Re: Comments on t.c.'s as transmission lines
> Original Poster: Kennan C Herrick <kcha1-at-juno-dot-com>
> The Corums in their recently-posted paper on Tesla coils characterize
> them as transmission lines. Altho I learned my College rf upwards of
> 50 years ago & haven't used it all that much in the interim, my
> recollection of a transmission line is that it is a mechanism for
> transferring rf from one place to another, and it fundamentally
> incorporates a means for minimizing its radiation to space. A Tesla
> coil, on the other hand, incorporates no such means.
Actually, the physical length is so much shorter than the operating
wavelength that TCs in general are appallingly inefficient radiators
at their fundamental. The high Q's (in the hundreds) that one can
obtain with a good secondary are testimony to that.
> I would think that a t.c. would more accurately be characterized as
> partially a transformer and partially a highly-loaded 1/4 wave
> antenna. The lowest part, nearest the primary, is of course the
> transformer, with the upper (major) part acting as an antenna.
> By "loaded" I mean that the antenna incorporates a lot of inductance
> along its length, viz. the coil itself, making it physically a whole
> lot shorter than 1/4 wave of its resonant frequency.
> As the voltage rises on the t.c./antenna, the coil starts to radiate
> energy to space just like an ordinary antenna. That radiation energy
> loss, plus smaller losses within the coil, is what keeps the voltage
> at the top from rising to infinity given finite input energy.
You are charging the structural capacitance. That alone means that
limted energy = limited Vout.
> If there is no spark (no corona and the top electrode having a
> sufficiently large minimum radius facing space), then, after the
> primary excitation is removed, the voltage exponentially decays--with
> a rate, I suppose, dependent primarily on the intrinsic Q of the coil.
> When there is a spark, then what happens is merely this: The energy
> that would otherwise have radiated away into space subsequent to that
> time, exponentially decaying in amplitude should the excitation have
> been stopped at that time, just gets "radiated" very quickly, i.e. via
> the spark. But all the time during which the voltage is building up,
> our t.c. "antennas" are doing their thing, pumping energy into space,
> and that's where a lot of the energy from the primary goes. We'd like
> it, instead, to go into the ZAP but first principals would seem to
> deny us that.
> Observing the secondary-voltage waveform of my FET-driven coil, it
> appears that the "area under the curve" of the building-up portion of
> the wave is more or less the same as that of the decay portion
> subsequent to removal of the (pulse-burst) excitation, absent a spark.
> That tells me that, if I cut off the excitation just at the time of
> the spark (which I do utilizing a crafty little circuit I designed for
> the task), then the amount of energy I've "wasted" in radiation is the
> same as that that I've dumped into the spark. 50% efficiency at best!
> So sad.
I have measured transfer efficiencies approaching 90% in a small
coil. Such figures are routinely obtained in specially constructed
TCs for use as linear accelerators.
> I'd be happy to be corrected on any of this. And lastly, I don't
> understand a lot of the terms the Corums use in their paper.
> "Velocity inhibited", "slow-wave", "partially coherent", "frequency
> dispersive resonator", "guide phase constant", "inhibited velocity
> factor", "equation of continuity", "line splitting passband
> broadening", "degree of coherence of the up and back resonator waves"?
> Those terms are new to me and I would value either precise
> definitions or citations as to where those could be found.
They are likening the spectral response of the coupled system to an
optical situation. I have reason to wonder just how accurate the
> Ken Herrick
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