[RFI] Introduction - power line noise

Roger (K8RI) k8ri at rogerhalstead.com
Mon Jun 25 21:08:45 PDT 2012

On 6/25/2012 11:16 AM, Jim Brown wrote:
> 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.

Another point is some guy wires may be relatively hot with RF at some 
frequency and produce no RFI, BUT can under the right circumstances.
At one time I had to local repeater in my garage.  Originally it worked 
fine, but I noticed some desense at times.  The tower was 90' with guys 
at 3 levels.  It had a tribander at the top with the repeater 
antenna/vertical on a mast above that.  The guy lines were broken up 
with insulators  IIRC the top guy broken up  on the order of about 1', 
6', 10, 20', 30' and progressively longer segments until they reached 
the insulator just shy of the turnbuckle.

With some experimenting I found I could go out with an 8" screwdriver 
and just rub it on the guy wire closest to the turnbuckle, but on the 
side away from ground. It would produce so much noise it'd wipe out a 50 
watt mobile from a mile away.  The same thing can be true of a 
neighbor's fence with a little rust, but it'll only bother when a strong 
signal is present. An electric fence can excite another fence which will 
then cause the RFI, so they can be very difficult to trace down to the 
actual source.

Most stations will not hear their own RFI as their receivers are muted 
while transmitting.  This can be particularly interesting when you have 
a number of hams within a mile or so, operating a contest, or chasing 
DX. You not only have to give each other room, but several QRO stations 
in relatively close proximity can create a LOT of RFI to each other.  
Those door bell transformers and old, unused (but connected) TV preamps 
can really do a number when in the near field of a QRO station and 
generate RFI that seems unrelated to the frequencies of the nearby stations.


Roge (K8RI)

> 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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