[RFI] Power Line Noise
k9ma at sdellington.us
Thu Apr 16 23:50:32 EDT 2020
In many cases, I think the current causing noise to radiate is flowing
in only one of the lines. For example, if a lightning arrestor were
arcing (very common), the current flows primarily in the one line it's
connected to and the ground wire. In the horizontal line, the current
flows about equally in both directions, so the far field largely cancels.
Another case I've seen often is arcing between a couple poorly bonded
pieces of hardware not directly connected to a power line. That current
is mostly capacitively coupled from an overhead HV line, flows through
the hardware, and either to a ground wire or is capacitively coupled to
ground or another line. If the ground wire is involved, the polarization
is likely to be mostly vertical. If between lines, perhaps mostly
horizontal. In that case, the wide spacing would certainly allow lots of
I believe near field for antennas is generally considered to be about
1/6 of a wavelength. That's only 11 feet on 20 meters.
Another observation that may prove useful: Poorly bonded hardware near a
single phase line is much more likely to be problematic than that near
three phase sets, unless it's much closer to one of the phase lines. At
any distance large compared to their spacing, the 60 Hz (or 50) fields
from the lines mostly cancel, so there's just less field to cause
arcing. (Discovered the hard way.)
On 4/16/2020 22:30, Jim Brown wrote:
> On 4/16/2020 2:32 PM, K9MA wrote:
>> The current from an arcing device like a lightning arrestor flows in
>> BOTH directions away from the source on the horizontal lines, so the
>> horizontal component largely cancels out. It's like the top of a "T"
> I'd buy that if the wires were close together, but NOT at the wide
> spacing typical of HV lines. Think about it -- twisted pair is FAR
> better (20-30dB) than "zip cord" at rejecting all forms of crosstalk
> and coupling, simply because interfering fields are close to one
> conductor than the other. That's a fairly small differential, but the
> two conductors in the AC distro system have far greater spacing.
> They're also typically 3-phase.
>> The radiation then mostly comes from the vertical ground wire.
> I'm good with this (and I've seen it), but for a different reason --
> it can be carrying the arcing of both conductors, perhaps from GFCIs?
>> Generally, I've found the HF beam heading to be pretty accurate for
>> more distant sources, perhaps because the vertical component is
>> attenuated more quickly.
> I agree with Mike that antenna arrays develop their directivity in the
> far field, and can be VERY different in the near field. Polarization
> of the RX antenna might be part of it.
> With the VHF tracker (135 MHz), I do find I have to
>> sometimes turn it vertically, but not consistently up close. I expect
>> the shorter wavelength has something to do with that.
>> In any case, the moral of the story is don't just look in the
>> direction your HF beam thinks it's coming from!
> I'm not suggesting that anyone in this thread might be making this
> mistake, BUT -- remember that antenna arrays are all based on spacing,
> phase relationships, and spatial relationships between elements as
> ELECTRICAL length. A tri-bander only exhibits it's design directivity
> on those three bands. Its directivity on any other band, including 2M,
> should be thought of as "random," like the infinite number of monkeys
> and typewriters. :)
> 73, Jim K9YC
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> RFI at contesting.com
k9ma at sdellington.us
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