> The NEC surface wave includes low-angle fields well above zero degrees
> elevation that do not appear in a NEC far-field plot, and they are in fact
> space waves (see link below).
And that's the piece that probably not been underscored in this discussion.
Some have probably concluded that the surface wave plot in NEC only includes
ground wave conduction.
An interesting exercise I went through a couple days ago (using NEC/4.2):
distance was varied between 1km and 10km from a 160m 1/4-wave vertical
radiator over a 60-radial field. I then modified the ground conductivity
between poor and very good while observing the surface wave plot with
changing distance. Even over average ground, and unlike the far-field
analysis, the surface wave and the space wave are reasonably close out to
about 4-5 km. That's roughly 25-30 wavelengths on 160m. Over poor earth,
you start seeing the effects low-angle field strength attenuation much
earlier as is expected.
So, just how much distance is required to effectively launch a 160m field
into the ionosphere to the point where the ground surface and conductivity
is no longer relevant? By 25 wavelengths, can we safely say that "it's
launched" absent some other factor like a mountain range? How about 5-10
wavelengths -- is that enough?
Clearly, the traditional far-field plots are at odds with the NEC surface
wave plots (that also include the space wave as Richard mentions) in that
unless the ground is extremely poor, the far field plots are not accurate as
far as NEC is concerned. Yes, the far field plot does show the lobe from a
ground-mounted vertical radiator coming down closer and closer to the earth
as ground conduction improves but only gets there with super-conductive
ground like salt water, leaving us to conclude that with normal earth
conductivity there's no field at all below the far-field curve. A surface
wave plot over salt water does show extremely close field strength over very
long distances between the far field and surface wave as one might expect.
So, if the NEC surface wave tool is accurate, we've not been looking at the
entire picture when considering only far-field analysis.
Paul, W9AC
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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