Hi Charlie,
There isn't a lot of info out there, however there are a few things have noticed and picked up over the years.
1. Negligible limits as published by many manufactures tells me that metal airplane follow all the same rules. For antennas, I care about holes. Any new hole (non fastener) "should be" two diameters from any other fastener line or underlying structure, and smaller than 2 inches. Use common sense, because elsewhere in in the negligible limits it will say "If there are no limits published for the part you are assessing..." Spars, engine and gear mount structure ect..
2. Stringers are length wise, the doubler will have two vertical webs spanwise about a half inch high. The doubler can joggle over the flange of a stringer or slide underneath. Trim the vertical web as close as possible to existing intersecting webs. Add a small 90 degree angle to tie the webs together for an indestructible installation (long antennas, close to prop wash).The nutplates can go through the skin, I usually just put them in the doubler only and take my chances on no nutplate failures. The webs of the doubler should be as close to the nutplate as possible. This is what really stiffens up the mount and prevents fatigue cracking. Additional fasteners are of same type and spacing as the existing rivet line. If two existing lines differ, use the stronger provided it makes sense. Smaller flat antennas like GPS located out in the open away from propeller vibration can be a simple flat plate, provided negligible limits can be met. The taller antennas will need those webs to prevent fatigue cracking.
First photo shows added CI268- mount in green aft of aft spar attach, and factory CI26- mount just aft of skin in yellow. Second photo is mount in green.
https://lh5.googleusercontent.com/-0...0/P3100023.JPG
https://lh5.googleusercontent.com/-9...0/P3150004.JPG
Antenna Doubler Specifics
Linear Mode
