[TowerTalk] Weird RFI at remote location

[K9MA]

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 at contesting.com] On Behalf Of K9MA
> Sent: Saturday, January 02, 2021 10:36 PM
> To: towertalk at 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 at sdellington.us