Frank wrote:
So far yours is the only response.

Hi Frank, your results look reasonable and thanks for your input.

I was also puzzled by
the slight increase in current just under the loading coil, but suspect it
was caused by coupling between the lower conductor and the base of the coil.

A number of us have reported the same thing. In my EZNEC octcoil.ez simulation,
(available from my web page below) the net current decreased by 2% from the feedpoint
to the coil and then in the first part of the bottom of the coil, it increased by 5%.
(Whoops, I almost said it "dropped" by 2% and "rose" by 5% which would have elicited
endless nitpicking. :-)

There is, no doubt, an impedance discontinuity at each end of the coil. The
net current standing-wave values at each end of the coil that differ from just
a straight wire are probably due to interference effects among the various
forward and reflected wave components. A conceptual idea of what is happening
might be the following solvable example (S-parameter analysis).

a1-- | --a2
--b1 | b2--

-----Z1-----+-----Z2-----+-----Z3-----open

bottom coil top
wire wire

The Z1 to Z2 impedance discontinuity will cause reflections in both directions.

b1 = s11*a1 + s12*a2 b2 = s21*a1 + s22*a2

The Z2 to Z3 impedance discontinuity will also cause reflections in both directions.

same equations as above with different parameters

The net current at the bottom and top of the coil cannot be expected to be the
same as in a straight piece of wire with no step functions in the impedance.
--
73, Cecil http://www.qsl.net/w5dxp


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