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From: Sam Dellit VK4ZSS
Date: 14 Mar 2001
Remote Name: 188.8.131.52
G'day to all the Antennex Antenna Theory Forum gang
I have been away from this stuff for far too long, but I can recall some of the relevant factors.
The 5/8 vertical (grounded monopole) is a favourite with am broadcasters and generally called an anti-fading radiator. For a grounded vertical monopole (or an elevated vertical ground plane monopole). The horizontal gain increases very slightly as antenna height electrically increases from low to 1/4 wave (in practice there is a substantial increase in this range due to much reduced earth losses), the horizontal gain continues to increase steadily up to and reaching a maximum at 1/2 wave, then commences to very slowly fall. there is negligible reduction in horizontal gain up to 5/8 wave, but two very desireable attributes are obtained. it is difficult to match an exact 1/2 wave antenna as terminal reactance is swinging between capacitive and inductive, but mainly the terminal impedance resistive component is quite high (1000 ohms plus depending on the electrical thickness of the antenna), the 5/8 wave antenna is consistently capacitive and of moderate terminal impedance (few hundred ohms). finally in the am broadcast environment, on a clear channel a station's night-time coverage is limited by self-fading (interference between groundwave signals and skywave signals). as the distance from the transmitter increase the groundwave signal steadily decreases while the skywave signal is relatively constant or at least a much slower rate of decrease. at around 50 miles radius the 2 signals are typically about equal and massive fading occurs because of the rapidly changing phase and amplitude of the skywave signal and the combination with the groundwave signal. above 1/2 wave a second lobe appears at high angle in the vertical radiation pattern and there is an associated radiation minimum (null) between the secondary lobe and the main lobe. as the height increase the minima angle with respect to the vertical increases. at around 5/8 wave the minima angle corresponds typically to the takeoff angle for skywave at the fading radius and the reduced radiation in this direction effectively pushes the fade free radius out a further 20 or 30 miles. i have in the back of my mind also that the horizontal gain even increases slightly going from 1/2 wave to 5/8 wave due to the reduced overall non-horizontal radiation
sam dellit vk4zss