It is reasonable for it to be a genuine change of phase as
the solar zenith angle varies through the day and D-region
ionisation varies with it.

For comparison I attach a plot of GBZ (distance 155km) from
today.  Recombination in the D-region is fairly quick, so you
usually get a more or less symmetrical phase change around noon.

MSF (co-sited with GBZ) shows a more or less flat phase
during the day here.  Also attached.  At these short ranges, the
received signal is a rich mixture of ground wave and several
sky rays. Consequently the diurnal changes are quite sensitive
to location and frequency.

Of course, local effects can also affect the phase.  Temperature
of active antenna, ground conductivity, and so on.

In the old days, it was common to lock receive frequency or
sample rate to these off-air signals!

--
Paul Nicholson
--

gbz-180330.png
Description: PNG image

msf-180330.png
Description: PNG image