NECLIST: Anomaly?

Re: Grant Bingeman, Anomaly?

The results in this case look very reasonable if I understand the
question correctly. NEC shows an attenuation rate of .455 and 2.21
dB/m for 0.001 and 0.005 S/m conductivity, respectively, and
Exp(-j*k*R) gives .453 and 2.21 dB/m. I also tried it down to a depth
of 10 m, and the attenuation rate remained the same.

It is certainly possible to get wrong results when computing the near
field in the ground when the depth exceeds the range of the table
lookup/parameter estimation schemes used. These evaluation schemes
and table ranges are described around page 44 in the NEC-4 Theory
manual. When the evaluation or source points fall outside of the
table ranges the code uses asymptotic approximations with higher order
saddle point and surface wave terms. These approximations are very
accurate with increasing radial distance near the interface. When the
path between source and evaluation points is nearly vertical the
asymptotic approximations have more trouble. The worst case is when
the path is near the critical angle in the ground, where a wave coming
from a buried source would be totally reflected. In this case the the
basic assumption of the asymptotic approximations, that the reflection
or transmission coefficients are slowly varying relative to the
exponential part of the integrals, is no longer valid. Strange things
like lateral shifts in reflected wave can occur. The NEC formulas in
this range may give a field that grows exponentially with increasing
depth. The easiest fix would probably to switch to numerical
evaluation of the Sommerfeld integrals in these regions. That would
be relatively slow but accurate to a distance of about 20 wavelengths
in the ground with the evaluation presently used. However, I would
not want to mess with the ground evaluation without studying it a good
bit to refresh my memory on the various things that are done.

About all that can be said now is to take a critical look at results
for fields at any significant depth in the ground. Section 5.2.5 of
the NEC-4 Theory Manual summarized some of the limitations that may be
encountered. At least ground stakes (interactions straight down) or
ground screens (radial but not too deep) should be fairly safe.

Jerry Burke
LLNL
Received on Tue Jun 30 1998 - 09:48:57 EDT