I believe it is more complicated than that. This implies you would shift the effective primary left or right along the secondary (assuming you don't want to change the inductance). I don't believe any net changes in coupling will occur. I am currently building a conventional bipolar which is in essence two quarter-wave secondaries with the bases bolted together with a spacer in between. The spacer width (hope to be corrected here if my logic is faulty) was determined by through JAVATC by taking a helical primary and locating it to the recommended K. With the diameter I chose, this pushed the "bottom" of the helical primary below the base of the "bottom" of the secondary, so much so that over half of the primary was below the secondary. That value (the distance between the halfway point of the primary, and the end of the secondary) was doubled, and then inserted as a spacer between the bases of the two attached secondaries. Once I get the coil running, I can see if it was too small a gap, and increase it by adding additional spacers between the two secondaries.
Terry
On Saturday, October 17, 2020, 12:28:45 PM CDT, deano <deano@xxxxxxxxxxxxxxxx> wrote:
On 2020-10-17 06:01, mike tucknott via Tesla wrote:
> Hi All
>
> With a bipola coil would having 2 movable tap point work for changing
> the coupling ?
>
> Say the coil need 5 turns to be in tune, you could built the primary
> with 10 turns and you could move the inner and outer taps points in or
> out but still keeping the 5 turns needed to keep the coil in tune, the
> inner and outer turns passed the tap point would be electrialy dead so
> to speak.
>
> Thoughts on this one guys.
>
> Cheers Mike T
>
I think that changing the coupling by changing the diameter of the
primary is the way to go. What you propose would accomplish that
effectively. However I think the unused are not dead. They are still
coupled and generating potential at the open ends. When you are tapped
at the outermost turns for minimum coupling there may be some unexpected
"coupling" in the form of D'Arsenval currents. Of course if you could
remove the excess turns it would work fine:-)
Perhaps a solenoidal spring wound from beryllium copper or phosphor
bronze. Wound tightly would be small diameter with most turns. untwist
for large diameter and least turns. Someone else can figure the change
in inductance due to change in diameter vs due to change in number of
turns:-)
In any case your idea of changing the diameter, that is the ticket.
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