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Hi Ermi –
Re your post “Tubes work efficiently with Part 15 AM because a short grounded monopole can provide a high-impedance load to the transmitter. A tube can work well with a high impedance load because it can be operated with high plate voltage and low plate current.”
But the reason that the short antenna has very high feedpoint impedance is due to its capacitive reactance. When that reactance is offset by the proper inductance in a loading coil, then the transmitter sees an external load consisting of the ~ 1/10th of an ohm of antenna radiation resistance, plus the r-f resistance of the coil and ground system. Probably the net impedance seen by a typical Part 15 AM transmitter ranges from 15 to maybe 50 ohms, j 0 for a resonant 3-meter antenna system, which isn’t really “high impedance.”
Are you thinking that a Part 15 tube transmitter can work into a 3 meter monopole without the use of a loading coil, because the tube tx can operate with high plate voltage and low plate current? Does it not still require a non-reactive net load for efficient power transfer?