[RFI] Introduction - power line noise

Jim Brown jim at audiosystemsgroup.com
Mon Jun 25 08:16:56 PDT 2012

On 6/25/2012 4:14 AM, N1BUG wrote:
> But so far I always get more noise when pointing toward the
> stuff on top of a pole than when holding the antenna nearly touching
> a ground or guy wire.

The way to think about this is to realize that most noise we hear is 
radiated by some antenna into our antennas, NOT conducted into our gear 
by the connection of our gear to the power line.  Power line faults can 
generate noise current on the power lines themselves, and some can 
generate noise current on the ground wire. Like any other antenna, these 
wires will radiate more efficiently on some frequencies than others. If 
the fault puts its arcing on the ground wire, it will radiate, and a 
receiver near it will hear it. That ground wire also acts as an RX 
antenna. I've gone poking for 160M noise in my neighborhood with a 
talkie that works on 160,  and have head strong CW signals when I 
coupled the talkie's RX loopstick to a pole's ground wire.

The reason that VHF and UHF are so effective for isolating the fault to 
its source is that the higher frequency current is attenuated as it 
moves away from the source, so wiring nearest the fault gets most of the 
VHF/UHF noise current. At lower frequencies, longer sections of the line 
carry more of that noise, so the source is much broader, and like any 
antenna, will have peaks and nulls of current related to wavelength.

Another troubleshooting hint.  There's lots of RF noise that is 
generated within someone's premises (ours and our neighbors). Some of it 
may get past a improperly (or poorly) grounded service entrance, be 
carried out onto the power company's wiring, and radiated by the power 
line.  Most, but not all, of this sort of noise comes from some sort of 
oscillator or digital clock, and will have periodic characteristics -- 
that is, carriers every 10-20 kHz that are modulated by noise, and that 
drift around. The noise modulation and drifting are intentional, to get 
by FCC regs that limit the strength of noise on any one frequency by 
spreading out into sidebands and moving the carrier around.  Switching 
power supplies of all sorts are a common, and very nasty, source of this 
sort of noise -- battery chargers, 12V and 24V supplies for low voltage 
lighting, wall warts for electronics, perhaps even doorbell transformers 
(if you go to an electric supply store looking for a transformer for 
these uses you will get a switching power supply with no name on it, and 
it will be a NASTY noise  generator.  They will call it an "electronic 
transformer," and it will be the only thing small enough to fit in the 
backbox for the lighting equipment.  REAL transformers with sufficient 
current rating are FAR too large to fit.

Fluorescent lighting can also be a nasty noise generator, both from 
arcing within the tubes radiating directly from the fixture, and from 
electronic ballasts (more switching power supplies) that run them.

A spectrum display (like a panadapter) can be quite helpful in 
identifying these electronic sources.

73, Jim K9YC

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