The one-meter "rule of thumb" I mentioned was for radials under a
40-meter vertical. Double that for 80-meters and quadruple it for
160-meters.
Using data from the table in chapter three of the ARRL Antenna Book, if
90 radials (each 1/4-wave long) are used, the spacing of radials at the
perimeter of the circle would be about 2.9 meters on 160m. If only 16
radials are used the table says they need only be 0.1 wavelength long,
which is a spacing of about 6.5 meters on 160m. Data in the table
assumes a 1/4-wave vertical radiator over "average" conducting soil.
>From that table, the spacing aqt the ends of the radials varies from
about 3 meters to 6.5 meters, with the wider spacing at the perimenter of
the radial circle applicable to systems with only 16 radials.
To quote from the ARRL Antenna Book, "The reason that short radials are
sufficient when few are used is that at the perimeter of the circle to
which the ground systems extends, the few wires are spread so far apart
that most of the return currents are already in the ground between the
wires rather than in the wires themselves. As more wires are added, the
spaces between them are reduced and longer length helps to provide a path
for currents still farther out."
73, de Earl, K6SE
|