For those of you contemplating this, I've made some rough power measurements. Sitting in my shop with a clear view towards the airport, the TT21 is reporting nearly constant interrogations, and drawing between 120 and 150 ma. The TN72 alone draws about 70 ma. When both are powered and broadcasting ADS-B, the power is about 170 - 210 ma. All measurements taken with a clamp on hall effect sensor, +/- 10ma. So roughly, the extra power to transmit ADS-B is lost in the noise. The TN72 takes measurable power, but not much. This is a pretty cheap way to make yourself seen.