When researching to find a location for the COM and XPDR antenna I didn't find that much information. Eventually I discovered AC 43.13-2A Figure 3-6 which showed a doubler riveted to the skin and two stringers, using a flange and joggle. Under the seat behind the main spar centre section I found a suitable point ... just behind the aileron trim brackets between F-814 and F-815. At this point the floor is not rivetted (see black cleco for most aft nutplate in the floor) so access for inspection/maintenance will be OK.
Accepted practice says the antenna should be 36" apart but that is very difficult on an 8 and in practice many RV8 owners say 18" separation seems to be fine, particularly if the XPDR is a low power device (normal).
I used some 0.025" sheet left over from making the wing doubler and cut two peices 7" x 10 1/2".
I drew a line 0.7" from one end and placed this under my back rivet plate to bend the flange that wil be rivetted to F-814 - allow a set back of 1/16" for the bend.
