NEC-LIST: Modeling an inductor

Chuck,

A long time ago I tried modeling an inductor at the base of a
monopole. The inductor had ten turns and a diameter of 1.1 cm, and
the length was varied from 1 cm to 8 cm. The monopole was adjusted
to keep the total length constant at 32 cm and the wire diameter was
0.02 cm. It was easy to set up the models using the GH and GW
commands. The impedance of the monopole with no load was
11.25 - j249.3 ohms at 150 MHz. I subtracted this reactance from the
reactance of the monopole with each inductor to get an the reactance
of the inductor, then computed the inductance and compared it with
the value given by a handbook formula. The results are summarized
below.

length NEC Handbook Avg.
  (cm) (uH) (uH) Gain
   8 0.21 0.14 1.93
   7 0.23 0.16 1.91
   6 0.24 0.18 1.87
   5 0.26 0.22 1.83
   4 0.29 0.26 1.77
   3 0.34 0.34 1.68
   2 0.450 0.476 1.56
   1 0.782 0.795 1.35

NEC seemed to be modeling the inductance fairly accurately. The
greater difference for large lengths was probably because the
handbook formula was becoming less accurate due to varying current,
radiation, etc. I also computed average gain which should be 2.0
over the PEC ground. It was closest for the long inductors. For the
shorter cases average gain was substantially less than 2.0. This
indicates that the NEC solution was becoming less accurate for the
closely spaced wires, at least for input resistance, but the
reactance and resulting inductance still looked good. For a tightly
wound inductor I think I would trust the handbook formula more than
NEC.

Jerry Burke
LLNL

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