LF: Re: 500 Khz - QSO

["James Moritz" <james.moritz@btopenworld.com>]

Dear John, LF Group,

----- Original Message ----- 
From: "John P-G" <gm4slv@sighthound.demon.co.uk>
> Mal's comments about G3XIZ and M0JXM does raise a question, in that the
> inability for XIZ and JXM to hear me isn't only to do with the sea-path
> vs. land-path difference in path-loss.
If you draw a line on the map from Shetland to Bedfordshire, you will see 
that the signal comes ashore after roughly 680km near Scarborough ... then 
it has to travel a further 240km across land to Beds. I made a rough 
estimate of field strengths, applying the "Millington method" formulae to 
the ITU ground loss data, assuming that the ground conductivity of the land 
path was 10mS/m (a reasonable average according to the conductivity maps), 
and an ERP of 0.5W. This came up with field strengths at G3KEV of +7dBuV/m, 
and at G3XIZ/M0JXM of -12dBuV/m, so a 19dB reduction in signal level due to 
the extra distance over land - Quite a big difference. At these fairly large 
distances, the MF field strength falls away rapidly due to the combination 
of relatively high ground losses on land, and diffraction around the earth's 
curvature.
Previously, I have measured a noise floor at my QTH just south of 
Bedfordshire of about -10dBuV/m in a 300Hz bandwidth, so GM4SLV would 
certainly be expected to be down in the noise here - in Scarborough it would 
be a good signal well in the clear with the same noise floor. I doubt if 
this is the actual, natural, noise floor - there are various wideband noise 
sources here I can never entirely eliminate, and when I operated /P on 500k 
earlier this year, the noise level at the field location was certainly 
significantly lower than at home. So I guess a quiet location for GM4SLV's 
RX could account for being able to receive the southern stations in 
Shetland, but not the other way round.
Cheers, Jim Moritz
73 de M0BMU