On 06/03/15 23:02, Spike wrote:

Imagine a short rod vertical aerial not connected to ground, for the
(say) 160/80/60/40m bands, as might be found in a typical /M set-up, fed
with RF energy and operating over ground of average conductivity.

Many thanks to all who took the trouble to reply, with input ranging
from from the uncouth through the unhelpful to the deeply technical. The
modelling results and the graphs of the surface-wave propagation that
were provided will likely prove very useful for another propagation
project currently under study here.

One fact that has become apparent is that ground conductivity maps that
assign a value to region-wide areas are not to be trusted - there are
sometimes quite severe changes in local conductivity, and these could
encompass the ground that affects the radiation pattern from one's
antenna. However, models have now become sophisticated enough to
incorporate these into their predictions; the difficulty lies in
obtaining reliable conductivity figures for one's location, especially
those of poor conductivity where earth currents can run deep in the
soil. Although this isn't strictly an 'antenna' issue, it is
nevertheless fundamental to LF/MF/HF operation and highly pertinent as
to how the antenna contributes to the station performance.

--
Spike

"Hard cases, it has frequently been observed, are apt to introduce bad
law". Judge Rolfe