RF Skin Depth in the Ground vs. Frequency for Given Soil CharacteristicsAuthor: R.J.Edwards G4FGQ © 18th March 1999
Alternating currents in the ground are attenuated exponentially as depth increases, like Exp(-A * Depth). Where A is an attenuation factor depending on soil resistivity R, permittivity K, and on frequency.
Skin Depth is defined as that at which the current magnitude has fallen to 36.8 percent of that at the surface. A layer of soil deeper than 4 skin depths can be considered to be of infinite thickness with respect to its top surface.
R in ohm.metres is the resistance measured between opposite faces of a 1-metre cube of the material. K is the relative permittivity obtained by measuring the capacitance between opposite faces of the cube. Therefore soil has an impedance which may be represented by a resistance in shunt with a capacitance. Impedance magnitude decreases and its negative angle increases with frequency. Values of R and K can be considered to be independent of frequency from DC to UHF.
R and K strongly depend on the moisture content of the soil and on the amounts of the various salts dissolved in it. Soils with high agricultural fertility have the lowest impedances and vice-versa. Dry rocks have very high impedances.
|Terrains and Soils||R
|Oceans, salt seas, well away from river estuaries||0.22||81|
|Agricultural plains, moist, rich, highly fertile loam||30||22|
|Pastoral, low hills, rich soil, lawns, gardens, trees||80||15|
|Flat country, rivers, streams, marshes, wooded, gardens||130||12|
|Medium hills, forested, bushes, flowers, some clay, stones||170||12|
|Medium hills, heavy clay soil, weeds, grasses, mosses||250||13|
|Steep hills, well drained fine rocky soil, sparse vegetation||500||14|
|Flat, dry, sandy, some weeds, grasses, as for coastal country||500||10|
|Unpolluted freshwater lakes and rivers, reeds, fish, insects||1000||80|
|High-rise city streets, industrial areas, no vegetation||1-5,000||5|
|Arid sand deserts, cacti, little animal or bird life||5-20,000||3|
Skin depth vs frequency for copper is computed for interest and comparison. Enter frequencies between 10 Hz and 100 MHz. Measurement units are Kilo-Hertz. Results are tabulated over a frequency range of 10-to-1. The unit of conductivity, Siemens/metre = 1/Ohm.metre, is sometimes used.
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