[RFI] Fwd: 20 Meter Interference

r55stan at gmail.com r55stan at gmail.com
Fri Feb 26 18:05:35 EST 2021

Don't forget to also turn off any battery backup power, such as uninterruptable power supplies many of us use for computers, WiFi routers etc.  The inverters in them and the devices they power will still be on.  I once wasted a few hours searching for RFI that was still there after all of my circuit breakers were off.  Since the breakers were off I first assumed it must be from a neighbor.  It ended up being Ethernet on CAT5 cables in my own house, which was running on the backup power supply.

Good luck hunting,

Randy KQ6RS

-----Original Message-----
From: RFI <rfi-bounces+r55stan=gmail.com at contesting.com> On Behalf Of K9MA
Sent: Friday, February 26, 2021 2:37 PM
To: rfi at contesting.com
Subject: Re: [RFI] Fwd: 20 Meter Interference

Yes, I discovered it by running the radio on a battery and turning off circuit breakers.

The culprit definitely was the wall wart. The cyclic pattern apparently was due to the device intermittently loading it.

Scott K9MA

On 2/26/2021 2:13 PM, Dave Cole wrote:
> Did you discover this during a home power off test?
> 73, and thanks,
> Dave (NK7Z)
> https://www.nk7z.net
> ARRL Volunteer Examiner
> ARRL Technical Specialist, RFI
> ARRL Asst. Director, NW Division, Technical Resources
> On 2/26/21 10:04 AM, K9MA wrote:
>> It turned out to be a wireless doorbell extender, right in the shack. 
>> Once again proving that one should check your own home first. I'm not 
>> sure if it's the wall wart or the device itself. It seems to have 
>> magically fixed itself after power cycling, but I'm sure it will be 
>> back.
>> I suspect the 1.1 second interval between noise pulses is the polling 
>> period of the device, when it draws current from the wall wart. If a 
>> ferrite core on the power lead doesn't fix it, I'll just unplug it 
>> when operating.  Who wants to be disturbed by the doorbell then, anyway?
>> 73,
>> Scott K9MA
>> -------- Forwarded Message --------
>> Subject:     Re: [RFI] 20 Meter Interference
>> Date:     Tue, 23 Feb 2021 20:25:09 -0600
>> From:     K9MA <k9ma at sdellington.us>
>> To:     rfi at contesting.com
>> The spectra images didn't come through, so here are the links:
>> http://sdellington.us/hr/RFI/20m_K9MA.jpg
>> <http://sdellington.us/hr/RFI/20m_K9MA.jpg>
>> http://sdellington.us/hr/RFI/20m1_K9MA.jpg
>> <http://sdellington.us/hr/RFI/20m1_K9MA.jpg>
>> 73,
>> Scott K9MA
>> On 2/23/2021 7:46 PM, K9MA wrote:
>>> I've recently started seeing this signal on 20 meters, probably 
>>> local, as it is present late at night when the band is dead. It is a 
>>> swept signal, typically starting at about 13.625, sweeping upward to 
>>> a sharp cutoff at about 14.125. The total sweep seems to be about
>>> 500 kHz. The sweep slows down near the upper limit and seems to 
>>> dwell there. Sweep repeats about once per second. It seems to be 
>>> amplitude modulated during the sweep, so it shows up as a series of 
>>> pulses on the waterfall display. While the frequency range seems 
>>> fairly stable, it does move around a bit. I can't tell whether the 
>>> frequency change is systematic or random.
>>> In the attached images, the blue trace is the peak signal, 
>>> accumulated over many sweeps. The yellow one is the actual signal, 
>>> without averaging, which I caught near the upper end of the sweep.
>>> The waterfall shows the pulsed modulation. Resolution bandwidth is 
>>> about 440 Hz.
>>> Any idea what this might be?
>>> 73,
>>> Scott K9MA
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Scott  K9MA

k9ma at sdellington.us

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