"I thought this all started with an observation that some people hit it way too hard and generate unwanted harmonics. I have always thought the "cure" for that was to turn down the audio feed. It is not clear to me how using rig split or Fake It solves that problem."
When WSJT-X split mode is used the modulation frequency is always in the range 1,500 - 2,000 Hz. The lowest frequency of the second harmonic is 3,000 Hz which is outside the TX passband of most transceivers. When WSJT-X split mode is not used the lowest modulating frequency is 200 Hz. The harmonics of 200 Hz contaminate the normal WSJT-X operating frequency range. It is simply not possible to foul up the band with audio harmonics if split mode is used. There is no protection against fouling the band if split mode is not used and transmission are made at the low end of the DF range. This is not new knowledge. Long before WSJT-X was popular JT65 HF operating instructions cautioned against transmitting in the low DF range. Sure, if every operator set up their station to be free of audio harmonics, there would be no need for split mode. You only have to look at the FT8 frequencies on any day of the week to know that isn't going to happen. I wrote a paper on this topic years ago. It was intended for club use but I'll share it here - https://tinyurl.com/2err7nsz 73, Andy, k3wyc
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