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

Roy,

The limits will depend on how close a wire is to the ground and the
configuration. We have run into problems before in NEC-2 with a wire
over seawater at VLF. In that case it was fixed by shrinking the 1
wavelength radius of the Sommerfeld interpolation grid to put more
points over the small electrical size of the model. That cannot be
done easily in NEC-4, but your model is not too small. Also have
had trouble with a VLF antenna with a top load, where the top load
had to cancel a large reactance. That does not seem to be the case
here.

Another possible problem is the horizontal wires close to ground
joining with a vertical wire. You run into that with a radial wire
ground screen just above ground. The fix is to use short segments at
the junction, on the order of the height above ground, so that charge
can distribute the way it needs to. I modified your model as shown
below to use short segments at the junction. It changed the NEC-4
results, but R still goes negative for high conductivity. Have not
tried comparing the two models in NEC-2.

A comparison (below) of NEC-4 and 2 for the model with short segments
at the junction shows that NEC-2 looks reasonable to 5 S/m
conductivity, while NEC-4 goes really bad around 0.3 to 1.0 S/m. The
problem may be that NEC-2 uses a 2D interpolation grid for field with
source and evaluation points above ground, which is needed here.
NEC-4 has one 3D interpolation grid for source and evaluation points
on opposite sides of the interface. For source and evaluation points
above ground at heights z' and z, respectively, it evaluates the
field at height z' + z for source just below the interface and adds
the appropriate direct and image terms to get the field needed. That
may result in adding large numbers for a small difference with high
conductivity as here.

The one 3D grid was used to save a little time and storage, but I
would not do it now. I might get around to revising the NEC-4 ground
evaluation some day.

Jerry Burke
LLNL

===============================================
CE Model with short segments at the base junction
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.,1.,.0008138
GW 3,2,0.,0.,1.,0.,0.,.4,.0008138
GW 9,3,0.,0.,.4,0.,0.,.1524,.0008138
GW 4,3,0.,0.,.1524,0.,.3,.1524,.0008138
GW 4,2,0.,.3,.1524,0.,1.,.1524,.0008138
GW 4,21,0.,1.,.1524,0.,50.292,.1524,.0008138
GW 5,3,0.,0.,.1524,0.,-.3,.1524,.0008138
GW 5,2,0.,-.3,.1524,0.,-1.,.1524,.0008138
GW 5,21,0.,-1.,.1524,0.,-50.292,.1524,.0008138
GE 1
FR 0,1,0,0,1.83
GN 2,0,0,0,81.,5.
EX 0,9,2,0,1.,0.
XQ
EN

Results: (epsilon = 81)
cond. --------- NEC-4D -------------
------------- NEC-2D -----------
S/m Rin Xin Avg. Gain
Rin Xin Avg. Gain

0.001 7.69123E+01 1.26695E+02 0.160 7.90906E+01 1.29476E+02 0.155
0.003 7.21069E+01 1.25749E+02 0.167 7.38409E+01 1.28334E+02 0.162
0.01 6.06862E+01 1.19449E+02 0.218 6.02083E+01 1.22146E+02 0.202
0.03 4.92272E+01 1.02255E+02 0.289 4.36457E+01 1.07793E+02 0.323
0.1 4.10716E+01 7.86277E+01 0.409 2.84842E+01
9.36058E+01 0.583
0.3 2.78348E+01 6.67538E+01 0.664 1.96676E+01
8.87374E+01 0.929
1.0 -4.87995E+00 1.01535E+02 -4.060 1.59479E+01
8.70547E+01 1.218
5.0 -4.21415E+01 3.45929E+02 -0.492 1.42931E+01
8.59620E+01 1.472

>Jerry,
>
>Thanks very much for the information. What's the range of allowed
>complex permittivity for NEC-4?
>
>Roy
>
>Jerry Burke wrote:
>>Roy,
>>
>>That gives a complex permittivity of 81 - j* 4.9e4, which is out of
>>the range that we planned for. It might work to use reflection
>>coefficient ground or a two-medium ground with PEC in the inner
>>region and seawater for the radiation pattern. We have run into
>>that sort of problem with NEC-2 also, but the limits would be
>>different. With NEC-2 it is easier to adjust the parameters of the
>>Sommerfeld tables to try to make it work.
>>
>>Jerry
>>
>>>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
>>>
>>>--
>>>The NEC-List mailing list
>>>NEC-List_at_robomod.net
>>>http://www.robomod.net/mailman/listinfo/nec-list

-- 
The NEC-List mailing list
NEC-List_at_robomod.net
http://www.robomod.net/mailman/listinfo/nec-list