Noise from electronic devices can have that impulse nature, for which
noise blankers are effective. The AM audio often has harmonics of 60
and/or 120 Hz, as does that of power line noise. Most often, the
clearest difference is the RF spectrum: broadband for line noise,
harmonics of something below about 100 kHz for electronic devices.
Switching supplies, etc. usually has a rather broad peak centered on
each harmonic, maybe 5 kHz wide. Their spectra are usually asymmetrical,
and often they drift around a lot. There may be more than one such
source, in which case it's harder to figure out the fundamental frequency.
The narrow band harmonics from routers, etc, are entirely different
animals. They're narrow and stable, clustered around harmonics of some
fundamental. They're not on exactly the same frequency because of
crystal tolerances. When you turn your beam, you often see some go down
and others go up. Those are probably your neighbors' routers.
73,
Scott K9MA
On 1/2/2021 9:45 PM, Al Kozakiewicz wrote:
I had two intermittent noise sources that the NB was marginally effective on.
One was the dishwasher, the other a flat screen TV. Not plasma, but an older
LCD fluorescent backlit model.
Al
AB2ZY
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of K9MA
Sent: Saturday, January 02, 2021 10:36 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Weird RFI at remote location
On 1/2/2021 8:32 PM, Ignacy Misztal wrote:
1. The noise is originating from one location because otherwise NB
would not be able to blanket it.
I don't think so, unless you are using some sort of noise cancellation
with two antennas etc. Noise blankers can be very effective against
impulse noise, typical of line noise. However, they don't work well if
there are strong signals within a few kHz. While it's a good diagnostic
sometimes, a noise blanker that works on a quiet band probably won't be
useful during a contest.
Besides its impulse nature, line noise is usually quite broadband. While
it may affect one band more than another, there generally isn't much
variation in strength within a band. This is very unlike noise from
switching supplies, motor drives, etc, which usually show peaks at
harmonics of the switching frequency, often 10-100 kHz. As others have
suggested, the difference is most readily apparent with a spectrum
display. It's a lot harder to discern by tuning around looking at the
S-meter, though it can be done.
Because I live in the city surrounded by overhead power lines, I have
way more experience with line noise than I'd like. Hopefully that will
be of some use to others.
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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