Antenna Flex?

I installed my comm antenna on my Cruzer. Top mount, Comant-121. I mounted it directly behind the rivet line for the upper baggage compartment wall where it joins the upper fuselage skin.

I used a piece of .060 backer plate that extended behind the antenna by about an inch. To that, I cut and riveted a piece of L angle that extends to the diagonal stiffener for that bay.

I have not yet riveted the upper skin aft of the upper forward corner braces to allow access for running cables, etc.  When I cleco it all into position using every other rivet hole, I don't have audible oil canning, but I do still have some foreward / aft flex. Probably less than 1/4".

Am I over analyzing this and they all flex, or should this be rock-solid, no movement whatsoever?  Just concerned about ultimately stress cracking.

Thanks for any thoughts on this matter;

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Permalink Reply by Robert Riat on June 11, 2019 at 7:09pm

I mounted my comm antenna similar to the way you describe on my stol 750 and only have 90 hours on it, but it works fine.

Permalink Reply by Carl Orton on June 12, 2019 at 9:29am

Thanks, Robert, for the reply. I’m not too concerned about it functioning as far as RF, I’m mostly concerned if the fore/aft flex of the antenna base is an issue of concern. 

Permalink Reply by Robert Riat on June 12, 2019 at 12:24pm

Hi Carl,  I didn’t really say what I meant - the skin shows no sign of stress, but maybe it will with more hours on the plane.  I don’t think it’s a problem though.  Also, sometimes it seems like riveting stiffens things up over just cleco’s  which might make a little difference?

Permalink Reply by Carl Orton on June 12, 2019 at 1:46pm

Good point on full rivet stiffness. It definitely stiffened up when I cleco’d all holes between stiffeners. 

Permalink Reply by Robert Lemke on June 12, 2019 at 1:09pm

If you are really wondering if it will be o.k., check out this video from EAA. Once you know how much force is being placed on the antenna in flight, you can ground check it with the same force and check for frame deflection.

https://video.eaa.org/detail/video/3946101313001/calculating-antenn...

Permalink Reply by Carl Orton on June 12, 2019 at 1:47pm

Great resource!  I will definitely do that cake and test on my install!

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