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Re: Discharge impedance of a CW Tesla coil

In a message dated 3/10/00 6:41:05 AM Eastern Standard Time, tesla-at-pupman-dot-com 
writes:

> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>  
>  Hi All,
>  
>   I have been playing with my patched together RF generator and measurement
>  equipment.  Today I took off work early to do something important. ;-))  I
>  was able to measure the impedance of the brush discharge of my coil run
>  from the generator in CW (continuous wave) mode.

Hi Terry,

Thanks for these preliminary reports, I agree this is very interesting and
important work that should give us further insight for disruptive coils too
by bringing out underlying principles that govern all spark discharges in
TC's

>  It appears that the discharge is composed of a resistive region a few
>  inches in diameter (the part you see) that provides a relatively low
>  resistance path to a capacitive sphere around this discharge.  Thus the
>  discharge is a real resistor in series with a capacitance around the 
> discharge.

I wouldn't be surprised if the lower resistance seen in the CW coil is due
to the longer time period during which the sparks exist, which produces
more intense ionization over a smaller but "fuller" space.  Effectively
it's sort of like a disruptive TC which gives many streamers, but even
more intense because the sparks are there for a longer percentage of
time.  I suspect that disruptive TC's having multiple streamers will show
a lower than normal impedance also (but not as low as the CW coils).
It will be interesting to see if this impedance changes if the coil is pulsed.
Short pulses (short "on-times") may be needed to appreciably increase
the streamer Z.  If we can increase the Z by pulsing the coil properly,
we may lengthen the sparks.
 
>  I noticed that this coil produces almost no ozone or other noxious fumes in
>  operation.  I was very surprised by this!  Perhaps all CW coils are ozone
>  free and I just missed that bit of info.  Not gassing out the basement
>  during this testing is VERY pleasant as is the quiet discharge!!  It is
>  very cool but does not strike the "fear" those big disruptive coil
>  streamers, gaps, and all do...

I generally get quite a lot of ozone from my tube coils but I've never really
done a proper comparison with the disruptive TC's.  The power levels
probably have a lot to do with it.  In a small room, running a tube coil
can be brutal.
>  
>  The current at the top and bottom of the coil are almost perfectly in phase
>  but there is about a 60nS shift.  20nS of this should be caused by the load
>  impedance having a slight angle (87.7Deg. at 640K) and perhaps the other
>  40nS is due to the very slight transmission line effects.  Apparently, the
>  top and bottom currents in the secondary inductor of CW coils are just as
>  phase locked as disruptive coils.  The last coffin nail for those old 1/4
>  wave wire length theories ;-))

Good, let's get those old myths cleared away  :)
>  
>  I was very surprised that at
>  300 watts I was able to tune the coil with ZERO watts reflected (the
>  generator has a 50 ohm output through about 10 feet of RG-8).  My primitive
>  computer models suggested such things but I didn't believe them...
>  Apparently, primary inductance is not at all critical but coupling is...
>  Of course, the RF generator is much more predictable than tubes (at least
>  for me)...

A typical tube coil runs half wave rectified, so the various matching
aspects might not be as good?  I'll be interested to hear which coupling
k value you find to be the best.
>  
>  Much much fun was had today! ;-)))

Yes, I'm sure   :))   Looking forward to your further results, 

Cheers,
John Freau

>  
>  Cheers,
>  
>   Terry