Re: "High Frequency Therapeutic Apparatus".
It's not unusual to hear the rasping of what I assume are those industrial RF heaters around 13.5 and 27 MHz on the HF bands- I imagine that they can go rather a long way under the right conditions but I don't suppose that they do QSL cards even if they could be identified....
Another last outpost of thermionics, then- I imagine that their simplicity, and robustness under difficult-to-define loading, must count in favour here.
Colin
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