Dear LF Group,
I monitored 137.7894kHz again last night; nothing was visible until
0330, and the signal level then was quite low. The SNR was quite
good in the period before dawn. Good signals were observed at
0330-0400, 0450-0510, 0545-0720. The last period was the best,
as noted in the other reports. I found the noise level was pretty low
throught the night.
As far as mains noise goes, I did once try some experiments with
decoupling the house mains supply. On the whole, this was
unsuccessful, some combinations of filtering and grounding actually
made the noise worse. I concluded that the main problem was that
it is very hard to get a decent ground connection, and that
decoupling the mains noise to the ground connection simply means
that the ground connection is at the mains noise potential now.
There always seemed to be significant noise currents flowing in the
connections between different parts of the grounding system,
begging the question which one really is ground? I think for this
kind of filtering to work, a very good ground mat would be needed,
as previously remarked.
Most packaged mains filters are built with EMC regulations in mind,
and since most of these only apply to frequencies above 150kHz,
and the limits are more relaxed at the LF end of the range, the
filters are not very effective in the LF range.
I have had more luck with, firstly, eliminating what sources of noise
I could (going around turning things on and off revealed a
particularly noisy compact flourescent bulb), and secondly,
experimenting with different receive antennas in different positions.
The position of loop antennas can make a radical difference to the
noise they pick up.
Cheers, Jim Moritz
73 de M0BMU
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