LF: Phase of MSF through the day

[Andy Talbot <andy.g4jnt@gmail.com>]

The plot below and also to be found at http://www.g4jnt.com/DropF/clipboard_201803301751.png   shows the phase and amplitude of the MSF 60kHz signal received here on the south coast over a period of 5 hours.  Whilst I would expect phase changes to occur at dusk around 1800 - 1900 (still to be recorded)   the phase begins to shift noticeably downwards from around 1400z, dropping faster after that time.

What mechanism will be causing the afternoon change whiel the sun is still well up?

For info, the Rx setup is as follows:

The LF receiver as described here http://www.g4jnt.com/Coherent_LF_Receiver.pdf  with the 10MHz reference coming from a HP5061A caesium frequency standard.  

This was double-checked as a backup (because I don't turn the Cs on very often) by comparing the time delta of it divided down to give one second pulses against 1PPS signal from GPS in a counter-timer in delta time mode..  Over the duration of this test, the timing of the Cs derived pulses did not move by any discernable iota, ie significantly less than 100ns in the 12 hours the Cs standard was switched on

Frequency calculation rounding in the LF receiver DDS controller introduces an error of up to  +/-1.5*10^-11 on the tuned frequency due to truncation involved in the 64 bit integer maths.  At 60kHz that is equivalent to 0.9uHz or one degree per 50 minutes.  That error is perhaps just about detectable in the frequency plot early on, but the observed frequency change is greater than that, and non-linear over time.

We have had a lot, an awful lot, of rain today and although the Rx antenna is an active probe could there be a phase shifting mechanism due to ever-soggifying ground?   My 137kHz antenna runs overhead, but at so far off this frequency, I can't see its tuning affecting things

If the dusk plot shows anything exciting, I'll post another screen dump

Andy  G4JNT

http://www.g4jnt.com/DropF/clipboard_201803301751.png