On Wednesday, December 13, 2017 at 6:00:09 PM UTC-8, wrote:
Darryl, the one thing you are not talking about is power supply.

A K2 pack seems to be working for simple mode C. Is there a reason to expect things to be significantly worse for ADS-B. (Maybe due to higher power out, more GPS power, a higher interrogation rate, and/or longer output transmissions?)

Impossible to say for sure.. you could calculate power budgets and tell us :-)

But... I expect them to be generally better at least with a TN72 GPS. Other GPS sources like what Andrzej has used in his ADS-B Out install in his glider (or gliders --done that in two glider now?) may have significantly higher current draw. Huge credit to Andrzej for being a really early adopter of fully 2020 Complaint ADS-B out in his glider(s).

The Trig TT21/TT22 are lower power draw than most old Mode C transponders and their separate encoders. The book numbers for an active interrogated TT22 (with built in encoder) and a TN72 is about 440mA @14V. A little less, but close, to the popular old combination of a Becker ATC 4401 Mode C transponder and ACK A-30 Encoder at 490 mA, and that is an old 175W not 250W output transponder.

The factor in power consumption when adding ADS-B out to a an existing transponder is just adding the power draw of the GPS receiver. There is no "interrogation" involved in ADS-B. There is a ~1 per second extended squitter broadcast, that one little broadcast is irrelevant in terms of power consumption. When you transponder is is a very active environment it is being interrogated many hundreds to around a thousand times per second. The little blinking indicator showing interrogations in a transponder is a big 'ol lie, it's just slowed down so dumb humans can see it blink.