Help with CCS (chronic comm static)

Even though the forums have given me tons of ideas for trouble shooting and solutions over the last few years, I've been working on this comm problem off and on for over a year with no improvement.
No problem with transmit.. ATC and other planes hear me loud and clear 50 miles+. My problem is any signal received has background static that blocks weaker incoming ATC, planes, atis, etc. (Chicago center not amused on my way to OSH) Closer, stronger signals are readable but static is still in the background. I have perfectly clear reception with engine off.

Heres my main comm/electrical equipment list:
QT Halo Headsets
Garmin GMA 327 audio panel
Garmin Sl-30
Garmin GNS 480
XM weather receiver
B&C 60 amp and 20 amp alternators and regulators
Dual Comant antennas belly mounted, grounded through stainless bolts per instructions.

What I've tried:
-All new shielded mag wiring grounded only at mags, Mags have 140 hrs since OH.
-Rerouted RG 58 coax away from high energy wires.
-Have two ground cables and braided ground straps from engine to firewall.
-Added ground cable from battery to sub-panel and grounded everything to a
central tab forest.
- Disconnected aux audio wire(pin 24 on B connector) from GRT EFIS to audio panel.
-Installed Miracle Antenna Smoothie filter on just SL-30
- Readjusted SL-30 internal side tone,squelch, etc

Tests and results:
- Ground and in flight mag checks - no change
- Central grounding to Battery cable - no change
- Smoothie filter- voltage drop shut off SL-30 on transmit. uninstalled
- SL-30 adjustment- no change in RX static but TX and normal
operation is very quite now. No feed back, cant hear strobes, 4 75w landing lights, or pitot heat in system when operated.
- Audio panel shut off - no change, sl-30 and 480 both get RX static
- Alternators off - no change
-
Conclusions and Questions
- Obviously engine related
- I dont believe it's alternator whine, not high pitched following RPMs
- Noise is more of a course, white noise static, somewhat less a low rpms
- Dosent seem to be isolated to one mag with a mag check
- I suspect ignition wires and or plugs, but not sure how its getting in to system. Champion wires and plugs. Runs great, good mag checks,
- I notice previous owner has longer plug wires folded in bundles and zip tied
near mags. Can this cause RF problems?
- Any ideas on isolating radio interference from the ignition?
- What resistance readings should I see when testing plugs and wires?

I would appreciate any insight on this problem as Ive tried just about everything I know and can find on the forums. Thanks ahead of time.

Bill,

Sounds like you've started looking at some of the common culprits. Here's some other ideas to look at as well.

Have you tried turning your alternator off and running on battery only? If the noise goes away, consider adding an alternator filter.

What type of regulator are you using? Even though you have a B&C alternator, the regulator may be inducing some noise. Check out the B&C external regulator or the Plane Power external regulator. These are both solid state regulators and minimize/prevent the noise made by older style mechanical regulators.

You said that you've already tried the mags, but you may be getting some bleed-over. A magneto filter can help clean up that noise.

I installed a single capacitor that attaches to the positive lead of the battery and to the ground tabs on the firewall. This was done prior to first engine start, and I've never run without it, so I can't say definitively, but I don't have any noise issues on my radio.



Obviously you're not the first person to ever suffer from static. Let us know what you find.

Where are you antennas and what type are they? Have you checked the ground for the antennas?

Put a high value capacitor on the wire to the radio. 4700uf 16+ volt. Thats an idea. If that makes some difference then also put another say 1000uf and see what happens.

Jim
Austec Avionics

To clarify that is on the + (+ on capacitor) to the power to the radio. - on the cap to a common ground - (-neg plane chassis or ground wire on Comm).

There are only two ways interference (which can sound like static) can get into a radio:
a) Antenna (i.e. via the receiver)
b) Via one or more attached wires - PTT, intercom/headset, power supply

Switch on radio, open squelch.
Remove antenna.
You should hear a drop in background noise. Not a huge amount but noticeable. If you get almost no change, chances are that either the antenna cable is shorted at the radio side or the core of the antenna cable is not connected. If this is deemed OK and you are also absolutely sure your antenna connection is to spec, you can consider a faulty receiver. This does happen - a avionics shop should be able to test your receivers sensitivity fairly easily.
If you have done the above and the difference is a lot, start turning off all electrical users one by one until you only have the radio left. If any of these is the source you will hear the result of switching off the offending party immediately. The usual culprits are anything digital, worse if a LCD display is used. If you can locate the source this way you can then target a solution without guessing.
You can do this with and without antenna connected and that will tell you how the noise is getting into your radio.

Bottom line: If you switch everything but the radio off and still can't get good RX and your antenna is good (yes, I know - check again) - send the radio to the shop. Perhaps it is deaf.

Rainier
CEO MGL Avionics

Have you tried a different headset?

Like Rainer I would tend to go looking at antenna cables, connections and antenna mounting (best way to mount is no gasket or paint under the antenna mounting base).

