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Old 01-09-2022, 10:02 AM
Tony_T's Avatar
Tony_T Tony_T is offline
Join Date: Dec 2008
Location: Lacey, WA
Posts: 1,435
Question Comm Antenna - best mounting

Another thread mentioned finding loose screws mounting the comm antenna reminding me I had not checked those lately. I was surprised to find the 4 screws fairly loose, not snug by any means. My ship is an early build so here was the mounting dwg. which is now obsolete.

Note the RUBBER PAD.

The drawing was revised in 2014 and now the rubber pad is no longer shown.

Question 1 is: Does the antenna perform better without the rubber pad?

Now this drawing is the antenna manufacturer's:

Shown on that drawing is the rubber pad and also a 0.060" alum back-up plate.

Question 2 is: Will the antenna perform better with the back-up plate? Or is the plate just for structural considerations?

The 4 mounting nuts and split washers are a little hard to reach through just one of the access plates. I am thinking of installing the back up plate and using 8/32 one lug nut plates mounted on the back up plate to prevent loosening of the little nuts and split washers supplied with the antenna. This will also alleviate the access problem to check the screw tightness.

Question 3 is: Any possible problem using nut plates in the application?

BTW, this post does not apply directly to the current RV12iS which uses a different antenna which mounts differently.
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Old 01-09-2022, 10:18 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 3,359

#1. No, the pad makes no difference. Take it out if you like.
#2. Use the backup plate. It has nothing to do with antenna performance, just mounting strength.
#3. Nutplates work, but for you the back fit would mess up your paint.

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Old 01-09-2022, 10:47 AM
RVDan RVDan is offline
Join Date: Jun 2012
Location: Frederick, MD
Posts: 946

If the skin is flat where you are mounting the antenna, you can leave the pad out. If you leave the rubber pad out, you should filet seal the antenna to skin joint to prevent water entry which will result in corrosion. If you can put the nutplates on the aluminum doubler plate that would eliminate riveting them to the skin. Make sure the antenna base has good ground to the skin for RF performance.
Dan Morris
Frederick, MD
Hph 304CZ
RV6 built and sold
N199EC RV6A flying
Retired Aerospace Engineer and A&P/IA
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Old 01-13-2022, 10:57 AM
gemiller gemiller is offline
Join Date: Oct 2021
Location: Auburn
Posts: 3

The spec value for the resistance between the antenna base and the fuselage is 5 milliohm. One needs special equipment to measure such a resistance. That being said, by taking extra care to ensure the mounting surfaces are uncontaminated and then sealing the surfaces to prevent moisture from intruding, one should get and maintain an acceptably low resistance.
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Old 01-13-2022, 05:20 PM
rgmwa rgmwa is offline
Join Date: Feb 2010
Location: Perth, Western Australia
Posts: 1,687

Re Q3. FWIW, I installed nut plates during the build. Pretty sure Joe Gores did it and I just copied his idea.
RV-12LR 912ULS
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Old 01-14-2022, 11:09 AM
flion's Avatar
flion flion is offline
Join Date: Dec 2007
Location: Flagstaff, AZ
Posts: 2,709

Another +1 for nutplates. Even with a backing plate it makes installation much easier and you can always check tightness on every preflight.
Patrick Kelley - Flagstaff, AZ
RV-6A N156PK - Flying too much to paint
RV-10 14MX(reserved) - Fuselage on gear
EAA Technical Counselor #5357
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