Now that I'm over half-way through Phase I in my 750/Jab 3300, I'm starting to troubleshoot minor glitches. I've got a MGL V6 comm with a Comant antenna, and it works great - ATC, etc., reads me "loud and clear" and nearby transmitters and ATC are received equally well. However, if I get a weak or distant transmission, there is a strong "ignition" noise in the background - it defintely varies with engine rpm. The noise was equal on either mag. (The p leads are shielded and grounded to the engine and the ground on the mag switch.)

I researched this on the web to the nth degree. My engine is of relatively recent production and has resistor spark plug leads. I decided I might get some benefit from resistor plugs, so I went with the NGK DR9EIX - they're resistor, but the iridium means they'll fire at lower voltage, so the additional spark plug wire resistance shouldn't present a problem.

They worked ... sort of! The right mag became COMPLETELY quiet! However, the left sounded about the same! While checking the distributors for secure connections, etc., I noticed the right mag lead was labelled "High Temperature Suppressive", while the left was labelled "Electra Cable ECA 7". Now I'm wondering if the left lead is NOT rf suppressed (or maybe Electra Cable makes crummy cables)??? I had some ferrite beads and added four on the left lead - this made it quieter, but not completely quiet.

Does anyone know if the mag leads can be changed out with a quality rf suppressive lead such as one from Accel, etc? How are they attached to the mag? Can the existing lead be removed without damaging the mag?

I currently (pun intended!) have a #4 ground cable from the starter to the battery, but I "think" I only used a #10 from the battery to airframe. I think I'll upgrade the grounds with a braided strap between the battery and airframe and also add a braided strap between the engine and airframe to ensure good grounds and have some redundancy. I also read in the engine manual about grounding the carb to the engine - apparently it can generate rf noise? - guess I'll had a ground strap there, too.

Any insights or other ideas?



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Most likely the the charging output wire from the engine to the battery (maybe you ran it to the power panel 1st?) is routed too close to the radio or antenna cable. If routed to the power panel 1st and then through a split MASTER switch I have seen the exact problem / radio issue. Try running the charging / output wire direct to the battery, if the problem goes away that was the cause.

John, I installed resistor plugs to deal with RF noise and I think it worked, but I also installed aluminum cooling plenums. Not sure if the cooling plenums are providing additional shielding. At minimum, I know that the sparkplugs were contributing noise to the system. Downside is that the factory manual says resistor plugs could cause starting issues when cold, that's not a problem now. I'm going to reinstall standard plugs, someday.

I had a problem with the alternator wires from the engine, added some additional protection, that helped. And I installed ferrite clamps on everything coming through the firewall, including throttle, choke, carb heat cables. Overall the system is quiet, but on occasion it does make some noise.

I'd pursue Bob's suggestion before changing to braided straps, and ferrite clamps. All those metal cables are just antennas passing noise along.

Thanks for the insight, Bob & Jake.  I thought charging system noise was usually irregular or semi-regular "popping"? This noise is a high-frequency buzz directly related to engine rpm, so I assumed it was output noise from the high tension wiring. The resistor plugs did completely eliminate the noise on one of the mags. I don't think there will be any starting issues with iridium plugs - there's numerous examples of people using them without any problems.

Bob's idea is simple enough to test, though. (I've got to dig out my circuit drawings, but I do think I wired the +12v output to the power panel first.) To be clear, instead run the +12v output from the voltage regulator (red wire) directly to the battery, then run a wire from the + terminal of the battery to the power panel?

I am going to change out the small battery ground wire to a braided strap ... it's just too small. The other won't be "changing out", I'm adding an additional direct engine-to-battery airframe ground. As I understand it, it's good practice in the rare event the starter ground goes bad - keeps high current from burning out other, inadequate ground paths. There's also a theory that even a #4 starter ground cable might be inadequate for high-frequency noise - apparently it can run along the surface of the conductor, in effect the conductor is inadequately sized to handle the rf. The braid has such a tremendous surface area it handles rf much better.

Apparently, grounding the carb is to prevent the choke and throttle cables from conducting rf through the firewall - the factory install manual recommends it, so it certainly seems worth a try. I might even re-use the #10 ground wire off the battery that I'm replacing with the braid!

Still would like to know how to change-out that mag lead - it's definitely not identical to the other "quiet" one that's clearly labelled "suppressive" and putting ferrite clamps on it definitely helped.

John, I had a Microair radio in my Rotax 912 CH601HDS.  It had a problem with alternator noise.  I know it was alternator noise because I could turn the alternator off and the noise would go away.  The radio performed perfectly except for the alternator noise for the first year.  Then I started getting complaints from ATC.  I tried everything to fix the noise issue.  Rerouting the charging wires, new antenna, ferrite filters, moving grounds etc. etc.  Nothing worked.  Finally I bought a Flightline 760 radio which has the same plug wiring and fits in the same panel hole.  Shizam all my problems have disappeared.  I'm using the same wiring harness, antenna and wire routing as with the Microair and no issues.

