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Old April 12th 06, 09:32 AM posted to rec.radio.amateur.antenna
 
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Default Current across the antenna loading coil - from scratch


Mike Coslo wrote:

Yuri Blanarovich wrote:
The current in a typical loading coil in the shortened antennas drops
across the coil roughly corresponding to the segment of the radiator
it replaces.


Quote from your page.

I would not expect anything else. If the loading coil is making the
antenna act like a physically longer antenna, other "qualities" of that
simulation are likely to be similar.

Is there a reason why the coil would *not* do this?
- 73 de Mike KB3EIA -


Every reason in the world. This misconception is EXACTLY what started
this whole thing years ago.

The loading coil doesn't "replace" missing electrical degrees, it
primarily corrects power factor by compensating reactance. Anything
else is generally secondary, and is related to flaws in the system
rather than something necessary.

That idea repeats one of the worse myths about loading coils.

The truth is the loading inductor almost never has the same phase shift
in current as the missing antenna area it replaces, and it almost never
has the same "current drop". "Current drop" isn't even a good English
description of what happens in any circuit.

I can have an antenna of given dimensions with a loading coil at one
fixed spot. The difference in current flowing into one end and out the
other can go all over the place, depending only on the coil's physical
design wity the antenna resonant on the same frequency. This happens
ONLY by changing the coil.

If I had a coil that was compact and not against the groundplane with
low stray capacitance compared to the antenna area above the coil,
current difference between each terminal or through the coil could be
immeasurable with reasonably good instrumentation. Phase shift in
current could also be nearly zero. None of this would be anywhere near
the area the coil replaces.

Without any change in anything except the coil, I could change all
that. If I replaced the coil with a long stub or very large single
turn, it would indeed act more like the antenna area it "replaces". The
reason for this is the coil's capacitance to ground and capacitance to
space around the antenna, NOT the electrical degrees it replaces and
certainly not the standing waves. This all can and has been proven over
and over again. The very few people offering the long drawn out
arguments against this really are violating basic electrical laws of
how systems really work, and enforcing the myth that a loading coil is
so many electrical degrees long or that displacement currents are not
at work and current magically vanishes, or magically flows two
directions at the same time at one point in a conductor.

A few people have taken the model of standing waves, not understood the
limits or boundary conditions of that model, and thought it to be an
actual literal description of what really is happening. They appear to
actually think current can flow two directions at the same time at the
same point. They somehow thing we can have charge drift velocity in two
directions at the same instant of time at one point in a conductor, or
that current can just vanish into thin air without actually being
diverted through a second path.

A few people have violated the rules of charge conservation, charge
movement, and misapplied the concept of standing waves, but the single
largest error is standing behind the myth or misconception that that
loading coil somehow acts like the "missing area of antenna".

73 Tom