Finalized the location of the transponder antenna under the co-pilot seat. Laid out the holes based on the pattern for the RAMI antenna, it appears the same as the Commant. Sized the doubler and number of rivets per AC43-13 and laid them out of a 0.063" sheet. Drilled, cut and positioned in the fuselage so that the bolts and coax fitting lined up with the joggle that I put in the fuel line. Drilled a couple alignment holes in the bottom skin, and then cleco'd the doubler onto the underside of the skin and match drilled everything. Upsized the bolt holes and center coax hole and then trial fit the Commant antenna in place - fits perfect. Then being stupid, I'm looking at it from the top and it appears that I missed the four holes right around the coax hole as I can see primer through them. I thought I got them, but I guess I'll just redrill them from the top. When I looked under the belly, I immediately realized that I wasn't seeing primer, I was seeing the antenna and I drilled #40 holes right through the base. Just cosmetic damage. I guess it is too late and time to go to bed. Before checking out for the night though I thought I would check how stiff my doubler is and pushed on the side of the antenna. With a disappointingly small amount of force, the base of the antenna blade broke right off from the base. Well, that's official, I just trashed an antenna. 11/13: Alodined the doubler plate. Didn't come out nearly as dark as previous but I think it is adequate to protect the doubler. I initially planned to remove the primer below the doubler and alodine that portion of the skin too, but when I tried to remove the primer with some acetone, it didn't budge. Nice durability EkoPoxy. 11/15: countersunk the holes n the bottom skin since it is 0,040”. Touched up w the holes with primer and then riveted the doubler in place with my wife's help. Tested the continuity between the doubler and some other parts of the airframe and had good results despite not removing the primer below the doubler. This little side project is done and ready for a new antenna install.