I've decided to switch up my antenna plans a little and put one COM and the transponder up under the seat pans, then the other COM and ADSB behind the baggage bulkhead. That seems to be the best of all worlds in terms of separation and coax runs.
Drilling holes in the skins never gets easier though! I was careful to set the aft doublers parallel to the direction of flight (the antennas are fins, so best not to get them canted off to one side). If I had put the transponder and ADSB both back here, I would have had to slide the antennas out to the edge as far as they could go to meet the 2 foot separation requirement. That hits the curve of the skin, so less than ideal for the doublers. Based on what I learned online, including a post I saw on the Dynon forum by their tech support a few years ago, I decided to move those locations in a few inches so they'd land on the flat portion of the skin. Apparently the COM and ADSB antennas don't have the same issue with interference as the transponder antenna and COM does. The transponder is a transmission powerhouse, but the ADSB unit is just a receiver (coupled with the fact that it operates on very different frequencies than COMs do).
All that being said, no sooner had I drilled all of the holes than I got reply from Dynon tech support that I had sent a while ago. This guy said that ADSB and COMs should be 3 feet apart, just like transponders and COMs. This contradicts what they've said before, so now I don't know who to believe. My holes are already drilled, so I'm hoping this guy was just groggy from his holiday vacation and not thinking straight! That got me really digging into their installation manuals, and that made the confusion even worse. While their manuals may be great relative to the rest of the industry, they're still full of contradictory recommendations. Depending on which section I was reading, I found three vastly different antenna separation recommendations. They sure don't make it easy on a guy! For now, I'm leaving things as-is since I think the ADSB and COM are likely fine without much separation. If I learn otherwise, I guess I'll get to use the doubler just to patch up the holes and have to make another doubler for use further back in the tailcone.
