Sam Dellit wrote:
> just wondering if anyone has modelled the development
> of wave tilt for a skywave signal as it passes over and
> in the vicinity of a sea/land interface. i presume NEC
> has this capability.
You can set up a model for such a thing in NEC (for example, EZNEC allows
definition of two different ground media in the vicinity of an antenna), but
the result may not be accurate. I once tried to model a situation like this
in EZNEC, but the results did not make sense. Roy, W7EL, (author of EZNEC)
and I had some correspondence on this, and to make a long story short, Roy
concluded that the modelling results are not accurate when the two-media
ground model for highly differing grounds is used and the boundary of the
media is in the near-field of the antenna.
The problem occurs in how NEC handles field reflections at the boundary of
two very different ground media, where complex effects can occur. NEC
calculates the effects by modelling the incident fields as simple rays that
reflect off the ground and become reflected rays. The coefficient of
reflection depends on the ground characteristics and the polarization of the
field. However, the ray model is only a simplified approximation of what
actually happens. Reflection is really a result of the incident field
generating fields in the reflecting medium, and these fields in turn
producing the "reflected" field.
To quote from W7EL:
"The [ray] approximation works quite well when the ground is homogeneous or
when the boundary is far from the antenna, but not for highly differing
ground conductivities close to the antenna."
73, John W1FV
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