On Monday, December 21, 2020 at 6:23:20 PM UTC-7, wrote:
Get the TT22. Don't consider the TT21. It could save you about $150 on a $2500 USD purchase but does not provide full ADS-B compliance. In the USA this means a TT21 does not allow you to fly in transponder-required airspace.. The TT22 provides a full ADS-B out solution and more transmit power, yet its power consumption is identical to the TT21, according to Trig's spec sheets.

For an experimental glider you can use the same GPS receiver and antenna that you would find in a TT21 ADS-B out package (Trig TN72 and Trig TA50), so the only added cost is the $150 to upgrade to the TT22.

...david
On Monday, December 21, 2020 at 7:19:22 PM UTC-5, John Johnson wrote:
Should I assume it is not legal in the US to use a TT21 with a TN72 for ADS-B out in an experimental glider? That combination is ok in Europe (I think) with the difference being the US wants 2x the xmit power for ADS-B (TT22 at 250W) vs the 125W output of the TT21. I would like to think that having otherwise compliant, lower power ADS-B out is better (and not illegal) than no ADS-B out in an aircraft that is legal without ADS-B.
Probably not a realistic hope - but it would save me the not trivial expense of upgrading my TT21 to a TT22.

JJ
Fully agree a new purchase should be a TT22. Unfortunately, my glider has been setup with mode-S TT21 for some time now. I understand it costs ~$1000 or so to have an existing TT21 upgraded to TT22 specs. Since I can't see any practical need for my glider to have the 2x xmit power compliance, I'd like to save the $1000 and, perhaps, spend it in the future when the next wave of new transponder compliance requirements come about. I could always perform the upgrade later if that made sense for me.

thx, JJ