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  #11  
Old 03-05-2018, 08:18 PM
Canadian_JOY Canadian_JOY is offline
 
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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.
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  #12  
Old 03-05-2018, 08:21 PM
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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.
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  #13  
Old 03-05-2018, 09:45 PM
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Itís 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 donít complain if you hear your LED landing lights in your headset audio!
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  #14  
Old 03-06-2018, 11:16 AM
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Quote:
Originally Posted by fl-mike View Post
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.
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  #15  
Old 03-06-2018, 11:45 AM
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Quote:
Originally Posted by DanH View Post
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.
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  #16  
Old 03-07-2018, 07:53 AM
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Alex Peterson sent a good paper, which led me to another paper available in the NASA technical server.

https://ntrs.nasa.gov/archive/nasa/c...0080021261.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.
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  #17  
Old 03-07-2018, 08:18 AM
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It looks like they (garmin) are standardizing to braided drains too.
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  #18  
Old 03-07-2018, 10:50 AM
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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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