While this is all solid, there's a LOT more to it than that. First, we
must understand the difference between impulse noise generated by arcing
on the power system, which is extremely broadband, and is radiated by
the wires connected to the source (the arc). Because it's an arc, it's
spectrum is theoretically infinite. But because the highest frequency
components are most strongly radiated by the wires close to the source,
the most effective way to find the source is at the highest frequency
where it can be heard. Lower frequencies are radiated more strongly by
longer sections of the power line.
Noise is often attenuated by transformers, which helps localize the
source a bit more.
Unfortunately, noise sources inside a house often DO go much further
than the house. They are mostly radiated as common mode signals on
wiring connected to the source. These are NOT usually impulse sources,
but rather electronic sources. They mostly fall into three categories:
1) clocks in microprocessor systems, which tend to be quite stable in
frequency and appear as a steady or pulsed carrier; and 2) switch-mode
power supplies, variable speed motor controllers that rectify the power
line, do almost no filtering of the DC, then switch that DC at high
audio frequencies (10-20 kHz is common) to produce a square wave, again
do only enough filtering to take the ripple out of the DC (and often
regulate it). While it is POSSIBLE (even easy using smart circuit design
and layout) to prevent the strong and rich harmonics of that square
wave, it is rarely done (or done very well) for most consumer products.
In general, these power-handling devices are free-running and rarely
frequency stable, and the poor filtering the DC produces an
amplitude-modulated "hump" of noise at harmonics of the switching
frequency that drift (because they're not frequency controlled by a
clock). This mush of noise is conducted as a common mode signal to
wiring to which it is connected, again as a result of poor design and/or
construction, and is radiated by the wiring (just like any other RF
current would do on any other antenna). THESE signals can often travel a
significant distance (thousands of feet) or even miles for strong
sources that produce large enough currents.
Now, noise CAN be radiated by wiring internal to products due again to
poor design, but THIS component of noise generally does NOT travel very
far because the wires are too short. Plasma TV sets are an example of
this -- noise is produced by the switched currents driving the display.
I've written a lot more about noise, and it's on my website.
k9yc.com/publish.htm
BTW -- what the EMC rules call common mode is the voltage between
neutral and ground, which is NOT what hams call common mode, and it's
not what radiates. Rather, common mode current can best be described as
the algebraic sum of all currents on a cable, and it's that common mode
current that radiates. This often happens when the "green wire" is not
properly connected to the shielding enclosure. EMC line filters external
to equipment won't do anything to kill this current -- it's on the green
wire, and the green wire goes right past the filter, which is between
phase and neutral and neutral and ground (green). The only way to kill
this current is a ferrite choke. For the HF bands, we need multiple
turns of the cable through a suitable core. Lots of details at the link
posted above.
73, Jim K9YC
First, think On 6/11/2018 3:20 PM, David Eckhardt wrote:
from a device located in a neighbors home over long distances before being
attenuated?
This is why its so difficult to isolate arcing or corona using RF
emissioins and walking the length of the power line. Generally sources
inside a neighbors home don't go much further than the house. However, if
they are strong sources, they will. The reason we do conducted emissions
in regulatory testing is that lengths of conductors in housing are good
radiators of RF energy below 30 MHz. So, we measure conducted emissions
that become radiated emissions once they couple onto the house wiring.
Conducted emissions are measured and regulated from 150 kHz to 30 MHz for
most home electronic equipment. The generator is usually not a good
radiator due to small size wrt wavelength.
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