Noisy Radio EMI Fix

[nlappos 413]

  My Avid MKIV had really noisy radio transmission when the engine was running, not readable by ground stations with lots of buzzing and hash. My sidetone was the same. Engine off, the transmit was ok. I had a Yaesu 450 hand held, as well as an Icom A24. I used an installed VHF antenna on the turtledeck, as well as the short rubber antennas with the same transmit. Reception was excellent, when squelch was set pretty high, but transmit was a mess.

 

I noticed the noise tracked engine power/rpm, and reasoned it was magneto/ignition electro-magnetic interference (EMI).  A few good web sites discussed shielding the plugs and coils, so I installed some copper/tin braided shielding on the spark plug wires (see the pic). I used the stuff that Aircraft Spruce sells, and found the widest stuff 5/8" was able to be stretched over the plug caps, and the narrower ignition shielding stuff  went over the plug wires nicely, when started with a pencil to open it up. I pulled the wires out of the Ducatti coils, which took some effort (and some courage), and then reinserted the wire so it was pierced by the central barb that is in the receptacle. A spot of crazy glue on reassembly helped retain the wire.

I also wrapped the coils themselves in copper mesh, and tie wrapped it into place. The spark plug wire shielding is in contact with the copper mesh, and all are grounded to the engine/airframe, as checked with my ohm-meter.

I'll post some more photos of the coils as wrapped in a day or so.

The radios are now dead quiet, and transmit nicely. I used my Yeasu to talk to a tower 30 NM away, through my big airframe antenna. I hadn't expected such success, frankly!

 

Copper mesh:  http://www.amazon.com/gp/product/B00KKUZ6PY?psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00

Shielding:  http://www.aircraftspruce.com/catalog/elpages/copperbraid.php?clickkey=378115

Shielding:  http://www.aircraftspruce.com/catalog/elpages/shieldbraid.php?clickkey=378115

 

 

[ChrisBolkan 130]

An alternate approach is to install metal Bosch resistor plug caps in lieu of shielding the wires. Guaranteed fix. Some will say the resistor plug caps reduce spark energy. This is true. However, the added capacitance of shielding the plug wires also reduces spark energy. I do not know which reduces spark energy more.

 

I removed all of the shielding the original builder of my MKIV installed and used resistor caps successfully for well over 1000 hours and never had a problem. I felt is was an overall cleaner solution that did not require shielding and the associated mess to properly ground all of the shields.

 

This is NOT meant to be critical of the shielding solution! Shielding is a valid solution. I am only mentioning another solution that also works very well.

 

I assume everyone knows to use NGK BR8ES SOLID TOP plugs. I had a plug wire fall off early in my 582 learning curve because I installed BR8ES plugs with aluminum screw on tops before I knew the difference.  

 

It also helps a lot to shield the ignition kill wires.

[ChrisBolkan 130]

Image of Bosch Resistor Cap

[TJay 1041]

I had all kinds of engine noise with my hand held so I hard wired one of these between the power source and the radio, crystal clear and quiet all I can hear is the music coming through the intercom. Life is good. http://www.ebay.com/itm/12V-NOISE-FILTER-HUM-KILLER-ENGINE-SUPRESSOR-POWER-WIRE-IN-CAR-STEREO-AUDIO-/260741326774?hash=item3cb564e7b6&vxp=mtr

[nlappos 413]

Good info all, thanks.

[1avidflyer 1983]

Can't comment on radio noise fixes, but didn't worry about plug wires falling off on inverted engines. Made a loop with safty wire and then tied it around the sparkplug where you can still put a socket on the spark plug to put it in or take it out. Ran a zip tie around the spark plug wire and then through the wire loop and tighten it up. If I used 8" ties, I would wrap them around the plug wire as many times as I could and if I wanted to pull the plugs, I would cut the zip tie just where it went into the holding end. One less wrap and the zip tie is fine to reuse then. Jim Chuk

[nlappos 413]

For the record, the Avid Flyer Mk IV that I shielded has a Rotax 582 blue head with Ducatti ignition, and the coils and wires were completely open prior. The EMI hash was a complex noise, not just a snapping but a white noise characteristic that was obviusly RPM sensitive. In addition to complete interference with VHF radio transmit, the noise was also bombing my intercom, forcing it to break squelch and provide continuous noise in cruise.

After the fix, all is nearly quiet as can be, and the Yaesu hand held  VHF has about 30 NM range.

The coils were wide open. In the Rotax factory shielding kit that doesnt seem to be available any more, the coils and wires are also put in a faraday cage. I seriously dubt that the noise was spark plug snap alone. Here are a few shots of the simple copper wrapping that I did to the coils. I made sure the copper was in intimate contact with the cil mounts and the plug wires that had been wrapped, so resistance between all the shielding was a fraction of an Ohm, and so too was it all to aircraft ground.