I have started the process of equipping my 1929 Waco ATO with ADS-B out by installing a Trig transponder. One box and the antenna and I can be compliant. My concern is not a letter or phone call; my concern is that the FAA is not yet providing guidance on where the end requirements will be. I have no interest in additional boxes, antennas or other technology that might be required as a "solution" to the aerobatic problem. If the FAA were to state that if equipment complies except for aerobatic flight they will not require modification or replacement I will install that last box.