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The same method used for evaluating continuously-loaded helical antennas can be used for analyzing an inductively top-loaded antenna. I am not satisfied with the results I get with NEC-2 (the resonant frequency is higher than expected); but since PhilB likes his result, he might consider evaluating a coil 12 inches in diameter, 12 inches high, with 60 turns of #18 wire along its length, on top of a rod 106 inches high, using his NEC-2 program. This gives a total height of nearly 3 meters for the inductively top-loaded antenna.
A polyethylene bucket may be an actual low-loss coil form for a real top-loading coil. I expect the actual resonant frequency of the antenna I just described to be near 1.6 MHz, but NEC gives me a resonant frequency of 2.1956 MHz.