I have long had a suspicion that some fraction of the power in surface
wave must be "converted" to skywave as the surface wave encounters
discontinuities in the ground medium (both physical geometry and
dielectric parameters). Some folks I know from work were doing research
on low-loss dielectric waveguides for microwave and millimeter waves
(somewhat analogous to an optical fiber for RF). The degree to which
these dielectric waveguides won't radiate despite not having a
conductive outer shield is a function of how smooth the bends are and
the degree to which discontinuities are minimized.
Another way to look at this is to ask the question - how would a
vertical perform if it were installed on a perfectly uniform ground of
average conductivity that conformed to the ideal geoid as compared with
a more realistic installation - hills, mountains, lakes, and other
abrupt changes in characteristics of the surface wave medium? In the
former case, would the surface wave contribute less or more to the
skywave power at low angles than in the latter case?
73, Mike W4EF......................
On 10/3/2012 9:05 AM, Richard Fry wrote:
The elevation patterns of vertical monopoles over real earth has been
discussed in recent threads here
(http://lists.contesting.com/archives//html/Topband/2012-10/msg00140.html).
The common belief based on NEC far-field elevation patterns for those
conditions shows little relative field at low vertical angles, and
zero field in the horizontal plane. However the surface wave must be
included in a complete analysis of monopole performance, because it
contributes substantial low-angle radiation that will reach the
ionosphere to generate skywave service under the right conditions.
Below on this topic is part of an e-mail exchange of a few months ago
between Gerald Burke of Lawrence-Livermore National Laboratory and me,
and is quoted with his written permission. As many will recognize,
Mr. Burke is co-author of the software engines used in NEC computer
programs.
This text applies to NEC surface wave plots attached to my e-mail to him.
Hello Mr. Burke -
Would you expect the fields at elevation angles of 1 to 10 degrees
in these plots to continue on to the ionosphere, and under the right
conditions be reflected back to the earth as skywaves?
R. Fry
The low angle 1/R fields should reach the ionosphere, although
perhaps not
accurately predicted by NEC, since it does not include the effects of
earth
curvature and the ionosphere.
Regards,
Jerry Burke
LLNL
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