What Darryl said.

You might like to know that the avionics technician who did my VFR
transponder checks went to his computer and got the proper ICAO code
from the FAA. The reason is the "S" in Mode S stands for selective,
and they want to be darn sure ATC is broadcasting to the proper
aircraft.

I was being tongue in cheek with my previous post. There's no reason,
given the price and low power draw of the Trig TT21, that anyone
should even consider buying anything else right now (unless they're
replacing an existing transponder with the same type).

-John

On Dec 8, 3:37 pm, Andy wrote:
As to broadcasting the tail number - the TT21 installation menus
allow the aircraft ICAO code and the registration to be set but I
don't think there is an equipment requirement for them to be set. In
other words I think it will work if the defaults are left unchanged.
Is there any regulation that requires a mode S transponder to transmit
that data in US?