Re: 1/4 wave TC (fwd)

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Date: Thu, 26 Jul 2007 23:08:44 -0700
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: 1/4 wave TC (fwd)

Hi Skip, Matt, All, I've got a few comments to add;

>
> Skip wrote:
> My intention with this coil was to build a secondary that would resonate
> at the at the 1/4 wave frequency of the wire length used in the
> secondary. Indeed, with no top load, the overall excited resonant
> frequency of the secondary is only slightly below the non powered
> resonant frequency
>
> Matt wrote:
> 1) You have made a coil whose resonant frequency has a quarter wavelength
> that is equal to the straight wire quarter wavelength, but as soon as 
> you put
> the first bend in the wire, the "EM distance" between the two ends is 
> no longer
> that of the straight wire. Once you have put ~380 bends in it such 
> that the
> straight-line distance in three dimensions from end to end is only ~19
> inches, then that quarter wave number really seems to lose any 
> significance.
>
> Skip wrote:
> Interestingly, the unpowered resonance of the coil is approximately the
> same frequency as the 1/4 wavelength of the wire....even with all of the
> bends, just as JAVATC predicted. I actually designed the coil to be at a
> slightly higher frequency knowing full well that the discharges would
> lower the overall frequency. I expected the powered resonance to be
> lower due to the discharges.


Note what Matt is identifying: Once you wound the coil, the straight 
wire inductance is no longer valid because the wire is no longer 
straight. Thus, how are the two inductances electrically related? In my 
mind, their not.

> Matt wrote:
> 2) As the streamers form, they drop the resonant frequency of the 
> secondary.
> The streamers themselves become a constantly growing and shrinking 
> topload,
> as they form, grow, disappear, and new ones re-grow. When there is no 
> other
> topload, the percentage by which the streamers affect the frequency is
> greater. Since, when anything is happening, the frequency is 
> constantly changing,
> what is the relevance of the wavelength at which nothing is happening?
>
> Skip wrote:
> I can find no change in the frequency of the coil when powered at
> different levels that cause the discharges to be at different lengths.
> The scope gives a pretty clear indication that the oscillations are pure
> and not varible.


Very hard to tell. Small sparks will have little affect. The streamer 
geometry and position as capacitively coupled to the coil is a loading 
physical object and there's no denying that. Some systems due to their 
geometry suffer greater amounts of spark loading than other coils. It's 
usually not a great deal but some systems (narrow band tuning) are 
affected far greater than other coils (wide band tuning). What makes a 
coil narrow or wide is the ratio of C to L. The loading will shift the 
frequency. Matt is stating the fact that even if you build to a 
particular frequency, as soon as you run the coil, the frequency will be 
shifting during the run.

Now, if you had built a system that is not intended to spark but to sit 
there and radiate or whatever, then yes you won't have a frequency shift 
caused by spark loading (although you will still have physical objects 
around the coil capacitively loading). Tesla's writings on 1/4 wave 
coils had nothing to do with spark coil applications. You are building a 
spark generating coil and because of that, the application of the coil 
is outside of 1/4 wave theory and cannot apply to them.

I think when we term our coils as Tesla Coils, it's only the fact that 
Tesla sparked a few of his coils (period). To be more precise as far as 
our coils are concerned, they are "spark generators driven by resonant 
transformation". Because of that, building to 1/4 wave wirelength 
frequency or building away from 1/4 wave wirelength frequency will make 
no difference. The deciding factors for spark performance is far more 
complex and much to do with geometry, gap efficiency, power throughput, 
etc..

Take care,
Bart