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# Re: Coupling Questions & coil measurements

```In a message dated 99-08-09 06:16:07 EDT, you write:

<<
> In my reading, I have found several references that discuss "Magic K"
> values.  Brent's  book explanation says that Magic Ks relate to even
> integers (multiples) of "RF cycles". That makes no sense to me because
> "RF cycles" are different for each coil and it's unique Fsec.

Ross,

Magic k's occur when there's an exact number of RF half cycles
per beat.  Beats occur due to overcoupling.  The time duration of
a beat depends on frequency and coupling, so the coupling can be
adjusted to make the length of a beat equal to a given number of
RF half cycles.

> Can someone explain why some of these K values are supposedly better.
> Has anyone seen evidence of superior performance after moving from a sub
> optimal K to a "magic" K?  In other words, is it an artifact of the math
> or does it really matter?

Some say they've seen a benefit at magic k coupling, I myself have
not (although I really haven't explored this very much).  By using
magic k, all the energy is transfered to the secondary
at the notch, because notch and end of RF half cycle coincide.

> On a similar topic, I'm not sure that I understand the "double hump"
> response.  Can someone please describe how the resonant freq of the
> primary/secondary system is different from the resonant freq of either
> circuit measured independently and how this relates to coupling?

The double hump response occurs when coils are overcoupled
which causes frequency splitting of the coils' responses.

A reference is:   Radio Engineering, McGraw Hill, 1947 Third Edition,
Terman, page 58 - 69

Cheers,
John Freau

> Once I get more of this figured out, I hope to devote a web page to
> proper Tesla Coil measurement techniques and interpretation.  I intend
> to discuss it in layman's terms and then spend a little time describing
> the differential equations and showing how a few things are derived (for
> us engineer/math types).
--
> Ross Overstreet >>

```