From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Telsa Coils
Date: 26 Sep 92 12:35:44 GMT
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <1992Sep25.055726.22492@mintaka.lcs.mit.edu> regnad@hal.gnu.ai.mit.edu (Paul Prescott) writes:
>In article <1992Sep24.174214.16291@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>>In article <1992Sep24.014521.633@mintaka.lcs.mit.edu> regnad@hal.gnu.ai.mit.edu (Paul Prescott) writes:
>>>
>>>coil to be 2 to 2-1/2 times the diameter.  The next is a lot more tricky to
>>>work out.  The length of wire used in the secondary should be equal to a
>>>quarter wavelength of the resonant frequency.  This way you have a voltage
>>>maximum at the free end of the coil.  From what I can tell (I've never
>>
>>This is false. Since we are dealing with a transformer primary coil, and 
>>not a transmission line or antenna here, using 1/4 wavelengths of wire 
>>has no advantage. The voltage goes through a complete 180 degree reversal
>>at opposing ends of the coil regardless of it's length. The important
>
>Let me try and explain this because I think what I said is correct.

I think there's a misunderstanding here.

>First off, yes, there is a 180 degree phase reversal between primary and
>secondary, but this is not what I was refering to. 

Nor was I. Any *resonant* coil is exactly 180 *electrical* degrees long.
The two ends of the coil will have their voltages *exactly* 180 degrees
out of phase. With a Tesla secondary, one end is grounded and the other
is left "free". So the potential referenced to ground will always be 
greatest  at the "free" end, regardless of the physical length of the
wire. The only things that affect what the magnitude of that voltage
will be are the turns ratio between primary and secondary, the Q of
the coil, and the breakdown characteristics of the windings.

>                                                 I was talking about the
>voltage standing waves in the secondary, itself.  Since this is (ideally) a
>single frequency resonant coil, by using 1/4 wavelength (or some odd
>multiple thereof) you insure that the voltage at the free end is at its
>maximum value, rather than having this maximum appear somewhere else along
>the secondary windings.  

The coil is *not* an antenna or transmission line. It's the secondary of
an air core transformer operating at it's *self resonant* frequency. By
*definition* a self resonant coil is 180 electrical degrees long. Only
if you try to operate the Tesla coil off of it's self resonant frequency
can you have any other voltage distribution. Self resonant coils have a 
*fixed* standing wave with the voltage maxima at the ends. Electrically, 
it behaves like a perfectly cut half wave antenna with a very low radiation 
resistance. That RL constant can cause a *slight* phase shift across the
coil, but varying the physical length of the wire used to wind the coil
only affects the *number of turns* you can wind on the coil.

>The 1/4 wavelength secondary is cited by Tesla as a necessary element in
>producing greatest possible arc length in his notes from his Colorado
>Springs experiments, and also in the book "Tesla Coil Secrets" by
>R. A. Ford.

That may be, but remember the state of the art of the time. Tesla didn't
have the benefit of modern electrical theory or computer modeling. Nor
were his insulating materials very good, wood and cotton wrapped wire,
and he was operating his coils excited by spark gaps that weren't spectrally
pure. No doubt there were many different frequencies operating in his
coils at the same time.

Gary KE4ZV