Re: NEC-LIST: compute time & RAM allocation

At 10:32 AM 7/30/2008, you wrote:
>Hi-
>Is there a way to calculate computation time and/or memory
>requirements in advance of simulation? I'm particularly interested in
>the requirements for electrically small geometries of variable extent,
>near earth-ground, and using the Sommerfeld-Norton approximation.

Yes and no..

The NEC user's Guide tells you what the computational requirements
are (which things go as the square, cube, what-have-you of the number
of segments)..See section V. Execution Time (roughly page 121) of the
manual. I assume you've looked at this (based on your comments below)..

>Backstory:
> After discovering that NEC simulations fail when wires overlap, I
>wrote a script that creates my desired input geometry from up to 100k
>wires. Ultimately, I'm interested in simulating the affect on antenna
>response due to the addition of a ground screen. Some of the possible
>candidates for ground screens have dimensions well below the
>definition of electrically small; that is, the grid spacings are much
>less than 0.1*wavelength. I've read a bit of the NEC manual, and I
>realize that the approximations made by NEC are less accurate in this
>regime. (My thinking is that inaccurate simulation yields more
>information than no simulations.)

> The problem that I'm having is that I don't completely understand the
>memory and time requirements for the method of moments calculations.
>As I understand it, time for LU decomposition should go like
>(segments)^3. Since the number of segments is large -- the number of
>segments is large even if the wires aren't split into smaller segments
>for matrix inversion-- I would expect this matrix inversion to be the
>dominant time requirement. However, there appears to be some larger
>effect due to the extent of the geometry. For example, an antenna over
>a square ground screen with sides of 3m, composed of about 70k wires,
>takes several times 10^3 minutes, using 2GB of RAM for the duration of
>the simulation. For the same grid spacing, a screen larger in extent
>by a factor of ten simulates in less than eight minutes, and uses tiny
>amounts of memory. For calculating time of computation and memory
>allocation, I'd really like to get some information on the finer
>details of NEC, and it's benchmark testing.

There are some approximations used in NEC when segments are "far"
away from each other.. perhaps that's the source of the difference.
e.g. "APPROXIMATE INTEGRATION EMPLOYED FOR SEGMENTS MORE THAN 1.000
WAVELENGTHS APART"

Jim

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