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[quote=kk7cw]…And since the model 200B does need a radio frequency ground system to work, it is legal under Part 15 for AM.[/quote]
Did you intend to write “does NOT need” a ground?
In any case, here is a paste from the Isotron website about their MW AM antennas:
Antenna should be mounted on a metal mast with a maximum size of 1.5 inches. The mast can be ground-mounted with guy wires, mounted to a wooden pole, or mounted to a tower leg. Antenna should be mounted as high as possible for best performance.
This suggests that some, or even most of its gain could be supplied by radiation from the conductor and/or metal mast leading to true r-f ground below the surface of the earth.
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