Richard Clark wrote:
On Sun, 1 Oct 2006 16:16:44 -0500, "Richard Fry"
wrote:

AFAIK, neither NEC 2 nor NEC 4 from any source will show a composite
elevation pattern over a defined ground at a specified distance to include
the space wave and the surface wave in a single display. One must merge
them using his/her own understanding and resources.

This seems to ramble well off the earlier path encapsulated by Denny
and apparently subscribed to by Owen, in regard:
But to bring us back to the major complaint which seems to be that the
Nec engine doesn't model the last few degrees over ground very well, so
that the zero angle is discarded by the software... Richard seems on a
mission to prove the NEC engine wrong - well, I agree, the NEC engine
does have limitations for low angle signals which is why the authors
have installed an angle cut off... Per Richard's citations
To which I object to, to no notice (I wasn't surprised however).
. . .

I'm not aware of any such problem with NEC, or any "angle cut off"
intentionally included in NEC(*). As Richard says, EZNEC has no problem
extending analysis of any kind -- near field, far field, or (in pro
programs) far field with surface wave, down to ground level. The value
of zero for far field sky wave (that is, far field at distances beyond
which the surface wave has decayed to essentially zero) at zero
elevation angle for horizontally polarized waves over any ground and for
vertically polarized waves over non-perfect ground is a rigorously
correct result. It follows directly from calculation of ground
reflection coefficients, the simple formulas for which you can find in
Kraus' _Antennas_ and many other references.

I'd be very interested if such a limit exists, and would be very
grateful to anyone who could point to the place in the NEC code where it
occurs, or provide an example of a model producing a result where its
effect is evident. I strongly suspect that whatever effect is being
seen, it's due to misinterpretation or other causes and mistakenly
attributed to a limit which doesn't exist.

(*) There are many places in the NEC and EZNEC code where protection is
provided against divide-by-zero errors, which limits internal
calculations and perhaps a minimum or maximum field strength or angle.
However, these usually limit a minimum divisor value to something on the
order of 10^-10 to 10^-20 or so, beyond the point at which a calculated
result is significant or, often, even valid.

Roy Lewallen, W7EL