Total Harmonic Distortion is a measure of non-linearity in an audio system and is calculated as the RMS voltage of all the harmonics divided by the test tone RMS voltage multiplied by 100%.. The THD of my AM transmitter over the air signal as received by two receivers was measured.
The test tone was an 800 Hz sine applied uncompressed to the previously describe "high efficiency" transmitter and the first receiver was a BC-1004-C with the IF bandwidth set to 10 kHz. The transmitter modulation and receiver AVC and audio volume were set to yield the minimum THD. The received audio signal THD measured 0.72%.
A second receiver, a Yaesu FRG-100, was similarly adjusted and the audio THD measured was 1.04%.
The results for the two receivers may not be comparable because the FRG-100 has a narrower bandwidth than the BC-1004-C which would tend to reduce the THD reading for the FRG-100 since some higher harmonics are attenuated.
The majority of the distortion most likely originates in the signal path from the transmitter antenna to the receiver antenna (added noise) and in the receiver electronics. A better AM receiver/detector would be needed in order to separate the transmitter THD from the rest (maybe another winter project in the works?). The data do indicate that the transmitter THD is less than 0.72%
Nonetheless, a THD around 1% for the total chain is not too bad.