to protect the paint) on the skin close to an antenna, and hit it with the sander-----no sandpaper just the pad on the sander.

Do this engine off obviously, and have a helper run the sander, while you listen for the problem.

You can test various parts of the plane this way, simulating the engine vibration.

Be creative

Rainier Lamers · #16 02-10-2012, 12:33 AM

Read what I wrote. I mentioned periodic AND static as different kinds of noise.

Rainier

Quote:

Originally Posted by rocketbob

Periodic noise is not static.

RV6-O540 · #17 02-10-2012, 07:07 PM

Was only able to work outside about 30 mins before sleet and rain forced us back in the hanger, but testing with an icom handheld connected to the antenna coax on the radio end got some results that raised more questions than answers for a non EE like me who's just starting to understand ground planes, RFI and the like.
I have two Comant antennas mounted about 4" from the outside edge of the fuselage and 5" behind the spar, 36" apart. Both coax check good continuity.

All tests with engine off
Power off, connected handheld to comm#1 antenna, squelch at 0 - normal hiss.
Power on, no radios, no EFIS, no EIS - considerable static with regular popping or tapping sound.
Power on, EFIS on, radios off. - Lots of static, about twice as much.

Checked wiring conections and wiggled wires for 30 mins. while listening to static. - No affect.
Heres were I though i was living a Car Talk brain teaser episode...
When I kneeled on the wing to wiggle the antenna coax the noise became intermittent. (no jokes about my weight

) Coax wiggling had no affect, but if I pushed down on the end of the wing the static would nearly stop!!??
Fuel tank senders? AHRS changing position? Wires in the wing grounding?
Wires not grounding? Global warming?

Nope.
It was the wing skin overlapping the fuselage making contact when I pushed down on the wing. There is about 1/32" gap between the skins that nearly silence the static when I push it closed with my fingers. But holding on the the end of the antenna does this also.

Questions:
1. I know this affects the ground plane, but how is this related to my static
RX problem with the engine running?
2. Why does this not seem to affect the TX of the radios?

Ive moved both antenna coax away from high energy wires, mag leads, alternator to power buss, but still are bundled with the EFIS wiring.
3. Can these bleed over into the radios via the coax? Should they be isolated also?

Sorry for the long post, but I feel like Im finally making some progress with everyone's help, so I'm giving all the details I can.
Thanks guys.

PaigeHoffart · #18 02-10-2012, 07:23 PM

I'm not sure about the RV-6 design, but the RV-8 has nutplates and screws that join the lower fuselage skin to inboard wing ribs.

Paige

rocketbob · #19 02-10-2012, 09:28 PM

Noise and static are two different things as I pointed out previously. Based on your observations and descriptions Bill I thought you have an antenna and/or receiver issue. I deal with this sort of thing all of the time.

Now, why your antenna is behaving differently with the overlapping skin is contacting is straightforward. The antenna is either capacitively or inductively coupled to the ground plane. If the capacitance changes (poor ground connnection, ground plane changing size in this case) then something called complex impedance changes. Complex impedance changes will have a lot of effects on antenna performance. I can think of a couple other problems the skins not being well connected can cause but I digress.

Beyond the antenna issues I think I have heard in the past that the skin overlap screws are structural but you might want to call Van's on that one.

Frank Smidler · #20 02-10-2012, 09:49 PM

Quote:

Originally Posted by PaigeHoffart

I'm not sure about the RV-6 design, but the RV-8 has nutplates and screws that join the lower fuselage skin to inboard wing ribs.
Paige

The RV-6 also calls for nutplates on the inboard lower wing rib and screws connecting the bottom fuselage skin to the wing. I don't know what structural affect not having them would cause but it sounds like it will solve you noise problem. Is this the kit started by Mike Wonder? If so I saw it at Lee Bottom in 2010.