Cb radio manufacturers make these up and call them a magnet mount antenna, they would put a brass fitting in the center hole and a fiberglass ant. screwed into the top and a coax fastened into the bottom. Used to make them in 4",5",6" and 6 1/2" depending on the height of the antenna. In fact some of the magnets are stronger than others, the best guaranteed to stay on the vehicle at 100 mph.
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