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From: Alan G3NOQ
Date: 01 Apr 2003
Time: 03:35:18
Remote Name: 20.138.254.2
After reading Jack Belrose's interesting article I wondered why this antenna uses a lower plate, in other words why not place the lower plate on the ground and replace it by radial wires? Would this make any essential difference to the way the antenna operates? I don't think so as the antenna could hardly gain an advantage by using a lower plate of restricted size. . . . . Modelling the antenna in the simplified form of an inverted cone with height 1.865m and diameter 4m above a perfect ground plane gave an input impedance of 0.095-j350 at 1.6MHz, consistent with a short vertical radiator of height 1.45m and capacitance 280pF, not very different from Jack's results I think. Over perfect ground the antenna was of course 100% efficient because I assumed no losses, but the bandwidth was only 600Hz at 2:1 SWR. To pad out the bandwidth to the minimum requirement of 10kHz for AM broadcasting would need added resistance, reducing the efficiency to 6%. This seems low compared to the usual requirements. . . . 73s Alan