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Re: [TCML] New 3.5" Coil - A kick in the right direction please!



Thanks for the correction Paul. Yes, when I read my own post and the word "can't", I thought about emphasizing the statement, but decided to just let it go and see if there were any takers. I had the same train of thought as Grant when I first started building coils (had 1/4 wavelength in my head as a necessity for efficiency) and I remember my frustration when it came to inserting a top load. It was almost impossible back then to figure out it's affects on the coil and nothing more than Excel to play with. I think nowadays, this is certainly easier to look at. I should take a look at it again just to see maybe the common mechanical issues. I expect short wide coils result with normal sized top loads and as h/d grows, the top load just gets smaller and smaller.

Take care,
Bart

Paul Nicholson wrote:
Just a minor correction here...

Bart wrote:

> If you "really" want the secondary wire length to be 1/4
> wavelength (such as antenna theory, radio circuits, etc.)
> then realize you can't use a top load.

Actually the effect of winding the wire into a solenoid will
*increase* the resonant frequency above that of the straight
wire, by a factor of about 1.5 to 1.7 for a typical secondary.

Therefore it is possible to choose a topload capacitance which
will pull the resulting bare coil resonance back down to that
of the original straight wire resonance.  Nothing special or
useful happens if you do this, though.

For example, if Grant wound his 340 metres of wire into a
solenoid of h/d ratio 3.5, the resonant frequency of the
unloaded coil would be about 1.6 times 220kHz ~= 350kHz.
A large topload, with a capacitance about 2.5 times that of
the coil's self-capacitance would be needed to pull the
resonance down to 220kHz.

--
Paul Nicholson
--
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