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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