Dear LF Group,
At 17:09 22/03/2003 +1200, Bob wrote:
So my suggestion for selecting frequencies for long haul DX between amateur
stations is to avoid multiples of 50 or 60 Hz (50 Hz mains being used in
some regions, 60 Hz in others). 137.7100 kHz is an example of being clear
of 50 Hz harmonics, whereas 137.7000 kHz is dicing with the 2754th harmonic
of 50 Hz, plus or minus some tolerance for the national grid.
Not sure about ZL, but here in the UK the 50Hz nominal mains frequency
drifts around quite a bit - perhaps a few tenths of 1%. Seen on a fairly
coarse resolution spectrogram, the harmonics give a series of wavy lines
spaced by 50 or 100Hz, which drift enough to give overlapping coverage
during a period of a few minutes. But with very narrow resolution
spectrograms, this just adds to the "wide band" noise level, rather than
producing well-defined spectral lines, because the frequency does not
remain within one FFT box for long enough to produce a line. Most of the
stable spectral lines on LF at my QTH seem to be Loran, but a significant
number are not. I also see a very faint line at 137.700kHz sometimes -
wherever it comes from, it must be crystal controlled to have sufficient
stability.
Cheers, Jim Moritz
73 de M0BMU
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