Stop now. Please.

Reinforcing the hole "as per SRM" is probably good enough for the hole, but what about the loads from the antenna ? a 20kg antenna is not small. What are you doing for buffet loads ? A 20kg antenna is a scary, seriously scary, thing to be adding to the plane and needs a lot of internal support provisions.

DTA is just about the least of your problems. If there's no buffet stress, then you could use hoop stress as a hopelessly conservative 1/flt stress and run some DTA calcs at the dblr edge. So you'll probably end up with a LFEC inspection 1 every 5 or so years … not a problem in the mod world.

But you Need to have a sturdy internal provision to ensure that the installation is stiff enough to avoid problems (being too flexible). If this was a VHF antenna, I usually use an abuse load (200 lbs, 300, 400 ??) at the tip to demonstrate an acceptably stiff structure.

another day in paradise, or is paradise one day closer ?

I really don’t understand why these type of questions are posted here. You should be discussing this with your DER or your local ACO. And isn’t there an entire AC on this subject?

Following might be useful... lots of unexpected references [to me]...

FAA PS-AIR-25-17 STRUCTURAL CERTIFICATION CRITERIA FOR ANTENNAS, RADOMES, AND OTHER EXTERNAL MODIFICATIONS

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]