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Comm Antenna - best mounting

Tony_T

Well Known Member
Patron
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.

i-nXr4h8M-XL.png


Note the RUBBER PAD.

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

i-fHMpLff.png


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

Now this drawing is the antenna manufacturer's:

i-gbG3HKN-L.jpg


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.
 
#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.

Carl
 
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.
 
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.
 
Re Q3. FWIW, I installed nut plates during the build. Pretty sure Joe Gores did it and I just copied his idea.
 
Another +1 for nutplates. Even with a backing plate it makes installation much easier and you can always check tightness on every preflight.
 
Follow up

My original post was the result of finding loose nuts on the antenna mount and also having a squeal on transmitting and generally low performance with my GTR200 radio. I was also pretty sure the antenna had been damaged by bumping into it several times during the 12 years since it was installed. I installed a new antenna and installed it using the back up plate fitted with nutplates. I also used a new rubber pad. The nutplates made for a very solid and easy installation and it’s doubtful the screws into these nutplates would ever loosen. Easy enough to check without having to access the inside. Flew a test flight and both transmit and receive are very solid and clear.

Thanks for the useful comments.
 
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