Characteristics of a Buried Radial Wire from DC up to Radio FrequenciesAuthor: R.J.Edwards G4FGQ © 2nd February 1998
The earth electrode system of a radio transmitter using a vertical mast as the radiator consists of a number of buried wires radiating from a common connecting point directly under the antenna. For high efficiency power radiation the effective series resistance looking into the earthing system must be as low as possible. This program models a single isolated radial from DC up to 30 MHz.
Program input data consists of length, depth and diameter of the wire, local soil resistivity (R), relative permittivity (K), and the radio frequency. For average soils: R = 100 ohm-metres and K = 14. For poor, dry, sandy soil: R = 400 and K = 6. For fertile agricultural naturally-moist loam: R = 25, K = 20.
Analysis is that of a lossy-dielectric transmission line. As line length and overall attenuation increases, input impedance converges to the line's characteristic impedance. A further increase will then have no effect on input Z.
At 0.22-waves line-length there may be a minimum value of input resistance but without sharp resonance due to high loss. Relative velocity along the wire is much smaller than in free-space. Input reactance is always +ve due to wire inductance. A series capacitance will tune it out.
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