On 08/04/2011 22:01, Dr. Gerald N. Johnson wrote:
Careful about the EZNEC model for impedance. None of the NEC models do
coupling to the earth. Their earth model is only used for reflections
for the elevation pattern.
No - it also uses the earth model to generate feedpoint impedances. You
only have to see the effect that soil type has on the impedance to
realise that is true. You need to be careful with verticals and
associated ground systems where it has some shortcomings, but it's
pretty good on the impedance of horizontal dipoles and derivitives down
to heights of 0.005 lambda.
On my Smith chart slide rule, that 11-j930 a long ways from the low R
side of the chart spins over a quarter wave to about 4 + j2.75 though
the resistive component is practically on the outside edge of the chart.
I don't trust my Smith chart for those extreme impedances; I suggest you
don't either because it's giving you the wrong answer - your reactive
component is grossly in error. Much easier to use the simple maths for a
quarter-wave transformation:
Zin = Zo*Zo/Zload = 600*600/(11 - j930) = 4.6 + j386
73,
Steve G3TXQ
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