Total posts : 45366
On January 2nd, 2008 Ermi Roos says:
If antennas shorter than 1/10 wavelength are used, the RF field strength at the base of the antenna is very high, and the high electric field interacts with the soil, causing the ground resistance to be higher than for lower field strengths. The usual remedy for this situation is to shield the soil in the vicinity of the base of the antenna with sheet metal.
Much good information in your post, Ermi, to which I’ll add (being picky)… earth resistance is a function of soil characteristics rather than radiation from the monopole. Ground system losses near the monopole can be higher for very short antennas, but that is due to the relatively higher current flowing through that earth resistance from the greater induced r-f voltage there (greater I^2R loss).
This is also true for monopoles around 1/2-wavelength high, which have very high base voltages. Typical broadcast practice in such cases is to bury a 48 x 48 foot copper grid mesh centered at the tower base, which is connected to the usual buried radials. Another approach is to intersperse the 120 x 1/4-wave (or longer) buried radials with another set of buried radials about 50 feet long.
These techniques don’t act to shield the soil near the monopole, but to improve the conductivity encountered by the high r-f earth currents induced in that region.
Probably the average Part 15 AM operator isn’t too interested in this minutia, but some readers here may be.