Topband: Fwd: radals fer 160m vertcal

[Paul Christensen]

> These elevated systems are readily modeled using NEC-2.  However the
> radiation patterns shown by a typical NEC far-field analysis do not
> accurately show the fields actually "launched" by them, or by any vertical
> radiator with its base near the earth, because they do not include the
> surface wave.

As Richard points out, the surface wave plot in 4Nec2 is a key analytical 
tool when focusing in on what's happening at very low elevation angles. 
Otherwise, when only observing the elevation profile, one can easily 
conclude that little or no field strength is generated near the horizon. 
Even then, the only time the elevation profile appears accurate for 
ground-mounted vertical radiators is when placed over a super-conductive 
surface, like salt water.

After purchasing a NEC/4.2 license in December, I had approached Jerry Burke 
of Lawrence Livermore National Labs for an answer on this issue.  He's the 
author of the NEC code going back to the 1960s.  My hope was that NEC/4 
would have shown the far-field vertical profile descending down to the 
surface in a way that would capture the result of the surface wave plot.  In 
other words, a way to combine the results into a new vertical profile. 
Jerry's response was that it was not a NEC limitation, but rather a 
limitation in the user software.  Programs like EZNEC and 4Nec2 are nothing 
more than a user interface for the NEC data engine. These programs call upon 
the NEC engine, then NEC is responsible for the computation. After the 
computation, it hands over the data to EZNEC or 4Nec2 for presentation.  I'm 
sure this is an over-simplification of the process but it's probably an 
accurate high-level view of what's happening.

So, until one of the authors writes code to join the two processes, we're 
left with analyzing the two graphs independently (as shown by Richard).  I 
imagine that the code needed is not a trivial task to write and unless 
there's a high demand for it, that may never happen.  Even so, the surface 
wave calculator is still a great tool.  We just need to know that the 
far-field vertical elevation profile is not enough to get an accurate result 
near the horizon with base-fed vertical antennas.

Paul, W9AC