Shielding Terminations?

I have three sets of shielded cables to deal with in the fuselage - Mag P Leads, Strobe Leads and Headset Leads.

In The AeroElectric Connection (great source BTW) Bob Nuckolls says that the termination configuration - one, both or neither end - depends on the application and to consult the relevant instructions.

The details for the Mags and Strobes are covered in the Van's plans or Wheelen Installation Instructions.

What is standard practice for terminating the shield on the Headset leads?

Jim Sharkey
RV6 - Wiring etc

It should be covered by your intercom or radio installation manual. For mine, I have 2 x two conductor cables for each set of jacks. The shield is terminated at the radio, one conductor is ground for the jack, and the other is the signal wire.

Make sure you have insulating washers under the jacks. If you don't you'll probably be hearing your strobes, and anything else that makes noise due to common mode ground currents.

There are other possibilities such as single conductor shielded with the shield connected to ground at the radio, and to the isolated jack. Some even use twisted pairs instead of shielded wire. I went with what Garmin put in their manual, and I think it's probably the most bullet proof solution.

YMMV,
Paige
RV-8A

ground at intercome or radio

I did a fair amount of research on this when planning my wiring and all the information including from PS engineering is that the jacks are "floating" in the structure ( insulated via the fiber washers) and the shield is connected at either the intercom or the radio depending on your configuration. This is explained in the wiring instructions for my choice of intercom (PS 3000). If your jacks have the anti rotation stub I think you have to cut this off for it to work properly.

I always terminate at one end only

I always terminate shields at one end only to maintain "ground" potential of the shield without the chance of tying it to two "ground" points that may actually be at a different potential making the "shield" an active conductor. If the manufacturer does not show a preference I ground the delicate shields at the most convenient end and considering the abuse in the area and if the manufacture's drawing shows a preference that's where I put it.

Bob Axsom

Pick you method

Bob Axsom said:

I always terminate shields at one end only to maintain "ground" potential of the shield without the chance of tying it to two "ground" points that may actually be at a different potential making the "shield" an active conductor. If the manufacturer does not show a preference I ground the delicate shields at the most convenient end and considering the abuse in the area and if the manufacture's drawing shows a preference that's where I put it.

Bob Axsom

Click to expand...

I agree with Bob and have terminated both source OR destination. My suggestion is to shoot for the same way throughout your project. On my project (unless the manual said otherwise) will be shield only at source. The point is, troubleshooting in a few years will be easier if you are consistent.

I heard a nice ROT on this - use at your own discretion:

To keep noise out, ground at one end; to keep noise in, ground anywhere you can (or both ends).

An example of the former is the headset leads; an example of the latter is strobe wiring.

Do it right

Audio is special. Use shielded wire and the shielding wire is active part of the audio circuit and is connected at both ends. It is an actual ground for the audio circuit. At one end is the source or output (radio, mic or headphone/speaker). At the other is ground to the intercom.

In general the plugs should (must, better do it or you will have noise?) be isolated from the airframe. The Mic and Phone plugs used three wire and two wire respectively. The Mic has hot, gnd and ptt, so a two conductor shield with insulation is ideal, ACS sells it. For the phone/speaker, a single conductor shielded with insulation works nicely. You can get a single three conductor with shield for both. I prefer to run two cables out to each plug, mic/phone.

With this concept the PLUGS ARE ISOLATED (insulated) from the airframe with plastic shoulder washers or mounted on a non conductive surface. In both cases, the shield is grounded at plug and at intercom/radio.

Some models of intercoms have separate grounds for each circuit input (mic, phone, aux input & Com radio). Some intercoms don't provide a PIN for a ground for each input/output, so you have to daisy chain or gang the grounds together at the intercom and ground them to the single ground provided, which could be the power ground.

THE KEYS ARE: CENTRAL GROUND AND SHIELDED WIRES.

Audio is measured in millivolts. Long wires make nice antennas. Now run that wire to an amplifier (intercom) and poof, radio receiver. If the wire is the right length, it can "resonate" at the frequency of local RFI or EMI. RFI and EMI are basically the same thing. One is produced by a radio, one is a radio signal not made by a radio, but the noise affect is as undesirable regardless. The physics are the same. Radio waves are non ionizing magnetic radiation. If you get radio waves in ultra higher freqs, it becomes microwave and x-rays, which is harmful. Any way we digress (again).

Shielding is the key to keeping your audio wires from becoming radio receiver antennas.

Non audio wires that can become transmitters (like your Com's coax) must be shielded at both ends.

The quick & dirty way is to not use separate grounds, just local airframe grounds where convenient, the shotgun approach. That works for many things but NOT AUDIO EVER. YOU ARE ASKING FOR NOISE. That is my opinion and from experience it seems about right. It is very attractive to save wiring, Using shielded wire and many ground connections at the intercom and each jack connection is a pain, but worth it. Shield braid itself is hard to handle and you have to solder on a short pig tail. With that said, if you go quick and dirty by all means twist the wire pairs together and route them away from any thing making RFI or EMI (which is impossible).

Noisy end...

jdeas said:

I agree with Bob and have terminated both source OR destination. My suggestion is to shoot for the same way throughout your project. On my project (unless the manual said otherwise) will be shield only at source. The point is, troubleshooting in a few years will be easier if you are consistent.

Click to expand...

The default is to usually ground the shield at the "noise making end" - in the case of the strobes, the power supply.
As stated above, this may also be called the "source"...

gil A

az_gila said:

The default is to usually ground the shield at the "noise making end" - in the case of the strobes, the power supply.
As stated above, this may also be called the "source"...

gil A

Click to expand...

I agree with Gill 100%.

At first I ran ground wires from my strobe power supply and strobe wire grounds to the common ground on the firewall and this put a lot of noise in my audio system. The grounds were then pulled back and grounded next to the strobe power supply and all is good.

BTW, the Sigtronics intercom does not require grounded wires and I have no issues with it.