A buried radial wire behaves as a very lossy transmission line.


The soil surrounding the wire has a very low insulation resistance.


Because of soil permittivity and water content, the line capacitance is also
much higher than the free space value.


The velocity of propagation along a buried radial wire can be as small as
1/12 of the free space value.


This makes nonsence of cutting radial wires to particular resonant lengths.


Also, due to the high loss in the soil, the attenuation of the current along
a buried radial wire is very high. There is little point in having a radial
wire longer than about 8 dB because its input impedance, as a transmission
line, ceases to decrease for longer lengths. The input impedance simply
converges on the line impedance Zo.


For average soil characteristics, at the lower HF frequencies the optimum
length of a buried radial is of the order of 1/10th of the free-space
wavelength. But it varies widely.


Hence the old adage - if you have any wire to spare then increase the
number of radials rather than increase their length. But in any case forget
all about the magic number of 120. Stop at 8, 16 or 32, or when the
strength of RECEIVED signals stops increasing.


For a crude estimate of the performance of a single buried radial wire,
download program RADIOETH.


For the effect of multiple rods, plates and radials, download program
EARTHRES.


For the performance of an inverted-L antenna above a system of ground
radials, download program ENDFEED.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
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