[RFI] Powerline noise question

[Don Kirk]

Hi Richard,

I tried sending this early today but my attachments appeared too big, so
here it is with reduced file size attachments.

I looked at your RFI audio recording with my oscilloscope connected
directly to the audio out on my computer.  Using FFT on my scope the
predominant frequency component is indeed 120 Hz as you initially said.

I'm going to stick my neck out and post the following comments which might
turn out to be totally incorrect but please remember I'm definitely in the
camp that says it does not matter what device is generating the RFI (I
preach this all the time), what's important is to first locate the property
where the noise is originating and then you can figure out what device is
generating the RFI.

I looked at the actual waveform of your RFI recording in the time domain,
and assuming we are looking at power line RFI what I believe I'm seeing is
that at times there is an arc only 1 time per each half cycle of the sine
wave, sometimes 2 arcs per each half cycle of the sine wave, and
occasionally 3 arcs per each half cycle of the sine wave, and then there
are times (example 0.2 seconds) that there is no arcing at all.  In the
past I used to do this kind of waveform analysis a lot but with my improved
DFing techniques and availability of SDR receivers I now hardly ever revert
to it because it really does not save me time (but it's scientifically fun).

I have attached two pictures showing screen captures of my oscilloscope
looking at very brief sections of your RFI recording in which the yellow
trace is your RFI, and the blue trace is a 60 Hz sine wave that I fed into
the second channel of my scope so you could see the approximate
relationship in time between a 60 Hz sine wave and the arcing pattern.
Please understand my 60 Hz sine wave is not lined up perfectly but it
should still help you visualize what I believe the situation is.

Note: My technique looking at your recording is not perfect because the
bandwidth of the receivers filter as well as recording and playback
equipment is critical regarding frequency response and therefore what you
see, so take what I have said above with a big grain of salt.

Once again I'm pretty darn confident you are dealing with power line noise
and I would therefore pack AM VHF DF gear or at least have it available at
your house so once you get close to the source you can switch to VHF from
MF/HF in case it really does turn out to be power line noise, but with a
lot of attenuation and if out in the clear you should be able to get very
close to the suspect pole using HF.

73, and keep us posted on what you find assuming the RFI is still present
when you get your new DF hardware (sometimes even power line RFI disappears
over time).

Don (wd8dsb)


On Sat, Dec 31, 2022 at 5:18 PM Don Kirk <wd8dsb at gmail.com> wrote:

> Hi Richard,
>
> Great recording.
>
> Definitely does not sound like electric fence and I would initially say
> it’s poweline noise (listening on my phone) and not a regular pattern at
> all but when I get some free time in the next few days I will use some
> software to look at it.  Based on the sound and based on it going away when
> it rains I would say poweline noise with very high confidence but let’s see
> what other experts say.  Should be pretty darn easy to get very close to
> the suspect pole with the portable flag before going to VHF to determine
> the actual pole if it is power line noise, just stay away from overhead
> lines when determining direction and attenuation is your friend.  Even if
> it’s not poweline noise you should be able to find that bad boy easily
> because it’s not a constant noise (has variability to it which really helps
> distinguish it from other sources of noise).
>
> If possible look at the signal using a SDR display / pan adapter before
> heading out to determine what bands and frequency you hear it well, and
> this will be helpful to your decision making about what frequency or
> frequencies to use while out DFing it.
>
> 73,
> Don wd8dsb
>
>
>
>