shield ground run

I will be installing a PS Engineering 4000 audio panel and have a question about the shielded cable for the headphone jacks. The jacks require 3 conductor shielded cable. I have a tandem aircraft and would like to run the shield grounding wire from the forward end of the rear cables ( at the forward jack location) ( leaving the rear end (aft jacks) of the shield open ) and just jump it (the shield ground wire) over to the 3 conductor cable shield leaving the front jacks and going to the audio panel and there run the shield to a proper ground. Basically just jumping from one audio cable shield to the other audio shield and continuing to the audio panel and then to ground. Doing the same thing to the mic. side as well but not mixing them up.
If this will work as well as running them separately it would give me a cleaner install.
Thanks Ron

Are you running 6 conductor shielded from the intercom to the 1st pair of jacks? Looking at the wiring diagram, each input and output has its own pair of hi/lo terminals. The lo terminals are almost certainly tied together in the intercom, but the hi terminals will likely go to or be fed from different points. This is so the intercom can manage which voice is fed to the radios, and which audio out goes to which headphone.

No , I plan on using two runs of 3 conductor shielded cable. One for audio and one for mic.

But you need four; one for each mic and one for each headphone.

Minimizing ?ground loop? and other noise is a black art. You may do just fine no matter what you do, but best practice is to follow PS instructions: run all cables back to the audio panel, and ground the shields there and only there.

Agree that chasing ground loops can be tough. In this particular case, having that 'floating' shield daisy chained likely won't make any difference, but it implies paralleling the mics & headphone jacks. That will kill many of the functions the intercom is capable of.

In our experience shielding audio cables is pretty simple if you follow one golden rule: NEVER EVER use the shield as conductor. In other words - connect the shield to a ground point (any ground point anywhere) AT ONE END ONLY.
That usually means running the "audio ground" connection as an insulated wire inside the shield.
Other "oopsies":
If you mount the sockets on a surface that conducts - make sure you use isolation washers.
Keep the microphone and headphone audio lines separate (including their grounds). Like using two separate shielded cables or a cable type that has individual screens for every conductor contained. Running headphone and microphone cables in a single cable together (and not shielded form each other) can lead to really bad effects in particular with longer cable runs.

If your audio system is stereo and you have wired it as such - if you plug in a mono headset it will short one channel to ground. Not good. To fool proof this - wire resistors in series with the two audio channels. Unless your intercom has fairly high impedance outputs. I tend to use 10 ohm resistors. So if a channel is shorted the intercom output looks into a 10 ohm load which should not be an issue.
You can place resistors like that on each headphone socket so if you plug in a mono headset in one socket and a stereo in another (with a common intercom audio output for both) - the stereo headset will not be affected.

Rainier
CEO MGL Avionics

Rainier - Sorry if I am hijacking this thread, hope I can ask a follow up question on your first point.

I have been having transmit problems with a V6 radio and discovered the phone cables are shielded 2 wire cable using the shield to conduct the ground in violation of your direction. The mic cables are 3 wire shielded cable, no problem there.

The symptom is high levels of static (to the point the transmission is unintelligible) when transmitting on ~125MHz to 127MHz. The transmission is clean on other frequencies. Reception is clean on all frequencies.

Could the phone cables be causing this transmit problem?

thanks - lyle

Lyle RV8 said:

Rainier - Sorry if I am hijacking this thread, hope I can ask a follow up question on your first point.

I have been having transmit problems with a V6 radio and discovered the phone cables are shielded 2 wire cable using the shield to conduct the ground in violation of your direction. The mic cables are 3 wire shielded cable, no problem there.

The symptom is high levels of static (to the point the transmission is unintelligible) when transmitting on ~125MHz to 127MHz. The transmission is clean on other frequencies. Reception is clean on all frequencies.

Could the phone cables be causing this transmit problem?

thanks - lyle

Click to expand...

Can't help on a cause, but have a remediation idea. If you have access to the back of the intercom, you can swap the cables. Use the 3 cond for the phones (L, R, lo) and the 2 conductor for the mic (mic hi, mic lo). This will get the hi and lo signals inside the shield. If you wired and use PTT in the mic circuit, this wire can be run separately; It does not need to be shielded. Most RV's have no need for PTT access via the mic jack.

If RS-232 connectors with removeable pins were used, this should be straight forward (will need a pin extractor tool). If they were solder cup connectors, that is a tougher job.

Larry

Lyle RV8 said:

Rainier - Sorry if I am hijacking this thread, hope I can ask a follow up question on your first point.

I have been having transmit problems with a V6 radio and discovered the phone cables are shielded 2 wire cable using the shield to conduct the ground in violation of your direction. The mic cables are 3 wire shielded cable, no problem there.

The symptom is high levels of static (to the point the transmission is unintelligible) when transmitting on ~125MHz to 127MHz. The transmission is clean on other frequencies. Reception is clean on all frequencies.

Could the phone cables be causing this transmit problem?

thanks - lyle

Click to expand...

The headphone side is not sensitive to interference and very tolerant so likely not the issue (bit not completely impossible).

If things vary with frequency that usually is a solid hint that something is resonating as antenna which is connected to the radio somewhere in a way it should not be.
In most cases in my experience such issues are caused by one or more of the following:

a) Bad antenna tune (bad SWR). This tends to be very frequency dependent. Reflected energy from the antenna tends to contaminate the radio grounds with RF and this can leak into the audio circuits - in bad cases causing havoc.
This can be caused by the antenna itself, the cable (damage, sharp tuns (kinks) in the cable, incorrect cable) or bad terminations in connectors.

b) Direct radiation from antenna into headsets, active parts of ANR headsets or the microphone amplifiers inside headsets or into the headset cables (some types tend to have poor or even no screening at all). This is affected by distance to the antenna and the antenna radiation pattern.

c) RF coupling into the aircraft ground. In this case we have an innocent ground cable or structure connection somewhere that happens to form a loop (both ends connected to ground and there is considerable physical area inside the loop). If the loop happens to have the right dimensions (related to the wavelength of the transmitted signal) it acts pretty much as a secondary winding of a transformer and induces a low impedance RF power source into the ground. Now, depending on how that power travels and where it goes - it can again affect your radio. This one also often causes problems with other electronics (a typical one would be for example changing the altitude readout on an EFIS). Bad SWR can do similar though.

Radios are becoming more powerful and manufacturers are getting good at controlling signal modulation so the carrier wave tends to be well modulated. While this is good it unfortunately reveals any radio installation issues much easier.

Finding the causes usually is done by trail and error (using educated guesses and a SWR meter). Ferrites made to be effective at VHF frequencies can be used to prevent or at least suppress RF signals traveling on wires. While I don't like to use them (it feels like one is not fixing the root cause) - they can be helpful.

Rainier
CEO MGL Avionics