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One 15 meter antenna if all antennas were connected to the same transmitter, perhaps. But, if 5 transmitters each had it’s own separate antenna I don’t see how that would be considered as one system antenna exceeding the 3 meter rule.
Even so, it would seem if one transmitter with 5 antennas each 3 meters long in close proximity to each other (less than a few inches apart) and fed from a common point, that should be considered as one antenna 3 meters long. For instance, 5 – 3 meter long insulated wires such as a multi-conductor cable with common feed point and open at the other end would still be one-3 meter antenna.
I had an idea for a wide bandwidth antenna whereby 3 elements of different lengths (each within the 3 meter rule) fed from a common point, one being cut to the center frequency and the other two cut slightly above and below the center frequency to yield a bandwidth of 20 or 30 kHz. This would be similar to the ham radio “fan” antenna or perhaps a cousin of the “log periodic” antenna. That would allow the sidebands (where the money is) to radiate efficiently.
But then I wonder if the power divides among them, is the total field the sum of the individuals or of the single greatest field.