Re: 1/4 wave theory

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Michael,

At 11:05 AM 2/15/2002 -0600, you wrote:
>
>>From what I've gleaned from the list so far, I fear I may be about to start
>a holy war, but I'm going to ask anyway.
>
>Is it advantageous to design a coil such that the linear feet of wire in the
>secondary is equal to 1/4 of the operating wavelength?
>
>There, I've pulled the pin.  Let the shrapnel land where it may.

:-)   No Problem, this is an old war long since won.

Tesla thought in his work in Colorado Springs that the self resonant
frequency of a coil was much like a straight wire and thus the 1/4 wave
thing came about.  When a wire is wound into a coil, things change
drastically.  The frequency is controlled by the inductance of the coil and
the "self capacitance" of the coil.  There are also significant winding to
winding inductive coupling effects that come into play.  One can take the
inductance of the coil and find the "Medhurst" capacitance and then the
frequency is easily calculated.  In the end, the wire length as very little
to do with the frequency.  If you really want to see how it works see:

http://www.abelian.demon.co.uk/tssp/pn2511.html

Unzip the PN2511 file for everything there is to know about secondary
behavior...

The 1/4 wave idea has "stuck" hard since Tesla's day but it is wrong.  For
a time, it was a "holy war" but the truth finally won this one :-))  Many
more details, equations, programs, and ways to do it... but this is the
"quick" answer...

>
>Here's my situation.  I have a secondary that I wound a number of years ago
>that I would like to put into service.  I've never ran this secondary since
>I wound it.  I'd like to build a tank circuit to excite this thing, so I'm
>wondering if I should use the 1/4 wavelength theory to determine the
>operating frequency of my tank circuit or if there's some other method I
>should use for determining my operating frequency.
>
>Specs:
>
>Coil length - 23.625 inches
>Coil diameter - 8.375 inches
>number of turns - 536
>wound with #22 magnet wire
>spaced such that there are about 22.5 turns per inch
>linear feet of wire - 1174.6 ft.
>
>>From everything I'm seeing on this maillist (and associated websites), it
>looks to me like I may not be able to get very good performance out of this
>coil without rewinding it with more turns.  Comments?

It will be fine.  First find the inductance of the coil using the equations at:

http://home.earthlink-dot-net/~electronxlc/formulas.html

"Helical Coil Inductance"

Then find the "Medhurst" capacitance using that formula.

Then find the resonant frequency of your coil using the "Resonant Circuit
Formula".

When you put a top load on your coil, the terminal will alter the
capacitance and lower the frequency.  This program will still figure the Fo
frequency with a top terminal on the coil:

http://hot-streamer-dot-com/TeslaCoils/Programs/E-Tesla6.zip

See the text file with it for the explanation.

There are a number of design programs around but I don't have those links
handy (I should).  But this will get you started.  All the math stuff may
seem a little hard at first but it will soon get real clear when you see
what wonderful answers it gives you :-)

Cheers,

	Terry


>
>Michael McQuay
>