Re: 3/4 wavelength secondaries
>>From sgreiner-at-wwnet-dot-comThu Jul 18 22:44:31 1996
>Date: Thu, 18 Jul 1996 16:29:39 -0700
>From: Skip Greiner <sgreiner-at-wwnet-dot-com>
>To: tesla list <tesla-at-pupman-dot-com>
>Subject: 3/4 wavelength secondaries
>I wonder if anyone has ever tried to wind a secondary at 3/4 wavelength
>rather than 1/4 wavelength.
>It appears that it may not be possibe to wind a 1/4 wavelength coil that
>will resonate at the correct frequency without using the large terminal
>capacitance. Perhaps a 3/4 wave coil could be made.
>I would appreciate any comments.
There are no physical restrictions that I am aware of that would ever
prevent one from winding a secondary that was the entire 90
electrical degrees (1/4 wavelength) in length and resonating it without any sort of
topload to a given Tesla coil system. True , placing a large toroid
on top of the coil reduces the resonant frequency of the secondary,
and true, changing its size will effect frequency tuning, but so will
adding or reducing secondary turns, with or without a top load.
The benefit one gains from a smooth surfaced topload is its ability
to holdoff from making any leakage corona until the standing wave in
the secondary has reached very high voltage levels. With a large,
smooth radiussed topload the wave will travel up and down the
secondary a number of times, gaining energy each time as a new pulse
is coupled into the coil from the primary circuit before it reaches a
high enough voltage level to ionize the air around the topload. Finally the air
breaks down and a very high voltage, longer streamer is the result.
A discharge terminal of tiny dimentions (i.e. small radius of
curvature) like a 1/4-20 bolt, will form corona at relatively low
coil voltage, and this corona acts just like a gas filled voltage
regulator tube in an electronic power supply. It 'clamps' the coil's
output voltage to the level at which the corona was formed.
The pulse racing up the coil may break away from a pointy electrode
on the first trip. The system looses the opportunity to build power
through the magic of resonant rise. This is like kicking the kid off
the swing after the first pass, he'll cry harder if you get him
swinging as high as the bar after several passes before kicking him off, (I'm sorry
that's a terrible analogy, but it serves the point to be made here).
As for winding a coil of three-quarters electrical wavelength, you
will end up with a much larger coil which will produce less output
than a quarter wavelength coil tuned to the same frequency
because of losses introduced by the additional mass of the huge 1/2
wavength winding.. You will have a voltage maximum in the area of
1/3rd up the secondary, within the winding length which will want to break out
and destroy the coil from the corona or streamers escaping from this
area. Half a wavelength later at the top of the coil you will end up
with the same energy, less the losses occured at the 1/4 wavelength
point just described, plus the loss due to all the extra interturn
capacitance (bad), and radiative losses represented by the additional 1/2
wavelength of coil.
The worst part is that someone seeing your coil run is gonna think," Gee
that's an awful tiny spark from such a long coil!"
Hope this makes sense.
Happy Coiling!, rwstephens