[RFI] power lines, contd.

Hare,Ed, W1RFI Hare,Ed, W1RFI" <w1rfi@arrl.org
Tue, 16 Jan 2001 13:10:20 -0500

Hi, Pete,

The answer to the question is found in two part of the FCC rules. First,
power lines are incidental radiators -- they don't deliberately generate RF
signals, but incidentally do so as a side effect of their operation.  The
rules have no specific radiated or conducted emissions limits for incidental

What applies is a very general requirement to use good engineering practice
in the design and operation of incidental radiators and a requirement that
they not cause actual harmful interference.

The key word is "actual."  The FCC would generally take the position that
merely hearing this on your AM radio as you drive past the lines is not
necessarily harmful interference.  Even HF mobile may not be enough to
convince the FCC.  The FCC generally considers repeated disruption of
desired communications to be harmful interference.  To date, most of the
cases being worked by ARRL or the FCC involve S9+ level noise at ham

The other difficulty here involves the high-voltage employed.  The FCC has
taken two stances -- first, that "corona discharge" from such lines is a
natural phenomenon. They generally do not require utility companies to do
much to correct such noise.  On the other hand, they do ask that utility
companies repair correctable defects on high-tension lines.

Unfortunately, in some cases, the work has been painfully slow.  There are
just so many factors affecting what utility companies are willing, or able
to do.  This is very much compounded by miscommunication all around.

If you call their customer-service folks, you may or may not get the point
across that this is something they need to address. If a service call is
generated, it may or may not get to an RFI troubleshooter; many line repair
folks just do not have any knowledge in this area.  Even a "troubleshooter"
may or may be fully experienced in finding the noise.  I have seen some go
to locations with S7 80-meter noise (often indicating a source that could be
as much as a mile away) and point a VHF Yagi around and decide there is no
noise. Then, a work order may or may not be generated, and the crew may look
at the insulator and decide it is fine, and does not need to be replaced.
All this time, the RFI guy figures the case is solved. And, of course, no
one calls the ham to find out, or calls the ham to tell him or her that the
repair was rescheduled due to an emergency across town, leaving the ham
entirely in the dark (groan!)

The ARRL RFI Task Group discussed this very issue at an in-person meeting it
had in November.  The conclusion is that our "power-line" guy, Jody Boucher,
WA1ZBL, is working on an RFI article we are going to try to get published in
one of the power-line trade journals.

Reradiation is common, especially on HF.  A noise source (arc, motor, etc)
will couple into the lines differentially, then be radiated to some extent
by the lines.  The line is a transmission line, even at HF, with an SWR, and
the peaks and nulls on the line will radiate differently, showing peaks an
nulls as you drive down the street with an HF or AM receiver.

A good rule of thumb:  lower HF will get you to a neighborhood, upper HF
will get you closer, VHF will usually isolate to a pole and the use of an
ultrasonic detector or RF sniffer (mounted on a hot stick) will isolate the
source on the pole.

Don't mess with the poles -- loose hardware can be both noisy and dangerous.

Ed Hare, W1RFI
ARRL Laboratory Supervisor
225 Main St
Newington, CT 06111
Tel: 860-594-0318
FAX: 860-594-0259
Internet: w1rfi@arrl.org
ARRL Web: http://www.arrl.org
ARRL Technical Information Service: http://www.arrl.org/tis/

-----Original Message-----
From: Pete Smith [mailto:n4zr@contesting.com]
Sent: Tuesday, January 16, 2001 9:52 AM
To: rfi@contesting.com
Subject: [RFI] power lines, contd.

What is considered to be a normal, acceptable level of noise radiation from
high voltage transmission lines?  I was out looking for my 80m noise source
yesterday, and noticed when approaching a paired set of lines -- one EHV
500+ KV and one smaller, on wood poles -- that the noise level on my car
radio (set to a vacant AM channel) started to rise a half mile away and was
really intense near and under the lines.  I also found two old transformer
poles that were particularly noisy, well above the general level.  The
lines they are on are perpendicular to the transmission lines.  Is it
likely, since they are "bathed" in the noise field from the transmission
line, that they are simply reradiating it?

Fionally, about 3/8 mile away is a run of three relatively new poles
parallel to the transmission line.  My car radio indicated one, maybe two
of these as strong radiators.  Because they are parallel to the
transmission line, though at a considerable distance, is it likely this,
too, is reradiation?  How can I tell?

I've referred to the ARRL RFI book and Loftness's book, and found no
specific answers to these questions.  I'm not anxiously to call on the
power company on a false alarm.


73, Pete N4ZR
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