Roy Lewallen ) writes:
At 60 kHz, there shouldn't be any difference between daytime and
nighttime propagation. Certainly the ionosphere isn't involved.

Roy Lewallen, W7EL


Roy,

I believe that even at 60 kHz, propagation is within an earth-ionosphere
waveguide, the height of which is controlled by the sun (60 km during
daylight and 90 km during night).

Precise RF phase comparison is easy at these freqs because the phase
stability of the path is excellent during all daylight conditions, but
there is some random variation at night. It is best to avoid making
measurements (of phase) while the path is only partly in daylight because
of the diurnal shifts in phase as the sunrise-sunset terminator passes
over the radio path (due to the changing height of the waveguide).

The U.S. VLF OMEGA navigation system, the old WWVL (now SK) on 20 kHz, MSF
on 60 kHz, and WWVB on 60 kHz all suffer from these effects.

73,
.... Martin VE3OAT