Shield Ground Wire Length

[flickroll]

I am finishing up the wiring of my panel which will have a Dynon EFIS D-100 and an EMS D-120. The harness for the D-100 has a short wire stub of just a few inches which is used to ground the shields in that harness. What I would like to have is enough slack in the harness so that I can pull the D-100 from the mounting tray and then remove the connector from the back of the unit. If I do this the shield ground will have to increase in length to around 10" or so. Is this acceptable? I know that shield grounds are kept to a minimum for a reason, but wonder what the maximum length this wire can be without a danger of introducing noise to the wiring system. Thanks

Jim Shannon
RV-8 N52VV
Charlottesville, VA

[grover]

but in this case, where maintaintability will be dramatically improved, I'd opt for longer. had similar situation with my dynon, and it suffers no ill effects of longer than standard shield grounds.

[fl-mike]

No. In fact, the short stub already there is poor practice for high frequency attenuation. (it's all about impeadance, and long wires are inductive) The shields should be terminated to the (metal) connector backshell, so you should get the servicablility you desire. Do not run the shield termination over to some other ground point. Terminate shields at the connector interface.

Now, that being said, for audio-frequency shield terminations, short pigtails are often used. Where you start running into trouble is with the digital interfaces, RF, etc.

http://www.emiguru.com/kgb/sum1996.htm

[az_gila]

Originally Posted by grover

but in this case, where maintaintability will be dramatically improved, I'd opt for longer. had similar situation with my dynon, and it suffers no ill effects of longer than standard shield grounds.

...a 10 inch "shield ground connection" is actually a recieving antenna....

Since we are dealing with shields to negate the effects of stray RF, the extrapolation from one aircraft to another is not really valid. Unless all of the avionics are identical, and installed with identical harnesses, there is no real correlation.

Why not get a small push on tab connector, and connect the shield wire to a tab mounted on a screw on the back of the D100 unit itself? ...perhaps to one of the D-Sub mounting screws. It does not really have to go to airframe ground directly in this case.

I don't have the specification handy for the shield pigtail length, but it is in the order of 1.5 inches maximum for the frequencies we usually deal in.

[flickroll]

Great idea Gil, thanks. I have some push on connectors as well as some tabs. Will do that tomorrow!

Jim