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.

A friend had a very similar issue a couple of years back. On a whim we disconnected the antenna coax at both ends and checked it for continuity. Turns out there was a bad crimp on the antenna end that was shorting the braid to the center conductor. It's worth checking if you haven't done so yet.

Thanks guys for all the suggestions! Ive spent hours researching the forums for similar comm issues, but its great to get all this trouble shooting input on my specific problem.
I'll be working through your suggestion this week and I'll keep you posted. Thanks again!

Since this problem only occurs while the engine is running there's one more thing you could try to isolate the problem. I believe you've tried doing a mag check with no difference, but have you tried turning both mags off? If the noise goes away immediately when the ignition is turned off then perhaps you have some kind of corona or other HV leakage problem that's causing radio static. Also, make sure you're using resistor plugs, though now that I think about it is it even possible to get non-resistor aviation plugs?

Check the ignition wiring for signs of chafing and burning, which could indicate high voltage leakage. Even a little leakage can cause a lot of radio noise. I had a huge problem in my RV with radio noise - I had an electronic ignition using auto plugs - until I finally figured out the spark plug boots were dirty. After cleaning them and greasing them with dialectric grease the problem went away.

The P-leads for the magnetos are a known source of ignition noise. There are capacitors that can be added externally to filter this noise out.

Every time I have heard ignition noise it sounds like a rough buzzing sound that, depending on how strong it is, overrides weaker radio transmissions and if powerful enough can break the squelch of the radio. On my RV it started out as only being heard on very distant/weak signals and over time got worse, until it started to be heard over even strong transmissions.

Good luck on your search! My money is on your ignition since you seem to have eliminated the other usual causes of radio noise from a running engine.

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

rocketbob said:

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.

Click to expand...

Periodic noise is not static.

UPDATE

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.

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

Creative is right!

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!

Mike S said:

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

Click to expand...

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

Rainier

rocketbob said:

Periodic noise is not static.

Click to expand...

Todays Comm Test Results, Pics and Questions

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.

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

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.

Yes

PaigeHoffart said:

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

Click to expand...

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.

Thanks for the replies guys. You're right about the nutplate attachments. Pulled out the building instructions and plans and found it. 2 1/2" on center. I'll take care of that today and see how the comm issues go from there.

Frank - Yep, Mike had it pretty close to flying when it was flooded in Bloomfield.
Took me over a year to clean, partially rewire and replace everything. Flown it over 120 hrs. My goal is to have the leg fairings, wheel pants, this comm issue and painting done before Oshkosh this year. Love working on it. (well.. all except comm issues)

Quick question...are you using a gasket between the antenna and the airframe? Bob makes some excellent points.

No comment on the nutplates other than it's gonna be a bear to put them in with the wings permanently attached.....did nobody mention this to you before? Just curious.

Cheers,
Stein

SteinAir said:

Quick question...are you using a gasket between the antenna and the airframe? Bob makes some excellent points.

No comment on the nutplates other than it's gonna be a bear to put them in with the wings permanently attached.....did nobody mention this to you before? Just curious.

Cheers,
Stein

Click to expand...

Yes, the cork gasket that came with both Comant antennas. I made sure the countersunk holes on the base were clean and bare and the inside area around the nut is clean. I do intend to change the standard AN screws for stainless.
Ive read several posts about grounding problems on antenna base mounts. From your experience, any pros or cons on mounting bolt grounding vs bare base mounting?

Im thinking since the plans call for nut plates on 2 1/2" centers CherryMax rivits will do the job. I just know I'm not taking the wings off at this late date!

RV6-O540 said:

Yes, the cork gasket that came with both Comant antennas. I made sure the countersunk holes on the base were clean and bare and the inside area around the nut is clean. I do intend to change the standard AN screws for stainless.
Ive read several posts about grounding problems on antenna base mounts. From your experience, any pros or cons on mounting bolt grounding vs bare base mounting?

Im thinking since the plans call for nut plates on 2 1/2" centers CherryMax rivits will do the job. I just know I'm not taking the wings off at this late date!

Click to expand...

I'd try getting rid of the cork gaskets as a first step....as for the nutplates, how do you plan on getting the cherry's pulled underneath the fuse skin?

Cheers,
Stein

For Sure... get rid of the gasket.

SteinAir said:

I'd try getting rid of the cork gaskets as a first step....as for the nutplates, how do you plan on getting the cherry's pulled underneath the fuse skin?

Cheers,
Stein

Click to expand...

My only option at this point was to use the cherrys through the fuselage into the inside wing rib instead of the nutplates and screws. Which is what I did today after a good cleaning between the skins. It closed the tiny gap nicely.

It made a huge difference in background noise in the system even inside my all steel shop. I'll take off the gaskets also and hopefully get a test with the engine running outside tomorrow. Dont know if Ive got it solved yet, but I'm closing in on it.