"Richard Harrison" wrote
Fortunately, the length of radials does not need to be increased in
direct proportion to wavelength below the broadcast band as skin effect
varies with the square root of the frequency so as we go lower in
frequency we need to increase length of the radials by the square root
of the wavelength to keep the resistance of our contact constant.
____________

Here is a quote about this from RADIO ANTENNA ENGINEERING
by Edmund Laport:

"The distance from the antenna at which returning ground currents are of
such a low value as to be negligible is of the order of 0.5 wavelength."

At least across the AM broadcast band 530-1700 kHz, Laport, Terman, Kraus,
and Balanis show _no_ dependence of the lengths of buried radials with
frequency -- except, of course, that radials used with lower frequency
systems need to be physically longer to reach the desired radius from the
monopole, in free-space wavelengths.

The FCC uses a computer program (linked below) to calculate the groundwave
inverse distance field at 1 km and 1 mile from a MW monopole, based on the
radiator height and the number/length of buried radials.

On-line users of this program can enter their own system parameters to see
their effects on the radiated field. The program does restrict entries to
the minimum values acceptable to the FCC for commercial AM broadcast
stations.

== NOTE: The FCC program applet has no entry block for frequency.

Here is the link. The applet starts at the bottom of that web page.

http://www.fcc.gov/mb/audio/bickel/figure8.html

RF