Regarding Observation 4185285 …
right around 157 seconds my RPi4 (for satnogs station #2051) was unplugged to troubleshoot RFI. Then RF noise went away for nearby station #2134. 
Time to order a box of ferrite beads from the interweb 
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Ouch. My two stations are 4 feet apart… I’ll try out the ferrites…
The question is, is it the Pi or the USB C power block for it.
Well, disappointed (or pleased?) to find that both power supplies had ferrites on them already. I ended up snapping another on at the PS end. Didn’t do squat, according to the latest observations. Meh, worth a shot.