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
radiation.
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.)
73,
Scott K9MA
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"
antenna.
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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Scott K9MA
k9ma@sdellington.us
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