Re: NEC-LIST: compute time & RAM allocation

Hi Jim-
thanks for the quick reply.
We must have different manuals. The one I have is split into two
parts: users manual (part I) and program description (part II). They
are authored by Gerald Burk, have a publication date of January 1992,
and ID# UCRL-MA-109338. Is the version you have available on the web
somewhere?
Cheers
-Billy

On Jul 30, 2008, at 1:44 PM, Jim Lux wrote:

> 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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