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Re: 'quarter wave vs lumped models'

Subject:      Re: 'quarter wave vs lumped models'
       Date:  Fri, 9 May 1997 07:49:09 +1200
       From:  "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Organization:  Wellington Polytechnic, NZ
         To:   tesla-at-pupman-dot-com


Hi DWP, all,

>   From:   pierson-at-ggone.ENET.dec-dot-com

> 
> >One of the ways of looking at a Tesla coil is to think of it as a 
> >quarter wave transmision line.
> 
> >A PERFECT quarter wave line can be thought of as a current to voltage 
> >converter (or vice versa) where the voltage at the top is proportional
> >to the current at the bottom.
> 
> >There have been some heavy discussion on the list about quarter wave 
> >line vs. lumped constant models of Tesla coils,
> 
>         Philosophical rant on.  8)>>
> 
>         (NB: NO Directed AT anyone...)
>         There seems to be something in the human approach to solving
>         problems that leads to this _vs_ that, one _or_ the other.  In
>         some cases its applicable.  In Some Cases NOT.  In the case of
>         coiling:
> 
>         Fact:
>         Any coil will have a physical self-resonant frequency, based
>         on wire length and aspect ratio.
>         Fact:
>         Any coil will have a 'lumped constant' self-resonant frequency,
>         based on 'L' and 'C'.
> 
>         From which it follows that any coil has two resonant freqs [1].

Unfortunately, a resonator with a Q of 200+ which is typical and 
possibly on the low side is not going to exhibit more than one strong 
resonance. I have _never_ seen more than a single strong resonance in 
any of the secondaries I've wound. In some low-Q coils I have seen 
some other resonances appear but these were still rather weak.

      Also, I must correct my mention in my original "reflection" post
of VSWR dropping to 1. That is the same as saying Q drops to 0.5. It 
clearly never does (with a spark termination) and it has been measured 
as dropping to between 10 and 20 which makes plenty sense. Voltage 
production is still quite high but nowhere near the unloaded value. 
This might well be helping spark propagation too.
     I guess a key question is then: what secondary parameters are 
needed at a given power level to maximize loaded Q? If a spark is 
considerably lower (on average) than Zo, then it seems that a high
value of Zo explains why coils built on these lines works well.

Malcolm

> >         Speculation:
>         A key ingredient in The best performers is to have the _two_
> freqs
>         closest together, or equal, (other things being equal).  Where
>         the lumped_constant resonance and the physical_length resonance
>         are the same.
> 
>         I'm not speaking of primary/secondary interactions, but of any
>         coil (or coil plus cap, in the case of a primary) having two
>         freqs, which (for best performance) need to be the same.
> 
>         Or i could be talking rot...
> 
> ===========
> [1]
> Actually, there will be lots of freqs, notably the physical one will
> resonate at f, 2f, 3f, etc, etc.  But confining thoughts to the
> 'fundamental' (1/4 wave) case is reasonable.
> 
>         regards
>         dwp
>