Owen Duffy wrote:
K7ITM wrote in
:

...
Yes--and then if they were exactly equal, would that not imply only
transmission line current on the stub? Obviously, they are exactly

Thinking some more about it, my current thinking is that my analysis was
flawed. I was using the standing wave currents, when I should be using
the travelling wave components.

I suspect that when NEC models the conductor arrangement at my fig a), it
correctly accounts for propagation delay and the phase relationships
compute correctly.

If we replace the stub with a TL element, I suspect that NEC reduces the
TL to a two port network and loads a segment of the vertical with an
equivalent steady state impedance of the s/c stub network. If that is
done, the reduction to a lumped load means that there is zero delay to
travelling waves, and the computed currents (amplitude and phase) in the
vertical will be incorrect. This means that you cannot replace a resonant
stub with a high value of resistance, it doesn't work.

If that is the case, it suggests that NEC cannot model such phasing
schemes using TL elements.

Owen

It's easy to reason yourself into traps by dividing currents into
"standing wave" and "traveling wave" components. They're different
things and don't add or superpose. Results of attempts to make this
differentiation can be seen in a vast number of postings on this forum
in the past.

Rather, I recommend considering a current to be a single value or, at
most, made of differential and common mode components which *can* be
added to obtain the total current.

In a steady state single frequency analysis, which is what NEC performs,
there is no such thing as delay. All time relationships can be expressed
as phase difference, which can't be tied to a unique delay -- you can't
even tell if the phase difference was due to time delay or magical
prescience-caused time lead. In a steady state analysis there is no way
to distinguish a half wave lossless transmission line from a 1-1/2 wave
line; they act exactly the same in all ways. So does a magical -1/2
wavelength line whose output appears a half cycle *before* the input
appears. Only in a time-domain analysis will you be able to tell the
difference. So yes, NEC models the transmission line as a two port
network. It does force the correct voltage and current amplitude and
phase relationships between the input and output. And it's
indistinguishable in the steady state analysis from an ideal
transmission line which effects the phase difference by means of delay.
The NEC transmission line model is equivalent to a real (but lossless)
transmission line on which the current is purely differential, e.g., a
coax line with a large number of ferrite cores on the outside. The model
is accurate within the constraints of a steady state analysis. If you're
interested in looking at the effects of delay in a transient system,
you'll need to use an analysis tool other than NEC. But if you let your
transient analysis run until steady state is reached, the results will
be the same as NEC.

Roy Lewallen, W7EL