If you fly Experimental in airspace where you're not required to meet FAR 91.227 compliance, why wouldn't you configure your TN72 + TT21 to SIL=3? The only functional difference is that TN72 + TT22 (with SIL=3) puts out a stronger signal.

--------------email from Trig Avionics Sept 2017 posted below------------

Dear son_of_flubber,

Thank you for your enquiry, and for choosing to support Trig. The TT21 is fully certified to the latest ADS-B Out cert TSO C166b – so it can output all the required data. It is true that if you wish to meet ADS-B Out compliance FAR 91.227, you should have a higher power TT22. However, there is no way for the radar to tell if you’re using a TT21 or TT22. Inspection of your aircraft would be the only way to tell. As such I believe a number of experimental pilots have opted to ignore the requirement, but of course I cannot recommend this as it would be in violation of FAR 91.227 – the requirements for 2020 compliance.


The TN72 offers both a SIL =1 output, and also a SIL = 3 output. So this means that you can meet the FAA compliance check using your TN72 plus TT21. https://adsbperformance.faa.gov/PAPRRequest.aspx


-------------------email from Trig posted above ------------------