I think that there are items that are known to be a problem. Power line
adaptors that shove broadband signals down the mains to save the user
running a couple of wires are the worst thing, as although they have notches
at the ham band frequencies, they do not for most of the rest. They do bty
their mere design, send rf signals down the mains supply. However although
this contravenes the british electromagnetic interference specs, the EU
directive that allows these devices gives them a waiver.
The switch mode supplies for things like laptops, mobile phone chargers and
small domestinc devices like answering machines, ar often made in china, and
many have no parts inside to snuff out the spikes they send down the mains
wiring. They probably did at the start, hbut the Chinese are always looking
for cheap ways to do things, and who ever opens the devices after the first
run is made?

Plasma tvs use some form aof hf pulsing of the cells or pixels to
counteract the pulsating effects at low brightness they are often known for
in early samples, and this shoves out huge amounts of wide band noise.



The problem of course is that the devices to null out what used to be a
single source no longer really work when the transmission is coming from
mains wiring in an adjacent house.

Brian

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"Michael Black" wrote in message
ample.net...
On Sun, 25 Apr 2010, Brian Gaff wrote:

Well, it used to be so quiet here at my little hilltop in Greater London,
but of late it seems every form of annoying noise has come to this little
place.

And yet, in December of 1921, when Paul Godley went over to England to
receive US stations during the Transatlantic Test (the one that showed
that shortwave was useable, amateurs having been "exiled" to "200 metres
and down" because nobody believe those frequencies were useful), he
originally set up in London, only to move to a more rural location because
reception in London was awful. Too much interference, both man made and
natural.

What's really happening now is that there is so much stuff. Even forty
years ago, the average household had a tv set or two, a radio or two, and
maybe a stereo of some sort. (And it drops off even more the further back
you go.) Now, every household is loaded with electronic equipment,
computers, VCRs, digital clocks, microwave ovens, and on and on. It's not
so much that any one item is "noisy", it's that there is a collective
noise created by all that mass of electronic equipment. It also creates
a density issue, the noise generated by such equipment is often quite low,
but if you've got a good receiver next to one, it will cause interference.
Move the offending item a bit away from the receiver, and the interference
drops off fast, indicating it isn't a strong "noise". But the more stuff
you have in a room, the harder it is to get away from any of the units.
If I move a couple of feet away from this computer and monitor, that moves
me within a couple of feet of my DVD recorder and VCR.

Yes, some of it is noisier than it should, and obviously all of it
is often too noisy for good reception. But likely for much of the
equipment, reasonable steps have been made to quiet things down, and
it would take a whole lot more effort and money to make it even quieter.

And then you'd be stuck with the reality that the neighbors have their
own collection of electronic equipment that generates noise too.

Michael