N6GN says:
“…….In studying noise sources and in particular the coupling of these sources to the KiwiSDR I’ve come to realize that one of the dominant coupling mechanisms of signals that degrade receiver noise floor is via common-mode signals. These can exist almost everywhere in the system; they exist on the antenna
itself, within ground systems, on the feedline and other connections to the antenna and through the KiwiSDR itself. I’m presently of the opinion that for most amateur stations and even most KiwiSDRs that the dominant source of unnecessary SNR degradation, QRN and QRM, is due to common mode currents. Near-field coupling to a variety of types of local sources, interference which is attenuated as a function of distance faster than the inverse-square field of a radiated plane wave emanating from a distant source dominates a majority of amateur stations.
Although resignation to the existence of “all those noisy digital devices” and the mindset that interference has to be accepted seems to be the prevalent wisdom within the hobby, I’ve found that this is not the case. I have come to believe that the vast majority of amateur receive systems are not limited by either propagated noise, which would be the desired condition, or by radiated noise from local interferers, as is commonly espoused, but by coupling to near-field sources and in particular common mode noise…..”
source:
https://forum.kiwisdr.com/uploads/Uploader/5f/492f44efb5a272d715c5219da67b44.pdfThis pdf has further thoughts on this. (The active antenna he mentions was in the Oct 2018 issue of QST, not Sep 2017 as he states in the pdf.)