> 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 _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
