Spike wrote:
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.


While your local earth conductivity may well vary from that for you region,
in the scheme of things, especially if the path in question includes a
transit of sea water, to suggest it will play a significant role is
somewhat bold. To see this, look at the relative numbers I gave earlier for
North America.

Local conditions will, of course, impact antenna efficiency.

I suggest you do some more thinking before you challenge the work of the
eminent people who have studied this area and published papers etc.