Re: Interesting "source" of RF interference

Robert Lorenzini

I wonder if it could be a bad MOV in the strip?

Bob - wd6dod

On 12/31/2021 2:23 PM, Ken Sejkora wrote:

Thanks for the insights, Dave.  Following your advice, I placed some clamp-on ferrites onto the extension cord and moved them to various positions along the length of the cord.  The RFI hash never completely disappeared, but was attenuated the most when the ferrites were at the plug-end of the cord (nearest the power strip), and had little or no effect when placed at the receptacle end of the plug.  I should point out that in none of the cases I described was anything plugged into the extension cord, not during the initial tests or during the later tests.  The NiMH charger was not in use during these tests.  In fact, when I plugged the NiMH charger into the receptacle end of the cord, the RFI hash dropped in signal strength, presumably from the added load.


I left the unterminated cord plugged into the power strip, and began coiling it up on itself going toward the power strip.  The RFI hash decrteased substantially as the coil neared the power strip.  I can’t say for sure, but I suspect the reduction may have been due to the formation of a “cable choke” with the looped wires, but it may have been due to the decreased length of the extension cord effectively acting as an “antenna”.


Going a bit farther, I made up an AC plug with about 10-feet of single wire attached to one of the prongs.  I wanted to test if the RFI was being introduced on/from the neutral line or the “hot”/live line.  I could easily reverse which line the extended wire was connected to.  Although a small amount of RFI was introduced onto the waterfall, the orientation of the plug made no difference in RFI strength, and it was considerably less than when an equivalent length of dual-line extension cord was plugged in.  It is as though the paired/parallel strand extension cord was acting somewhat as a ‘tuned’ circuit coupling the RFI into or out-of the power strip.  I hooked up a multimeter and the extension cord measured out as about 150 pF if the receptacle-end was not shorted.  The maximum RFI hash seemed to be around 22500 kHz, and with extension cord plugged into the power strip the signal in the vicinity of the hash measured -115 dB dBm compared to a noise floor of -131 dBm, for an overall SNR of about 15 to 16 dBm.  Without the extension cord, the signal strength was about -130 dBm compared to a noise floor of -137 dBm, or SNR of about 7 dBm.  Looking at it a different way, plugging in the extension cord raised the RFI hash signal strength from about -130 dBm to -115 dBm, while increasing the noise floor from -137 dBm to -130 dBm.


I can’t say for sure whether the extension cord is coupling some RFI *OUT OF* the plug strip and ‘broadcasting’ out into the ether to be picked up by the SDR, or if the extension cord is picking up the RFI and coupling it back *INTO* the AC power to be passed through the PC-to-SDR interface.  Regardless of the mechanism, it is worth noting that plugging extension cords into an AC outlet can cause some strange things to happed, and are worth considering as a potential source of RFI when using an SDR or other receiver.


Thanks again Dave.  Have a great weekend and best of 2022.  73


Ken  -- WBØOCV


From: Dave (NK7Z)
Sent: Friday, December 31, 2021 11:39 AM
Subject: Re: [SDR-Radio] Interesting "source" of RF interference




I would put a high Z, ferrite suppression choke on the extension cord at

the plug end as a test.  If the RFI goes away or is reduced, then as you

suggested, the cord is probably a transmitting antenna for the RFI...


It is possible your not adding enough choking impedance to the main

antenna feedline, and it is still part of the main antenna system, and

picking up the radiation from the extension cord.  I might move the

feedline around and see if the RFI level is changing as a test of this.


If the extension cord test reduces the RFI level, I might add a few more

suppression devices to it, and test again.  If the RFI is reduced, then

I would conclude the extension cord is radiating the EMI from the

charger, and my feedline is picking it up.


73, and thanks,

Dave (NK7Z)


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