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Old March 25th 06, 02:04 AM posted to rec.radio.amateur.antenna
 
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Default Current through coils


John Popelish wrote:

But what is the need for such an argument? Just to prove that lumped
component networks can model real, distributed things? I get that.
As I see Cecil's point (and I hate to say this with him absent), it is
that real, large coils with all their poor turns coupling and stray
capacitance both turn to turn and more important, to ground, take a
lot of those lumped components to model, accurately, but only their
own self, described by distributed network concepts to model, accurately.


Cecil's point is rather obscure, but as I read it Cecil thinks the ONLY
way to model a loaded antenna is through reflected wave theory.

As I understand what Cecil writes, he seems to be saying if we use a
current meter we cannot measure current. If we look at an inductor's
properties he seems to say they change in the presence of standing
waves. He also seems to be saying a loading inductor replaces a certain
number of electrical degrees through some reflection property.

What most others seem to be saying is an inductor is an inductor. It
behaves the same way and has the same characteristics no matter how it
is used, so long as we don't change the displacement currents by
varying capacitive coupling to surroundings.

That is where the difference is.

I can easily build a loading coil that has no appreciable change in
current from end-to-end. My measurements of typical loading coils shows
it is the ratio of load (termination) impedance to capacitance to the
outside world that controls any difference in current, and not the
"electrical degrees" the coil replaces. It is also not the reflected
waves that cause the unequal currents, but rather the fact the inductor
has distributed capacitance to earth or other objects besides the coil.

Capacitance from the coil to itself won't cause these problems. The
change in phase of current at each end of a coil would depend heavily
on stray C of the coil to the outside world as compared to reactance of
the coil, and it would also depend on less than perfect flux linkage
across the inductor.

I measured a typical inductor and found it did have more phase delay in
current at each terminal than the actual spatial length of the coil
form would indicate. I measured a delay about equal to double the
length of the 10 inch coil form length. If the inductor was perfect,
the delay would be about equal to light speed across the length of the
inductor form.

The only thing in all of this I can't find agreement with is what Cecil
is saying. I'm not disputing currebnt can be different, and phase can
be different. What I am disputing are Cecil's claims that an inductor
behaves differently in an antenna than in a lumped system that
represents the antenna, and that the cause of inequality in currents or
phase delay is caused by reflected waves and cannot be understood
without applying reflected wave theory.

In my experience, either lumped circuits or reflected waves will work
IF applied correctly.

This is my take of the disagreement.

73 Tom