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Shield Termination Pigtail Loop

DanH

Legacy Member
Mentor
Garmin Shield Termination Pigtail Loop

Curiosity question....

I was reading a GTR-200 manual. The instructions for shield termination are specific; the pigtail must point toward the backshell where it exits the solder sleeve. Pointing away from the backshell and looping around to connect at the backshell bar is not allowed.

So...why?
 
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I think the method specified allows for a shorter ground wire pigtail.

However, in the GTN 650 manual the second method you describe is called an "alternate" method and the first method is described as the "preferred" method. :)

The GTN 650 manual has a note "Shield drains as short as practical (No longer than 3 inches)"
 
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Dan - the answer is as Gil mentioned... Shorter ground wires mean shorter antennas to either pick up radiated emissions or to become a radiating element for emissions conducted from within the radio.

I've worked with some avionics equipment which had very specific shield termination requirements. The device would only meet EMI/RFI requirements when all the shield pigtails were fully contained within the all-metal backshell of the connector. What a bear those connectors were to work!
 
Dan - the answer is as Gil mentioned... Shorter ground wires mean shorter antennas to either pick up radiated emissions or to become a radiating element for emissions conducted from within the radio.

Well now, there's why I got curious. I understand we want to minimize the length of a conductor not covered by a shield. Here we're talking about shield braid and its pigtail to ground. In terms of radiating or receiving, you're saying the pigtail is somehow different from the entire shield braid, which is every bit as exposed?
 
In terms of radiating or receiving, you're saying the pigtail is somehow different from the entire shield braid, which is every bit as exposed?

Exactly what I was wondering.

Sometimes these electronic doodads do strange things:confused::confused:
 
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Well now, there's why I got curious. I understand we want to minimize the length of a conductor not covered by a shield. Here we're talking about shield braid and its pigtail to ground. In terms of radiating or receiving, you're saying the pigtail is somehow different from the entire shield braid, which is every bit as exposed?

Yes.... they can become mini tuned antennae in short lengths. Keeping them really short limits the possible tuned frequencies.

At least that's how I see the black magic of antenna/EMI/EMC stuff. :)
 
I'd better ground my plane immediately (is there a pun there?).

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This is one of Garmin's exhibition demonstrator panels. They remove a short length of the outer insulation, keeping the shield intact. Then they use a solder sleeve to attach their pigtail. All that happens about 4" away from the backshell. The shielding ends just before the cables enter the backshell. Now please don't ask me for a painless way of removing that outer insulation that neatly!

garmin_demonstrator_panel_wiring.jpg


I took this photo in April 2017. If my memory serves me right (big "if" here), the panel they showed two years earlier had the wiring done the way they don't recommend now. I checked, but I didn't find any pictures.

Anyway, here is a link to one of their wiring videos showing how one can attach a pigtail to the shield. Pointing to the connector. Go figure ...

https://www.youtube.com/watch?v=jmVLVhGoGQs
 
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Funny because aircraft spruce, an authorized garmin dealer, does it the way Dan says garmin says is bad. Of course it behooves you to check and verify all connections as the 3 gtx345?s I?ve gotten from spruce with a harness all had miswired connections on an rs-232 port.
 
I think we're seeing "common practice", particularly in that last photo from Driving '67. "Common practice" and "recommended practice" are often two different things, with the recommendation coming from engineers who have never had to build or maintain a production wire harness.

Personally I tend to default toward extending the pigtails as exemplified by the green shield terminations in Driving '67's photos because it makes life much easier to do it that way. If the manufacturer of the avionics recommends doing it another way, they likely have a good technical reason to make that recommendation. Whether or not that technical recommendation is practically applicable is another question entirely.
 
Sometime over the past several years, Garmin changed their documentation.

I was able to find a copy of an older revision of an install manual.

In the older revision, their diagram for the ground to backshell is a loop. In the latest revision, the ground takeoff is further back and runs straight to the backshell.
 
It?s all about the impeadence (inductance really) of the shield drain wire.
As frequency goes up, the inductance of the wire becomes a larger contributor to the impedance of the wire. Above a certain frequency, the wire may as well not even be there.

So, that is probably how they had to terminate the shields to pass the DO-160 radiated emissions or radiated susceptibility levels they demonstrated compliance to.

Use other methods at your own risk. Just don?t complain if you hear your LED landing lights in your headset audio!
 
So, that is probably how they had to terminate the shields to pass the DO-160 radiated emissions or radiated susceptibility levels they demonstrated compliance to.

Mike, I got some private guidance, and yeah, turns out it's pretty much a standardization thing. Not unreasonable at all. There are a lot of ways to terminate a shield, and surely a few of them have EMI or mechanical issues.
 
Mike, I got some private guidance, and yeah, turns out it's pretty much a standardization thing. Not unreasonable at all. There are a lot of ways to terminate a shield, and surely a few of them have EMI or mechanical issues.

Yes many ways to do it, but shorter is definitely better. Do the best you can. :)
 
Alex Peterson sent a good paper, which led me to another paper available in the NASA technical server.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080021261.pdf

Germane to pigtail loops, here's what they found:

The next question we examined was noise coupling through
the pigtail connection. First we investigated how sensitive the
load noise was to pigtail loop area. What we found was that
the load noise can vary up to 3dB by simply widening or
narrowing the loop area of a 7.5 cm pigtail. Three things can
be done to reduce this coupling. The pigtail can be kept short
and tied flat against the cable thereby minimizing the pigtail
loop area. If the electromagnetic field is known, the pigtail can
also be oriented so that a minimum amount of magnetic flux
passes through the loop.


Note the reference to the area of the loop. What they are saying is that a 3" loop pigtail worked pretty well if tied tightly so both legs of the loop are together, i.e. minimal area between them. Note that the recently prescribed Garmin method has no loop at all, and minimizes exposure of the center conductor, as the braid extends well past the pigtail tie point. An additional table suggests the 3dB reduction from minimizing loop area is roughly equal to exposing one inch less center conductor. We all know to minimize exposed conductor, so now you know to also tie those pigtail loops flat and tight, if you choose to use them.
 
In addition to what Dan said, one of the papers noted strong reduction in EMI coupling when braid is inserted completely inside the metal back shell housing. That paper did clamp the metallic shield braid at that point, whereas we generally only tag the ground wire (attached to the shield) to the housing. Lots of emphasis on a 360 degree method of "masking" of the signal wires from EMI. I believe this will be accomplished by insuring the shield continues well into the back shell.

What wasn't a good shield was when the braid was terminated outside the back shell some distance, and then a ground wire strapping the braid to the back shell.

Bottom line: run the braid inside the back shell and keep any physical loops of the ground wire to a minimum area as Dan said. Use heat shrink to insure no errant braid strands are floating around in the connector.
 
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