Re: NEC-LIST: NEC-4 problem with salt water ground

Well, I think the problem is in the segmentation of the wires and
numerical convergence. In the original problem (Roy's), wires 1, 2
and 3 were segmented too finely in terms of electrical length (i.e.
relative to the wavelength), and wires 4 and 5 were segmented very
much larger. The MoM technique utilized in NEC is particularly
sensitive to extreme non-uniform segmentations (this is already
addressed in the NEC manual, I think), especially when the wires are
placed in close proximity to a lossy ground plane.

Furthermore, NEC 4 uses improved basis function formulations and
Green's function calculations for the far-field (these are also
already addressed in the manual), and will thus yield better accuracy
over NEC 2. Using the input parameters below, I obtained a reasonable
impedance of 2.48602E+01 3.95022E+01 Ohms from NEC 4 ( but I have not
done a convergence analysis for a converged solution, however).

CE
GW 1,7,0.,-15.5448,15.24,0.,0.,15.24,.0008138
GW 2,7,0.,0.,15.24,0.,15.5448,15.24,.0008138
GW 3,7,0.,0.,15.24,0.,0.,.1524,.0008138
GW 4,21,0.,0.,.1524,0.,50.292,.1524,.0008138
GW 5,21,0.,0.,.1524,0.,-50.292,.1524,.0008138
GE 1
FR 0,1,0,0,1.83
GN 2,0,0,0,81.,5.
EX 0,3,7,0,1.,0.
XQ
EN

Hope this helps.

Regards,
Kin Sze

At 04:04 1/22/2008, Tim Molteno wrote:
>If you run your example in nec2++ you get the following warnings
>
>ROM2 -- STEP SIZE LIMITED AT Z = -1.34142E-03
>About the above warning:
>Probably caused by a wire too close to the ground in the Somerfeld/
>Norton ground method. Execution continues but results may be inaccurate.
>
>Perhaps neither answer should be trusted.
>
>Cheers
>
>Tim
>
> > I recently ran into an example of an apparent problem with NEC-4 using
> > salt water ground characteristics (conductivity = 5 S/m, rel.
> > permittivity = 81), where it gives a negative input resistance for a
> > single-source model. NEC-2 gives a believable positive resistance for
> > the same model. Here's the model:
> >
> > CE
> > GW 1,21,0.,-15.5448,15.24,0.,0.,15.24,.0008138
> > GW 2,21,0.,0.,15.24,0.,15.5448,15.24,.0008138
> > GW 3,21,0.,0.,15.24,0.,0.,.1524,.0008138
> > GW 4,21,0.,0.,.1524,0.,50.292,.1524,.0008138
> > GW 5,21,0.,0.,.1524,0.,-50.292,.1524,.0008138
> > GE 1
> > FR 0,1,0,0,1.83
> > GN 2,0,0,0,81.,5.
> > EX 0,3,21,0,1.,0.
> > XQ
> > EN
> >
> > The result using NEC-4 is an impedance at the source of -61.4 + j621
> > ohms. NEC-2 reports a believable 14.8 + j105 ohms.
> >
> > The problem with NEC-4 is evidently the combination of highly conductive
> > ground and the low (0.0009 wavelength) radial wires, since raising the
> > radials or decreasing the conductivity restores apparently valid
> > operation.
> >
> > I recall from some past experience and correspondence that NEC-4 isn't
> > nearly as tolerant as NEC-2 of extreme ground conductivities. But it was
> > some time ago and I haven't been able to locate the information. Can
> > anyone shed some light on what guidelines should be followed to avoid
> > this problem?
> >
> > Thanks,
> > Roy Lewallen

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