I also had a Microair Transponder that was sent back to the factory on 3 separate occasions.  They fixed it twice and it worked for awhile.  They said they fix it the third time but they didn't.  So I bought a new Trig transponder and installed it.  It works great.

I'm not a real Microair fan as you can tell.


Hi John. The MGL radios are incredidably sensitive to correct installation. We do plenty installs of them on various aircraft.

Some simple rules with the MGL install of V6 or V10 radios.

Headphone jacks must be completely isolated from body

Use 2 core screened wires for all the jacks

The jack earth is wired diredctly to the pin connector of the radio,and not to chassis at all. There is a pin for this on the radio.

Also, install an transorb on the power in at the radio as per MGL Transorb specs on their install manuals. It takes away the noise as well if any that may come thru from a noisy power supply as well as protecting agains voltage spikes. And then finally, run your wiring thru a ferrite bead as well.


Phillip - I used the MGL-supplied harness to wire my radio. It's made by Approach Fast Stack Systems and appears incredibly professionally constructed, clearly labelled, and a diagram and test report is included! In addition, I used shielded wiring throughout my installation.

I power the radio through a "Mini Power Panel" by Composite Design. I'll ask Wayne Lanza if I should add the transorb - the MPP already has a dedicated, filtered power supply for the radio, but I don't know if it absorbs voltage spikes like the transorb.

The thing that really irks me is that the NGK DR9EIX (iridium, resistor) plugs completely eliminated the ignition noise on the right mag. The spark plug leads are labelled resistive and appear high quality. But, as I pointed out, the left (noisy) mag appears to have a different HT lead to the distributor - at least it doesn't have the same appearance or labelling as the right (quiet) mag. Also, ferrite beads helped the left (noisy) lead, but not completely.

I've had several advise "change that lead" - but no one has told me HOW! I know it's 7mm HT lead and can get a suppressed replacement wire, the distributor end is easy, but how is the wire removed and re-inserted into the mag? Does it screw in? Think I read somewhere some of the wires are glued-in - do you then have to replace the entire mag? Regards, John


An interim report!

While I had the plane down for the 25 hr cyl. head bolt torque and oil change, I went ahead and grounded the carb to the engine. Flew another hour today - the radio is getting quieter and quieter! Still a little ignition noise on reception of really weak or distant signals, but that's about it. I'm now noticing that other planes calling ATC are much more clearly received.

Wayne at Composite Design (the support there is the BEST!) said he agreed it was a good idea to try the transorb ... they're only $6 from Spruce (I think I'll put one of the Xtreme Mini EFIS while I'm at it, too). While I'm installing the transorb, I'll put a ferrite bead on the MGL V6's power wires, too.

If it gets much better, I think I'll just leave that coil HT lead alone and leave the ferrites on it.

It's gettin' there!

Final report! (famous last words! LOL!)

Installed the transorb and put a ferrite clip on the power wires to the com radio. I also beefed up the grounds with mil-spec braided copper from battery to airframe and engine to airframe. I really didn't see much further improvement, but the radio is really quiet except for the most distant and weak transmissions such as distant aircraft talking to ATC. I'm a happy camper! The transorb is a good idea, however, since it provides a voltage clamp to protect the radio from a sudden spike above 33v.

So, my opinion would be that the iridium resistor plugs helped the most, and the ferrite clips on the "noisy" left mag lead helped second-most. Some worry about the resistor plugs + resistor spark plug leads causing spark plug firing problems, but the iridium plugs fire at lower voltages, so I've noticed no difference in ease of starting or engine operation. I left the gaps as they came from the box at about .030.

By the way, the joke is on me! I was "fiddling" with the carb the other day and way up on top, I saw a white wire. Turns out, the carb was already grounded to the engine by the factory! Don't know how I missed that ground wire, but now the carb has two! HA! Guess my ground lead can come off the next time I do some maintenance on the carb!

Jerry - My radio is a MGL, not a Microair and I have a Jab 3300, not a Rotax. Switching the alternator off has no effect in my case. No complaints with my transmissions (always "loud and clear") but this rf noise is only picked up with weak receptions - nearby transmitters are also received by me "loud and clear". Thanks anyway.

Sorry about that.  I guess I didn't read your post carefully.  I have a MGL EFIS and have had really good luck with it.  Good luck with your problem.  It sounds like you're making progress.

Hi John

Clip on ferrite cores as others here have mentioned are designed to cancel RF noise. Put them on the panel end  of every bunch of wires. Also, when I installed my Icom A210 radio they supplied ferrites for one end of the RG58 cable. Even do your trim indicator wires. Also I found that if you run your RG58 aerial near your trim wires, when you operate the PTT, the trim indicators would illuminate. I tried ferrites on both ends of the trim indicator loom but had the same problem. Took them off the trim motor end and left them on the panel end and the problem nearly dissapeared but my squelch was being affected because my headset wires ran beside it also. Moved the RG58 away and ran it by itself and all problems dissapeared.




Thanks for the info, Phill

I used RG400 for my com and wondered about putting a ferrite on it - I haven't seen much about using ferrites on antenna cables - didn't know if there was any negative effects or not. But, if Icom recommends them, sounds like it must be OK!



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