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03-12-2019, 11:36 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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This is not a ground loop problem. This is basic RFI.
Get someone with an antenna analyzer to ring out our feed lines and antennas. If you have a bad connection this will show it. What you can do in the meantime is look at wire bundles - do you have feed lines bundled with coax?
Shielding instrument wires would not hurt. You may need to experiment with small disc capacitors across the analog gauge terminals ls - this will bypass the RFI to ground. This was standard practice in ?back in the day? radio work.
Carl
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03-12-2019, 01:53 PM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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Funny...
Funny, Carl, that you can make that statement with absolute certainty.
I had a similar issue in my C172...and it WAS a ground loop issue. That is why I suggested it...
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Bob
Aerospace Engineer '88
RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
EFII System 32 - Done
297 HP Barrett Hung
ShowPlanes Cowl with Skybolts Fitted - Beautiful
Wiring...
Dues+ Paid 2019,...Thanks DR+
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03-12-2019, 02:07 PM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,145
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Quote:
Originally Posted by rocketman1988
Funny, Carl, that you can make that statement with absolute certainty.
I had a similar issue in my C172...and it WAS a ground loop issue. That is why I suggested it...
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Was it though? Or was it just a mechanic telling you it was a ground loop issue because he didn't figure you were smart enough to know the difference?
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Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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03-12-2019, 03:54 PM
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Join Date: Jun 2010
Location: Sunman, IN
Posts: 2,189
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No
No it actually was a ground loop problem...
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Bob
Aerospace Engineer '88
RV-10
Structure - 90% Done
Cabin Top - Aaarrghhh...
EFII System 32 - Done
297 HP Barrett Hung
ShowPlanes Cowl with Skybolts Fitted - Beautiful
Wiring...
Dues+ Paid 2019,...Thanks DR+
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03-12-2019, 05:40 PM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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Importantly for analysis, 'ground loop' is only one way (of several ways) to have a ground problem.
Check your grounds and ground paths.
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03-13-2019, 07:33 AM
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Join Date: Jul 2015
Location: England
Posts: 74
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It is likely to be a RFI problem as many others have found in the past.
Depending on how much your time is worth it may be easier to install a different (better designed) gauge.
Pete
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03-13-2019, 02:28 PM
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Join Date: Mar 2018
Location: Conifer, CO
Posts: 33
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Thank you everyone for the help! The antennas are belly mount so I will check the connections. I tried running an independent ground for the ammeter and it made no difference, I did not try isolating the ground on the manifold pressure gauge but will when the weather gets better. I will look at the wire bundles and try to see if the coax runs with the electrical. I did have a previous owner reach out to me and said he had the same problem and just ignored/dealt with it. The radios did have some issues but that was when the "AMP" switch was in ALT. Also I still have no idea what the "AMP" switch does.
Thanks,
Dan
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RV-6A N60KD
Navy Vet
Donation made for 2020
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03-14-2019, 07:40 PM
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Join Date: Sep 2015
Location: Nevada City Ca
Posts: 198
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Mine has done it since it was built. Never could figure what causes it in 380 Hours.I always thought it was a bad ground but never figured it out. I guess I am used to it
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F1 EVO
Aviat Husky A1B
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03-14-2019, 08:14 PM
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Join Date: Jul 2009
Location: Longmont, CO
Posts: 236
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The first thing to check is if there is high SWR on the antenna. if this is high it causes the coax to radiate RF which causes all kinds of problems.
If you have high SWR there's all kinds of things you can do to fix it. I had a bad connector at the antenna end of my coax and it caused **** with my instruments. Fixed that and the problems went away.
However - as others have already posted - if you have instruments that are sensitive to RF you may have to experiment with bypass options at the instruments themselves.
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Mark Olson
RV-7A First flight 2005 Sold 2019
F1-EVO Rocket First flight 2010
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03-15-2019, 08:33 AM
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Join Date: Nov 2012
Location: Cincinnati,Ohio KHAO
Posts: 128
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Belly Mount
Everything Charlie and Mark X 2. As soon as he said "belly mount" my eyes perked up. Don't think I've ever seen better than 4 to 1 SWR on a Comant
bent whip ( the kind that is parallel to the fuse ) . A place to start. It is 400 coax, right? Dual shielding, it helps. Then start bypassing with disc caps, helps a little more. You will get it, but more than one treatment will be necessary.
John
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John R Prince
RV-7 want-a-b
Dues Paid for 20
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