Topband: Loading coil current distribution

[Michael Tope]

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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