Model and Predict Helically Wound Vertical AntennasAuthor: R.J.Edwards G4FGQ © 2nd August 1997
This program models and predicts the performance of a helically wound vertical antenna, mounted immediately above a ground plane, top-capacitance-loaded with a vertical rod or whip. The antenna is designed to be fed between its base and the ground plane operating at its 1/4-wavelength resonant frequency.
Input data consists of length and diameter of the helical coil former and the top rod or whip. By adjusting the number of turns on the helix the antenna can be resonated at any desired frequency as is done in practice with any suitable and available wire gauge. Fine tuning is done by trimming loading rod length. A warning is given if wire diameter exceeds wire pitch as set by the number of helical turns and the height of the helix.
The top capacitance loading may be omitted. The helix may be long and thin, or very short and fat - a lumped loading coil! A wide range of HF applications is therefore catered for. Accuracy of results falls off when the loading rod (it may be a wire) is greater than about 1/10 wavelength. The height of the helix can be as great as a 1/4 wavelength but this occurs when it has only one turn. It has then been reduced to a simple quarterwave vertical rod, wire or mast.
Small efficient antennas inevitably have high Q with limited bandwidth. On the MF and LF bands normal AM and SSB operation may not be possible: CW only.
When the feedpoint resistance is less than 50 ohms an impedance match to a 50 ohm feeder can be obtained over a band of frequencies simply by connecting a fixed capacitor between the feedpoint and ground. This results in an increase in resonant frequency which may be corrected by adding four or five turns to the helix or by increasing the length of the loading rod. The computed resonant frequency is the value before connecting the capacitor.
The most uncertain parameter is the effective HF resistance of the connection to earth or ground-plane. When the ground-plane is the steel roof of a road vehicle insulated from ground the loss resistance may be between 3 and 12 ohms being smaller for the larger vehicles. For an earthing system consisting of about six long buried radial strips, values from 5 to 30 ohms may be expected. Loss resistances up to several hundred ohms may occur using one earth plate or a heavy rod in poor dry soil. A square wire-netting earth mat with sides equal to twice antenna height of H metres would have a loss resistance crudely 20/H ohms when in intimate contact with compacted ordinary garden soil.
Large diameter wire or copper tube, using fewer turns per metre, reduces power loss in the antenna itself. This can be done without increasing overall height only by increasing the helix diameter. Ultimately the efficiency is limited by earthing losses. Without an increase in height there is little to be gained by increasing wire diameter when wire loss is already less than ground loss.
Note: See The Behaviour of a Single-Layer Solenoid Coil as a Transmission Line for a new 1/4 wave helical antenna design perspective.
Carefully observe measurement units when entering and reading data.
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