[RFI] Power Line Noise

K9MA k9ma at sdellington.us
Thu Apr 16 17:32:18 EDT 2020

I think I just learned something interesting. I'm in the city, 
surrounded by 14 kV overhead lines. I went out today to track down a 
noise source, which on the HF bands appears to be coming from the east. 
What I found, with a VHF tracker and an ultrasound tracker, is that it 
appears to be coming from two lightning arrestors across the street to 
the NORTH.  (The ultrasonic tracker points to one particular arrestor on 
each of two different poles.) So why does it seem to be from the east on 
HF? (Yes, I'm absolutely sure it's the right source.)

It has to be the polarization. My tribander is, of course, horizontally 
polarized, so it's most sensitive to vertical polarization off the side. 
But, something I never realized before, one should expect line noise to 
be mostly vertically polarized. 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. The radiation then mostly comes from the 
vertical ground wire.

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. 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!


Scott K9MA

Scott  K9MA

k9ma at sdellington.us

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