Peter,

 

well I've also been scratching my head about those abrupt changes at the beginning of May and November.

 

I wondered whether it might be an artificial effect, perhaps from scheduled retuning of the antenna for summer vs. winter impedances. But while that might induce a minor change in ERP, it would hardly have an effect on the vertical radiation pattern.

Regarding temperature: Frozen ground can have a significant effect on groundwave absorption. But it's hard to imagine that wide-spread frost would occur so suddenly and at a deterministic time, and that it would persist continuously for so many months into springtime.

 

So this remains an unsolved mystery...

 

BTW There is also a small effect of frost or snow on phase velocity. I noticed this when analysing emission times of dual-rated American Loran stations. Some of these were slaves which were synchronized over the air by different masters, and thus the emission times were steered via different groundwave paths. For some Northern and Canadian secondaries, there were seasonal variations of several hundred nanoseconds which were clearly correlated with snow coverage:

In contrast, European chains, which are using local clocks to steer both rates, do not show this effect:

http://df6nm.bplaced.net/LoranView/European%20dualrate%20phase%20offset%20100208.gif  

 

Best 73,

Markus (DF6NM)

 

Hi Alan & Johan, 

 

Thanks! 

 

Johan wrote: 

> ... 

> It is interesting to see the very abrupt signal drop in the morning, and how 

> well it correlates to sunrise, and compare it to the irregular return in the 

> evening. 

> ... 

 

Yes, apart from those sidereal dependent effects: 

from May until October all day-light levels seem to be somewhat blurry. 

May be it's an effect from temperature or thunderstorms ??? 

 

BTW: those data are from a system established in 1999 for observing the  

total solar eclipse at 75 kHz. I got the selective level meter from 

Gary (dk8kw) and the computer is an old "laptop", Toshiba, I486, 

~15 years old, running Linux kernel 2.0. That system still refuses 

to die running 24/7 since 2003. Meanwhile the keypad is defect, but I 

still can reach it via ssh... 

So, watch it running: 

http://df3lp.de/cgi-bin/hga22/show-hga22.cgi 

 

Peter, df3lp