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The concept of using a “massive ground” wire, metal roof, tower/flagpole etc as an effective r-f ground is operationally useful to Part 15 AM.
But as a fact of physics, true r-f grounds cannot radiate. All conductors carrying r-f current and that are not buried in the earth, will and do radiate — whether “massive” or not. Therefore the use as such conductors as an r-f ground is functionally invalid.
To illustrate, suppose a Part 15 AM transmitter and 3-m whip are mounted at the top of an unused, triangular broadcast tower having 3 ft face widths, 200 feet in height. The tower was designed to be series fed, so its base and guy wires are insulated from the earth. The Part 15 transmitter chassis is connected to the top of the tower by a “short ground lead” not greater than 15 inches long.
The 200 foot tower certainly is “massive,” but it is NOT an r-f ground. This system comprises an off-center fed dipole, and even if the transmitter was battery powered and used an on-board program source (no wires leading down the tower), its radiation efficiency would be much higher than if the transmitter and 3-m whip were mounted at earth level — even if the earth-based system used a buried r-f ground of zero ohms.
Clearly an exposed, massive conductor does not constitute a functional r-f ground, no matter how useful it may be to Part 15 AM users.