Thanks all for the replies with detailed information, and also the links, which provide a treasure of information. It looks like the nanoVNA will be extremely helpful in looking at the snap-on ferrite cores - I bought a whole assortment of them from Amazon last year when I was fighting an RFI issue - trying to prevent ingress of RF from my transmitter into a Tivo DVR that kept rebooting. The cores never seemed to have much of an effect. I have a suspicion that they are not the right material for the low frequencies involved (3.5-14 MHz). .. and back then my only tool was whether they worked or not...very unsatisfying. But now...with the nanoVNA I will get to find out what the ferrite cores can or cannot do - hooray!
