Rick,
When I read that article on Tom's website, I didn't get the impression
that he was claiming that the current was always equal on both ends. In
fact, if you scroll down far enough he shows scenarios where he measured
equal currents (e.g. toroid loading inductor) and scenarios where the
current at the top of the inductor was diminished significantly compared
to the current at the bottom (e.g. physically long inductor in close
proximity to a top hat or long inductor in close proximity to sheet
metal). The main claim he was trying to debunk was that the difference
in the current would be equal to the current taper in a linear section
of radiator that the loading coil replaced. IOW, some were claiming that
if you base loaded an 1/8 wavelength tall monopole with an inductor in
order to bring it to resonance, the current taper in the inductor had to
be equal to the current taper that would occur in the bottom 1/8
wavelength of a 1/4 wavelength tall monopole, since the inductor was in
effect replacing that bottom 1/8 wavelength. The measurements done by
W8JI and W7EL showed that wasn't true. The current taper in the inductor
depends on its distributed capacitance to the ground plane and other
parts of the antenna since that distributed C gives rise to displacement
current which is the source of the tapering. That is why there is so
little current taper in toroidal inductors. They tend to be very compact
with limited stray capacitance external to the toroid.
73, Mike W4EF.................
On 8/8/2024 6:32 PM, Richard Karlquist wrote:
The fact that Tom performed a hero experiment showing that a particular
coil in a particular environment had equal current at both ends by no
means proves that this is the case for all coils and environments. For
example, how would Tom explain Tesla coils? Surely the current at the
top (where the voltage is much higher) cannot be the same as the current
at the bottom (due to conservation of energy). You can calculate the
capacitance to free space for any coil using Medhurst's 1947 formula.
If one end is closer than the other to nearby conductors, the result
will be to step up or step down the current. I guess Tom would say this
effect "doesn't count" even though it is real.
---
Rick Karlquist
N6RK
On 2024-08-08 08:15, Tom Boucher wrote:
Thanks Dave KH6AQ for the reference to Tom W8JI's article on loading coil
currents. I just re-read it and it makes interesting reading. As I thought,
he showed that current out of a loading coil is essentially the same as the
current into it. However the results shown in the QEX article pointed out
by Rick N6RK don't seem to agree with Tom's, maybe because Tom constructed
a pair of all-plastic RF ammeters which didn't affect the loading.
Quote by W8JI: <*Measuring the current into and out of the loading coil
with a small thermocouple RF meter, I detect no difference This is in close
agreement with the model*.>
Of course as has been pointed out, there is bound to be some loss through
the coil.
The original question was how to match a sloper which is shorter than a
quarter wave length. If possible I believe it is better not to use
inductive loading at all, but to extend the top of the antenna making it a
full quarter wave sloping inverted 'L', which is what I have. Better still,
two top wires as a 'T'.
My inverted 'L' has its 90 ft vertical section supported by a tall tree.
It slopes away from the tree in order to minimise absorption by it. Also it
is slightly longer than a quarter wave making it slightly inductive and is
matched to the 50 ohm coax by a parallel fixed capacitor acting as an
L-match. This also has the advantage that the point of maximum current is
some way up the 'vertical' section.
All good stuff.
73,
Tom G3OLB
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