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

Can anyone explain these anomalous results I've gotten using the
Insulating Sheath (IS) command in NEC-4 ?

I defined a "transmission-line" structure comprising two parallel
wires of radius 0.0025 inch, separated center-to-center by 0.0250
inch, in free space. I used segment lengths of 0.0250 inch at first,
but got basically the same results with 0.0500- and 0.1000-inch
segments. Segments were aligned opposite each other on the two wires.

I "shorted" one end of this parallel-wire line with a 0.0250" wire,
excited the other end with a source, and used necplt to observe the
standing wave of current on the parallel-wire line.

The line was 1.4 inches line from source to shorted end, and I
observed a quarter-wave resonance (a current node at the source end)
at a frequency of about 2100 MHz, which is just right.

No anomaly so far.

Then I used the IS command to put a dielectric sheath on each of the
two parallel wires (not on the shorting wire). The sheath radius was
0.0040 inch, which is just under 1/6th of the center-to-center spacing
of the wires. I set the conductivity (sigma) of the sheath to zero,
and I gradually increased the relative permittivity (epsr) of the
sheath in steps from 1.000 to 1.319, watching the standing wave
pattern and varying the frequency to maintain a quarter-wave
resonance. At epsr = 1.319 I obtained a resonant frequency of 1575.42
MHz (the GPS L1 frequency), which happened to be my goal.

The anomaly is that the frequency ratio (2100/1575.42) = 1.333 implies
that the phase velocity of the transmission line has been reduced by a
factor (i.e., a divisor) of 1.333, whereas the _expected_ reduction
factor should be less than SQRT(1.319) = 1.15. I say "less than"
because the dielectric sheath does not fill all the space around the
parallel wires.

It's almost as though what the NEC-4 User's Manual calls "epsr" is
really an inverse velocity factor, or sqrt(epsr).

What's the explanation?

Thanks and 73 -Chuck W1HIS
Received on Mon Oct 18 1999 - 03:51:29 EDT