Transmit Static Increases With Airspeed?

Strange problem with my radio that I can't seem to get my head around:

GTN 650 that is perfectly quiet in Tx/Rx on the ground with battery power only. Engine running, alternator on does introduce a little whine with increasing electrical load but not objectionable. In flight, at any power setting with airspeed below about 90 KIAS the radio is fine. However, as soon as I get above this speed, the Tx picks up an electrical sounding crackle that increases with intensity up to about 135 and above, where the radio is all but useless as a transmitter. Rx is fine in all cases.

I have changed antennas. I have inspected and wiggled the headphone jacks. I have shut down the alternator. I have covered the mic boom with my hand. I have switched off the intercom - all with zero change in behavior.

However, I can fly along at pattern speed with good radio performance, but as soon as I drop the nose (same power and RPM) and pick up some speed, the crackle appears again.

This is a fairly new behavior. The only thing I've messed with recently however is changing out one of the (non radio) switches in the Infinity grip.

Any ideas? I'm stumped.

Any chance it is wind noise???

I considered wind/background noise, but covering the mic with my hand does not change the behavior one bit. and the sound is a distinct electrical noise... Sounds like someone is arc welding in my ear.

I'm trying not to get "tunnel vision" here, but the only constant I've uncovered so far is IAS.

Loose/dirty connection at the BNC to antenna location?

Or poor grounds at the antenna mounting screws?

Grip

My money is on a cold or broken solder joint in the stick. None of the radio buttons were changed but possibly effected by the work.

Is this noise present in the sidetone you hear, and confirmed by others in other airplanes who hear it directly from the transmitter?

Two ideas.
I had one of the thin fiberglass washers on the headphone jack break at the panel and it was creating an open mic issue when the vibration allowed contact with the panel/jack base.
(I over tighten everything so was no doubt installer error and not product error).

Second, and don't laugh, but my headset had corrosion on the jack leads that plug in at the panel. You couldn't see it. It was driving everyone crazy that rode with me in any plane as it made static (arc welding) noise, even while non-transmitting which got worse with RPM increases. I took these in to a dealer and he checked them out, ran a couple of tests, checked the wires internally then took a pencil eraser and scrubbed the jacks pretty good, handed them back to me and said "you had corrosion on the jacks, try this and see how it works now". Problem solved.

Pat Garboden
Katy, TX
RV9A
N942PT

Dues paid

The noise does transmit. My buddy flying off my wing hears it when I key.

The antenna was swapped in and out with others laying around and the antenna ground is in good (not pristine) shape. BNC is clean.

Mic jacks were inspected for security and wiring, but did not look too hard at the insulting washer. Corrosion on jacks is possible, but unlikely - I'll check.

Stick grip was the only maintenance issue that makes sense, but why is the problem related to speed?

So how about a bad coax?

Speed increases, antenna starts to buzz a little, coax makes/breaks connection at high frequency?

Sound reasonable?

Tug on every wire in your radio harness, I'll bet one pulls right out of a connector somewhere.

precipitation static? Do you hear it in your headset?

The increase with speed is funny.
I would bet that the noise increase is engine related and it only shows with a load on the engine. ie higher speed = higher load. To test to see if it is truly related to speed, you could throttle back and dive. I would bet you get a decrease without the engine loaded.
So if this is the case then this could bring you back to ignition noise. May even kill the alt. during dive to make sure it is not alt. related.

Transmit

With it being transmitted it is most likely being picked up by the mic's. Are you sure you don't have the other headset hanging somewhere by a vent? This usually happens in the summertime when we start opening up the vents. The ambient noise increases, especially with speed.

Vic

More testing, still no resolution.

Only one headset in the airplane, vents closed or open - no change.

Antenna removed and grounds cleaned - no change

Coax wiggled and pulled when transmitting, can't duplicate static on ground-rock solid and quiet. Built new coax anyway (RG400 this time), no change.

Opened up boot cowl and shook all wires - no duplication of static.

Cleaned main ground buss - no change

Tugged on wire harness from stick grip and banged on the grip and switch - no duplication of static on ground.

Switched headset jacks in flight - same behavior on both.

I did note that the noise is not directly related to IAS on the last flight... It follows MP. High MP settings cause the noise and it appears to be fairly linear. Constant airspeed and RPM, I can key the mic and go from quiet at low MP to loud at high MP. Thinking it was it ion related, I did a mag check and the behavior is the same L, R or both.

Still need to try different headsets and also kill the Alt in flight.

Running out of things to try.

What kind of ignitions are you running?

The factory installed Slicks. Total time on the engine and accessories is 98 hours

Bad grounds can do funny things, I would double check the 'eng-airframe-battery' ground straps. Don't just look at them, take them off, clean them and re-install.

That's good advice and I will go through all grounds with a fine toothed comb at next opportunity.

The battery-to-airframe ground has recently been serviced, so that should be in great shape. The airframe-to-engine has not had anything other than a visual and security check. I do get pretty decent spin on the starter though, so it must be fairly sound.

The discussion on magnetos stirred a memory though. Sometimes when doing a EOR mag check I will pick up a faint buzz in the headsets correspondong with single mag ops. This is intermittent and hasn't happened for a while though. And this buzz is nowhere near the intensity and much higher frequency than the noise this thread is discussing.

Follow up and I think I found the most significant problem:

Started getting very intermittent mic switch operation on my last flight which led me to install a second, panel mounted switch. I tied into the terminal strip to do this and my original stick mounted mic switch problem went away. Conclusion: The terminal strip connections were loose. So why the increased static at speed?

...Because the terminal block is mounted to the lower fuselage skin and the speed causes drumming. Several hours of high speed cruise confirms that these demons have been tamed.

Thanks for the help everyone.