you may well be right about the short ranges. The most simple picture looks only at the geometric path difference groundwave and one-hop skywave. For 180 km distance and 74 km reflection height this is 53 km, which happens to be 1.5 lambda (perfect antiphase) at 8.5 kHz. That is probably the reason for the midday minimum we are experiencing between Heidelberg and Nuernberg, and better propagation early after sunrise. Going to either 6 or 12 kHz would fix it (albeit my railway QRM is much more dense at lower frequencies). At the bottom of http://df6nm.bplaced.net/VLF/2006/vlf_DHO_dualfreq.htm there's a set of  curves for inphase (red) and antiphase (blue) frequencies versus distance.

 

All the best,

Markus

 
 > Paul's SNR seems to be about 8 dB better then mine, despite 
 > almost fivefold distance. 
 
The distance may actually be too short.   For medium ranges, 
say 100km to 500km the phase and amplitude can vary wildly 
due to the number of competing modes (or rays if you prefer 
the ray model).   When ray modeling I have to use over a dozen 
rays, sometimes 20 to get a realistic looking diurnal. 
 
 > QRG 8270.1 Hz 
 > 5 characters, 20 second symbols, 8K19A 
 > Start 6:00:02, end 8:50:42 
 
Copied 'HELLO'  Eb/N0 = +0.4dB constant reference phase 
and carrier S/N: 15.02 dB in 97.7 uHz 
 
A good snowfall here this morning so a lot of static from 
the E-field antenna. 
 
-- 
Paul Nicholson 
--