Richard Clark wrote:

First, several years ago, came the shocking observation that the
current into a coil is not the same as the current out of it. Somewhere
along the debate, this practical measurement was then expressed to be
in conflict with Kirchhoff's theories. However, Kirchhoff's current
law is for currents into and out of the same point intersection, not
component. The association with a point is found in that the "lumped"
inductance is a dimensionless load. The association with Kirchhoff was
strained to fit the load to then condemn the load instead of simply
rejecting that failed model and using the correct one.


So much has been said in this debate - and this is at least the third or
fourth re-make of the whole show - that I honestly cannot remember if
the exact words that Richard reports were ever used.

If they were, then they were excessively condensed, skipping some
essential steps in the explanation. Both sides of the debate have often
been guilty of skipping details that seemed "obvious" (at least to their
way of thinking) in order to get to their main point.

So please let me try to respond to Richard's criticism above. Since I
don't want to skip anything this time, this is going to take a little
longer.

If there's anything that someone doesn't agree with, please comment...
but please read the whole thing first. Many of the problems with this
debate are because people start to throw in comments before finding out
where the original poster is heading. This destroys any kind of
connected thinking, and reduces the "debate" into a series of
disconnected nit-picks.


The main electrical property of the thing we call a "coil" or "inductor"
is - obviously - inductance. But a real-life coil has many other
properties as well, and these complicate the picture.

If we're going to understand loading coils at all, we first need to
strip away all the complications, and understand what loading by pure
inductance would do. Then we can put back the complications and see what
difference they make.

If we want to understand real-life loading coils, it's absolutely vital
to understand which parts of the coil's behaviour are due to its
inductance, and which parts are due to other things.

Please have patience about this. If we cannot even agree what pure
inductance does, then this debate will run forever...

From the beginning, then:

"Lumped" inductance is another name for the pure electrical property of
inductance, applied at a single point in a circuit. It has none of the
complications of a real-life coil: no physical size, no distributed
self-capacitance, and no external electric or magnetic fields. Its only
connections with the antenna are through its two terminals. Lumped
inductance is just inductance and nothing else.

Unlike capacitance, inductance has NO ability to store charge. If you
push an electron into one terminal of a pure inductance, one electron
must instantaneously pop out from the other terminal. If there was any
delay in this process, it would mean that charge is being stored
somewhere... and then we'd no longer be talking about pure inductance
[1].

The inability to store charge means there can be no difference between
the instantaneous currents at the two terminals of a lumped/pure
inductance. Any difference in amplitude or phase at a given instant
would mean that charge is being stored or borrowed from some other time
in the RF cycle... which inductance cannot do. There is some kind of
difference in phase and amplitude in the voltage between its two
terminals, but not in the current.

Therefore any difference in currents between the two ends of a real-life
coil are NOT due to its inductance. They come from those OTHER
properties that make a real-life coil more complicated.

But let's stay with loading by pure lumped inductance for a little
longer, and look at a centre-loaded whip. The loading inductance is
responsible for almost all the features of the voltage and current
profiles along the antenna.

Starting at the bottom (the feedpoint), voltages are low and currents
are high, so the feedpoint impedance is low. Going up the lower part of
the whip, the magnitudes of the voltage and current remain almost
constant until we meet the loading inductance.

As we have seen, if the whip is loaded by pure inductance only, there is
no change in current between the two terminals of the inductance - but
there's a big step increase in voltage. At the upper terminal, the
current is the same but the voltage is very high, so we're into a much
higher-impedance environment.

As we go further up towards the top of the whip, current magnitude has
to taper off to zero at the very top. This also means that the voltage
magnitude has to increase even more as we approach the top of the whip.

Single-point loading by pure inductance has thus created almost all the
major features that we see in a practical centre-loaded whip -
particularly the big step change in voltage across the loading coil.

What we don't see in a practical antenna are exactly equal current
magnitudes and zero phase shift between the terminals of a real-life
loading coil - but that is ONLY because a real-life coil is not a pure
inductance. The harder we try to reach that ideal (by winding the coil
on a high-permeability toroidal core which confines the external fields
and allows the whole thing to become very small), the closer the
currents at the bottom of the coil come to being equal. Solid theory and
accurate measurements come together to support each other. The only gap
between theory and practice is due to our inability to construct a pure
inductance that has no other complicating properties.

If we can agree about pure inductive loading, we all have a firm place
to stand. Then we can then put back those "other" complicating
properties of a real-life loading coil, and see what difference they
make.





[1] This principle of "conservation of charge" is also the underlying
principle of Kirchhoff's current law. If you connect three ordinary
wires together, the current flowing into the junction from one wire must
be exactly and instantaneously balanced by the currents flowing in or
out on the other two wires. If this was not so, there would have to be
some means of adding, storing or losing electrons at the junction...
which contradicts our initial assumption of three simple wires with no
special properties.

It is not strictly accurate to say that Kirchhoff's current law applies
to pure inductance, but the underlying principle of "conservation of
charge" does apply.





--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek