NEC-LIST: Insulating Sheath (IS) command in NEC-4

Chuck,

I tried a model similar to what you described for the insulated
transmission line and got reasonable results. Don't know how close
this is to yours, but the data is included at the end of this message
for a transmission line separation of 0.003 m. I ran the model with
and without insulation and then read the output files with ZPLOT
(included with NEC) and interpolated the admittances to 3000
frequencies. I then set ZPLOT to plot from around 2000 to 2100 MHz
and estimated the 1 wavelength resonance in admittance, which was a
sharp spike. The results with wire radius 6.35e-5 m, sheath radius
1.016e-4 m and relative permittivity 1.319 were:

wire spacing f_res ins. f_res bare ratio
     m MHz MHz
   0.0005 2038 2098 1.029
   0.001 2048 2091 1.021
   0.003 2040 2070 1.015
   0.006 2012 2037.5 1.013
   0.012 1952 1973 1.011

It did not make any difference whether the source and termination
segments were insulated or bare.

Your use of NECPLT to estimate the quarter wave resonance seems like a
very imprecise method, but don't know if that accounts for the
difference.

For a result that should be more accurate than the above using ZPLOT,
I modeled a wire 10 wavelengths long with the same radius and
insulation. I wrote out the current coefficients A, B, C in the form
A + B*sin(ks) + C*cos(ks). Then ran a program that reads this data
and plots the current. That produces a large standing wave.
Estimating the propagation constant from the standing wave would
involve problems with end effects and determining the exact point of
minima or maxima. A better way is to estimate the slope of the phase
of the traveling wave with distance. However, with a standing wave
the phase vs. distance has big wiggles in it. The post processing
program eliminates the standing wave by numerically solving for a
matched load. Then the phase is a straight line versus distance and
the program fits a line to it to get the slope, averaging many
hundreds of current samples. This typically gives very close
agreement with analytic solutions, even when the propagation constant
differs from free space by something like 1 in 1.e-4. (Unfortunately
I could not check the analytic result for an infinite insulated wire,
since the solution algorithm gets lost trying to find the zero of a
determinant. It works fine when the outer medium has higher
permittivity than the insulation. Think I got the wrong program,
since I have done that before and got good agreement with NEC4 on the
surface wave for a dielectric sheath.)

I modeled a wire 10 wavelengths long with 400 segments and a source on
segment number 3. Then eliminated the standing wave and fit the slope
of the phase from segments 50 through 300. That gave a wavenumber
ki/k0=1.0082, where ki is for insulated wire and k0 for free space.
When I fit the slope of phase for segments 10 through 30, near the
source, the result was ki/k0=1.013. This seems typical, since the
current near the source shows effects of radiation, while the surface
dominates at larger distances.

So the transmission line result looks reasonable. As Keith said,
NEC-4 will not include the interaction of one wire with the sheath on
the adjacent wire, but that probably is not significant here.

Jerry Burke
LLNL

=======================================================
CE Transmission line to check propagation on insulated wire.
GW 1 40 0. 0. 0. 0. 0. .14276,6.35E-5
GW 2 40 .003 0. 0. .003 0. .14276,6.35E-5
GW3,1,0.,0.,0.,.003,0.,0.,6.35E-5
GW4,1,0.,0.,.14276,.003,0.,.14276,6.35E-5
GE0,
EX0,3,1,0,1.,
IS0,0,1,82,1.319,0.,1.016E-4,
FR0,20,0,0,1000.,100.,
XQ
EN
Received on Tue Oct 19 1999 - 00:17:09 EDT