Re: NEC-LIST: Low angle radiation?

On Feb 17, 10:58am, Dan Bathker wrote:
> Subject: Re: NEC: Gif for Low angle radiation
>
> . . .
>
> I would indeed like to know your final conclusion about your
> pattern. Apparently I missed a communication from Cebek that you
> referred to.

A quiz? I guess that's fair. The true test of my understanding of
something is my ability to explain it. So, at the risk of embrassing
myself, I'll try to paraphrase some of what I've learned. (I promise
to not peek at my notes 8) )

I was trying to figure out if I had botched my NEC-2 model, or had
exceeded an NEC-2 limitation. In reality, it was neither and both.
My model was fine and NEC-2 had done its job. I didn't know how to
interpret the results.

NEC-2 computes far field radiation patterns through a ray tracing
technique that combines the direct signal with its ground reflection.
The angle of reflection is important. My antenna was modeled over a
relatively small radial cliff. The high angle radiation pattern was
based upon ground reflections from the top of the cliff. Likewise,
the low angle radiation pattern was based upon ground reflections from
below the cliff. Right at the point where the traced rays peek over
the edge of the cliff and start reflecting from the lower medium
rather than the higher one, there is a sudden change in the signals
that combine in the far field. That corresponded exactly with the
discontinuity in my elevation plots.

In real life, such a sudden change in reflections does not occurr.
Most notably, NEC-2's ray tracing approach ignores diffraction at the
cliff edge. The results are correct on either side of the
discontinuity (by how far, I'm not sure), but the discontinuity itself
gets smoothed out in real life.

The moral is: Understanding WHAT NEC-2 does, and HOW, is crucial to
understanding the results.

I hope I passed the quiz.

Thanks again to everyone for their help.
Received on Thu Feb 17 2000 - 21:35:07 EST