Practical Implications of Capacitance, Ground Conductivity and AC Frequency

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Wherever there is direct electrical contact between line and ground, there will also be signal coupling; both forms of coupling will be reduced if the ground conductivity is low, e.g. dry sandy soil. This is not just because the higher resistance reduces the current flowing via electrical contact; it also reduces the capacitance effect, because while one side of the capacitor (the line) is a good conductor, the surrounding surface presented to it is a poor conductor, so that a much lower ‘charge’ can be stored at any moment than the equivalent volume of wet soil. While the conductivity of a point of contact between line and ground is not affected by frequency, the fact that the effective impedance of the line to the signal is a combination of resistance and capacitance (as well as a little inductance, which can be ignored at this point) means that it is always easier to couple a high frequency signal into a given line than a low frequency one.

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