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Some clarification to my last post is in order.
The Isotron has some flat plate elements above and below the base of the antenna loading coil. This configuration can be compared to “top hat” base loaded vertical antennas.
A standard base-loaded antenna has a linear antenna current that is maximum at the bottom and zero at the top. A “top hat” antenna also has a linear current that is maximum at the bottom, but slopes more gradually to the top where it abruptly goes to zero. This increases the radiation from the 3-meter antenna. The Isotron probably has a somewhat better (but unknown) radiation level than a simple base loaded vertical, but I don’t think it could measure up to a real top-hat vertical.
Even in the ideal case that the Isotron would be able to achieve an antenna current profile that is constant from bottom to top, it would still not make a significant difference because, as we all know by now, an elevated 3 M antenna radiates mostly from the ground run, so minor differences along the little 3 M section at the top are insignificant.