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Larry, N8KU wrote:
"I want to construct a 5/8 wave ground plane for 24 MHz.----Even the ARRL antenna book hasn`t been much help." At 1/2-wave, reactance nearly is zero and begins repeating as from zero.. With a 5/8-wave, you have a 1/8-wave connected to the feedpoint through a reactance repeating 1/2-wave line. A 5/8-wave antenna will have nearly the same reactance as a 1/8-wave. Value of the reactance depends on the characteristic impedance of the antenna and this depends on length to diameter ratio. Resistance of a vertical antenna increases with its length until it reaches nearly 1/2-wavelength where it peaks and starts to fall. Sharpness of this resonant peak depends on length to diameter ratio of the antenna. You might expect a radiation resistance of between 50 and 150 ohms. Reactance may be between 200 and 1200 ohms. It depends on how skinny the antenna is. Fat conductors have smaller impedance variations (a lower Q). My impedance numbers come from Capt. Paul H. Lee, USNR, K6TS`s "Vertical Antenna Handbook". 5/8-wave vertical ground-planes were once popular on 27 MHz and often used an autotransformer at the base to step up the impedance from the 50 ohms of the coax. Best regards, Richard Harrison, KB5WZI |
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