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

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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