[opendtv] FCC on broadband over power lines interference

From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Date: Fri, 17 Jul 2009 18:07:28 -0400
http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-09-60A1.pdf

This might be a "here we go again" moment.

The FCC is seeking comment on how to estimate the attenuation of
interfering broadband over powerline (BPL) signals, as a receiver is
moved away from the power line. Their current guideline says that 40 dB
of attenuation is to be used as the rule, below 30 MHz, when the
distance is increased by a factor of 10. They call this "40 dB per
decade." Looks like the ARRL petitioned to have this reduced to 20 dB
per decade, the FCC didn't budge, and now the courts got involved and
told the FCC to explain their reasoning. (BPL is superimposed on power
lines, at allowable frequencies from 1.7 to 80 MHz, by current FCC
rules.)

OFCOM made some measurements, which looked like something in the
neighborhood of 20 to 25 dB per decade was the answer, in real world
testing.

The issue at hand here is not so much to compute interference, but to
adjust the results of an interference measurements when the test
receiver cannot be located exactly as intended, e.g. for safety reasons.

Nevertheless ...

Take a point source radiating RF. It creates a field strength in a
sphere, which attenuates as the square of the distance. This is because
the surface area of the sphere, 4*pi*r^2, increases in proportion to the
square of the radius. So field strength, given as V/m at a given
distance from the radiator, will diminish as the square of the r
increase, because at that distance r, the field has spread over a larger
surface area.

So, for a 10:1 distance ratio, the field strength reduction is

20*log10(1 / 10^2) = -40 dB.

This is where the "40 dB per decade" rule originates.

Now we have a long powerline, e.g. traveling parallel to your house
along a roadway. Is it a point source? No. It is instead a line source.
The field strength surrounding line source is in the shape of a
cylinder. The surface area of a cylinder is 2*pi*r*l, where l is the
length of the cyclinder, and r is the radius. So in this case, field
strength is reduced in a linear relationship with the increase in r.

20*log10(1 / 10) = -20 dB.

Obviously, those trying to sell BPL solutions want the FCC to use a
criterion that is as lenient as possible, allowing them to transmit as
much power as possible. But it seems to be drop dead obvious that the
ARRL and OFCOM are on the right track, and that the FCC needs to justify
why they think a power line behaves like a point source? It looks like
for the time being, the FCC compromised by setting a 20 dB per decade
criterion above 30 MHz, and 40 dB per decade below 30 MHz.

Interference from buried power lines might be attenuated more, due to
absorption from the ground, but still, honestly, let's not create
another nuisance problem on purpose here.

Bert
 
 
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[opendtv] FCC on broadband over power lines interference

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