Shielding plug wires.

O

Oldguyflied

Filing Flight Plan
Joined
Dec 24, 2016
Messages
4
Display Name
Display name:
Oldguyflier
I have a FlyBaby with a Franklin engine. No electrics. Have handheld VHF with external antenna. Lots of ignition noise. I understand that the plug wires should be shielded and the shielding should be grounded at ONE end only. Question: Which end, plug or mag? Any special method to attach the grounded end?
Thanks,
John Collins
 
I don't know who told you about "one end only" because those of us who fly aircraft with shielded ignition use the plug on one end for a ground and the threads on the mag fora ground on the other end. BTW, how do you propose to fabricate the wire?
 
Bonding one or both ends of a shielded cable is one of those topics right up there with religion and politics, and on some forums, just as likely to get thread locked as political posts here. My current conclusion is follow accepted practice in your field. Or in this case, ground both ends.
 
To stop that noise you need to change to a grounded plug, and a shielded harness.

Which Mags do you have? which size plugs does your Franklin take?
 
weirdjim said:
Wrong.

Jim
Click to expand...
Yeah ,, really. you need to shield the plug to get reliable noise reduction. If any part of the high tension lead is exposed you will hear it.
 
weirdjim said:
I don't know who told you about "one end only" because those of us who fly aircraft with shielded ignition use the plug on one end for a ground and the threads on the mag fora ground on the other end. BTW, how do you propose to fabricate the wire?
Click to expand...
Thanks for the reply. I got that recommendation from an Australian article some time ago. I had planned to thread the plug wire through a braided shield. The mags are Eismann and the plugs are Champion auto-type. I am a ham (KH6GC) and remember using that type of shield in a homebrew HF antenna.
 
Oldguyflied said:
Thanks for the reply. I got that recommendation from an Australian article some time ago. I had planned to thread the plug wire through a braided shield. The mags are Eismann and the plugs are Champion auto-type. I am a ham (KH6GC) and remember using that type of shield in a homebrew HF antenna.
Click to expand...

Better to use a shielded wire harness made for the magnetos; I believe putting a shield on an unshielded wire an attenuate the energy going to the plugs. As Tom said you need shielded plugs, too, though there are covers made for standard (unshielded) plugs.

You'll also need to shield the P-leads, too.
 
I learned that RF shielding is grounded at both ends. Like radio antennae and spark plugs. AF shielding is at one end, like headset audio and mic jack wiring. Grounded at the audio panel, ungrounded at the jacks. I have cured some headset noise by installing insulating washers at the jacks to unground them; the audio ground path is through the shielding, and if stray currents run through it the mic wiring picks it up and feeds it to the audio amp. Stray currents can be the result of a ground loop, where a current flowing through the airframe finds a little easier path through the shielding, such as strobes and flashers and alternator ground currents.
 
All you are doing with grounding one end (either mag OR plug) is coupling all that spark energy more efficiently to an external braided antenna that will reradiate that energy MUCH more efficiently into your radio. Go into the theory of quarter wave sections if you like.

Jim
 
weirdjim said:
All you are doing with grounding one end (either mag OR plug) is coupling all that spark energy more efficiently to an external braided antenna that will reradiate that energy MUCH more efficiently into your radio. Go into the theory of quarter wave sections if you like.

Jim
Click to expand...
Understand. Will shield the whole mess. Happy to be working on an experimental aircraft. Any recommendations foranyone local (San Diego) to fab the harness and plug shield system? Thanks!
 
I have a vague recollection that simply changing to shielded wire causes some issues with the rise time of the voltage at the plug, and that part of the solution is a resistor plug. Of course this implies that a modern shielded aircraft plug is also likely a resistor plug, so this may be part of the solution. I think there may also be differences in the wire itself to also help offset the issues that come with being shielded.
 
Oldguyflied said:
Understand. Will shield the whole mess. Happy to be working on an experimental aircraft. Any recommendations foranyone local (San Diego) to fab the harness and plug shield system? Thanks!
Click to expand...
Never mind. Did some research and see I need to run up to Aircraft Spruce! Thanks for the help.
 
Dana said:
Better to use a shielded wire harness made for the magnetos; I believe putting a shield on an unshielded wire an attenuate the energy going to the plugs. As Tom said you need shielded plugs, too, though there are covers made for standard (unshielded) plugs.

You'll also need to shield the P-leads, too.
Click to expand...

I'm anything but an expert on shielded ignition systems, but my gut would tell me to go that way. What I use in my line of work is copper braid, silver plated (the OP can google it). But I'd be ... itchy ... about running spark plug wire through that. I think you're right, using a purpose-built shielded spark plug wire would probably be the better choice.

The braid would need to be plated, though. Otherwise, you'll have a hot green mess in very short order as the copper braid corrodes. (edit: but if you're a ham, you know that.) :)

I would also strongly recommend that it be grounded at both ends. The ignition is putting out radio frequencies, and if you just ground it at one end, as another poster here pointed out, you're simply giving it a better antenna. :)
 
I'm no expert on ignitions, but it sounds like you have the Eisemann AM-4 mags which I have on my Champ. If you want to run a shielded harness, like most other modern planes, then you should have the LA-4 mags. I'm not sure how you're going to run shielding on your harness with the AM-4s.
And, don't forget to use shielded wire from the mag switch to your mags too. That reduced the ignition noise on mine.
 
Last edited:
champ driver said:
I'm no expert on ignitions, but it sounds like you have the Eisemann AM-4 mags which I have on my Champ. If you want to run a shielded harness, like most other modern planes, then you should have the LA-4 mags. I'm not sure how you're going to run shielding on your harness with the AM-4s.
And, don't forget to use shielded wire from the mag switch to your mags too. That reduced the ignition noise on mine.
Click to expand...
My Jodel had Case unshielded mags. Very noisy. I used shielded plugs and most of a shielded harness and grounded the shields at the mags (and the plugs, of course). The Cases' distributor caps weren't shielded and I still had a lot of noise. You have to shield everything. That little rotor spinning around in there, flashing sparks to the distributor cap terminals, sends out an awesome signal.

Remember that the very first radio transmitters just made big sparks between electrodes.
 
You must log in or register to reply here.
Back
Top