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Re: secondary design
Hi Skip,
> I have been trying to design and build a secondary for the last couple of
> years that will resonate at a frequency that is equal to the 1/4
> wavelength frequency of the length of wire used to wind it. I do not wish
> to use a toroid or other terminal capacitance to lower the frequency. So
> far I have been drawing a total blank. In order to obtain sufficient
> inductance to resonate the self C of the winding, the diameter of the
> winding becomes becomes very large and the height of the winding must
> necessairly become quite short. In fact it begins to look like Tesla's
> flat spiral may be the only way to accomplish what I want to do.
I checked this out three years ago when I was testing various formulae
- in vain. I set up a spreadsheet to find out where the wirelength and
frequency converge for a helical winding. Answer : use a longwire
monopole! That's it. To the best of my knowledge, what you are trying
cannot be done! And this is precisely where those "wirelength"
formulae came unstuck which then turned my attention to the tradeoff
of high mutual inductance for lower self-C in the structure. Which
is when I then went hunting until I found Medhurst's formula for self-
C.
An interesting result I obtained from an analysis last year is
that for a given coil former (diameter and height), ANY coil you
wind on that requires exactly the same terminal capacitance to make
the coil resonate at the wire 1/4 wavelength. Here at last was the
reason the h/d ratio has figured in all those attempts.
I'd welcome a single example that proves this incorrect.
Malcolm
PS - Applying Medhurst's formula shows that you can in fact achieve
your goal but only with h/d ratios considerably less than 1. I think
you would indeed be looking at something approaching a spiral.
MW