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  #1  
Old 03-05-2018, 08:25 AM
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DanH DanH is offline
 
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Default 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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Last edited by DanH : 03-05-2018 at 10:05 AM.
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  #2  
Old 03-05-2018, 08:34 AM
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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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Last edited by az_gila : 03-05-2018 at 08:41 AM.
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  #3  
Old 03-05-2018, 11:56 AM
Canadian_JOY Canadian_JOY is offline
 
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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!
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  #4  
Old 03-05-2018, 01:17 PM
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Quote:
Originally Posted by Canadian_JOY View Post
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?
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  #5  
Old 03-05-2018, 01:29 PM
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Quote:
Originally Posted by DanH View Post
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
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  #6  
Old 03-05-2018, 01:38 PM
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Quote:
Originally Posted by DanH View Post
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.
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  #7  
Old 03-05-2018, 02:27 PM
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I'd better ground my plane immediately (is there a pun there?).

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  #8  
Old 03-05-2018, 02:50 PM
hendrik hendrik is offline
 
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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!



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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  #9  
Old 03-05-2018, 03:07 PM
Timberwolf Timberwolf is offline
 
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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 gtx345ís Iíve gotten from spruce with a harness all had miswired connections on an rs-232 port.
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  #10  
Old 03-05-2018, 04:13 PM
Driving '67 Driving '67 is offline
 
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Curious as well, this is from a promotional video of SteinAir.


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