Lola,
The FAA is not requiring antenna to antenna "certification". You don't actually need a TSO for a transponder- you need a transponder that "meets the performance requirements of TSO X". ADS-B is the same, and it's technically legal to put completely uncertified stuff in your experimental- assuming you have data that it meets the performance requirements.
The ADS-B mandate has one weird twist- "all ADS-B OUT installs must be STC'd." Which is impossible in an experimental, which has no TC to start with.
The FAA has actually been pretty clear how they will handle experimentals- they need data showing it "works". One data set they accept is any STC'd combination of a specific transponder and a specific GPS. They will accept other data sets, and Dynon is and has been in discussions with the FAA for years about our setup and demonstrating that it will comply. The Trig transponder and the FF GPS is actually the only combination that has been STC'd so far, so that's the easiest one to show compliance for, and is already accepted by the FAA via their "any STC'd combo rule". On the experimental side, the FAA is more than accepting of the fact that the standard part number for the Trig and the Dynon variant are the same, and thus acceptable. When you last asked you were wondering about putting the Dynon in a type certificated aircraft, and we were warning we have no experience with that and listing possible issues.
We do not use our own version of Ground/AIR. We use the fully certified algorithm in the transponder, which is both compliant and REQUIRED for 2020. The transponder by itself decides ground/air, with no decisions made in SkyView.
As far as we know, have been told, have heard, and have experienced, a Dynon transponder hooked to a FF GPS is fully compliant today and has no caveats or issues with 2020 compliance. If that is worth $3K today is a different issues, but that setup will work.