Hi Ed,Thanks for the additional information, including the two links you provided. I find the methods for coupling/decoupling the relatively low power LF carrier channel to the HV transmission lines particularly interesting. I found the following link which has a good primer on PLC:
electricaltech.in/wave-trap-coupling-capacitor/
I wonder if PLC RF is sufficiently contained in one HV transmission circuit so it doesn't bleed further into more distant, unintended parts of the grid. Obviously for the intended use--controlling specific fault trip devices--you'd want the signal from A to go only to B (or maybe to B + C), which suggests one should only hear PLC carriers from fairly "local" grids (which still could be several hundreds of miles I suppose). But considering that HV transmission lines could also act as really long RF radiators, with lengths relevant to wavelengths of LF frequencies, I assume some undesired radiated RF beyond the HV transmission line must occur. Maybe these are the much fainter LF carriers I hear? On the other hand, if one had two 180 degree out-of-phase PLC carriers running along two parallel HV lines, they might act like "open wire feeders" with any radiated RF canceling each other out, so no net overall RF radiation from the HV transmission lines. Fun to think about.
Finally, can an average person (say, an LF DXer/LWCA member) ID these carriers via their frequencies? Is there a public listing, or are PLC frequencies known only to utility companies and held as closely guarded secrets? Earlier in this thread I joked about PLC DX'ing after the last NDB disappears, but I do wonder if one could ID specific PLC transmitter locations/grid nodes as sources of specific LF carriers heard.
73, Bruce WA1HGJ