[GreenKeys] 10 Hz shift RTTY?

[Jim Haynes]

There were some experiments done in the early 1960s with 75 baud FSK
transmission on broadcast station carriers with a shift of 36 Hz.  AM
stations are required to stay within 20 Hz of their assigned carrier
frequencies, so the shift was 18 Hz from center.

I wonder how much shift was used in VLF operation.  I've read that the
antennas there are highly resonant and thus it is hard to shift the
frequency very much.  There were experiments with electronically switching
in reactance along with the FSK modulating signal to try to keep the
system in resonance.