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
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|