Half-Wavelength Vertical Antenna & TunerAuthor: R.J.Edwards G4FGQ © 12th May 2005
This program analyses the performance of resonant 1/2-wavelength antennas. The antenna may be vertical or sloping. It is matched to a 50-ohm feedline by an L and C parallel tuned circuit which is connected between the base of the antenna and ground. The coax feedline is tapped down the tuning coil. The ground connection can be via plates, rods or buried radial wires which need not be extensive. A half-wave vertical antenna requires only a few, short, ground radials to obtain a high radiating efficiency. This is due to the high input impedance of the bottom-end antenna feedpoint.
Input data includes antenna height from which the 1/2-wave resonant frequency is calculated, taking into account "end effect." Performance data is then calculated for the resonant frequency to which the antenna should be 'pruned'.
The setting of the tuning capacitor is also input to the program from which the coil inductance is calculated. Coil loss is smaller as the capacitor setting is is reduced. When choosing capacitor setting take into account its minimum and maximum settings and the need to tune to a lower frequency than the resonant frequency of the antenna.
The program automatically takes residual antenna reactance and coil self-capacitance into account. Wire diameter is approximately 0.707 of coil winding pitch.
Coil loss also reduces as the physical size of the tuning coil is increased. Coil Q increases in direct proportion to it's physical size.
Computed results apply to an insulated lattice mast and, approximately, to an inverted-L antenna and a simple sloping wire of the same length. But to use a 1/2-wave antenna at frequencies much below resonance requires a different tuner arrangement using a variable inductance in addition to a variable C. The advantage of a relatively poor ground system is then lost.
Unless a choke is inserted in the coax line an additional ground connection will be acquired over the line itself. This may be considered to be an advantage at the expense of allowing RF to enter the station.
A link coupling round the coil, with a few more turns than a tap, can be used instead of a coaxial choke to prevent RF from entering the station.
For the performance of a system of shallow-buried ground radials in various soils refer to program Radials2 by the same author.
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