Re: Resonator base impedance

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Paul and Richie,

	I think the base impedance of a CW coil will depend greatly on the plasma
load at the top of the coil since that is were most of the power is going.
I can tune my 50 ohm RF generator to almost zero reflected power by varying
the source frequency.  Thus, in some carefully controlled instances, the
impedance can be 50 ohms resistive.  Having said that based on observation,
I really don't see why that is true or understand the details as much as I
would like to.  Below is an old post on this subject and some pictures and
diagrams are at:

http://hot-streamer-dot-com/TeslaCoils/MyCoils/CWCoil/ 

This coil of mine has a grounded secondary with a driven primary winding.
the generator controls power and frequency.

http://hot-streamer-dot-com/TeslaCoils/MyCoils/CWCoil/CWDig.jpg 

Searching the archives at www.pupman-dot-com for "Discharge impedance of a CW
Tesla coil" gives the threads on this subject.  I have not yet been able to
pursue this area further than the work I did then.

I am probably providing more questions than answers here but little is
known about all this and I don't have many answers to give...

Cheers,

	Terry

Old post "Discharge impedance of a CW Tesla coil"
===============================================================
Date: Thu, 09 Mar 2000 20:55:19 -0700
To: tesla-at-pupman-dot-com
Subject: Discharge impedance of a CW Tesla coil

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.

The secondary is 1180 turns of #24 wire on a 4.25 diameter PVC form 26.125
inches long with an inductance of 22.1mH (from my little 9kV/30mA coil).
The primary is 10 turns of 0.092 inch Litz that is 6 inches in diameter.
That is about 2uH.  I used the generator to drive the coil at 100, 200,
300, and 400 watts delivered power compensating for the small reflected
power.  All the coil components stayed cool so I assume almost all the real
power was going to the discharge.  I can adjust the frequency to get the
lowest reflected power so it is very well in tune at each level.  I ran out
of tuning range at 400 watts with the fiber optic probe transducer on the
top of the coil (added top capacitance).  I will need to wind a new
slightly higher frequency secondary to get the generator's full 900 watts
forward power in tune with the probe in place.  Hopefully, nothing will
melt down at that level...

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.

Obviously, I need to write one of my papers on this to fully explain all
the details, but for a point discharge at around 330kHz, I got a discharge
impedance of:

Z = 20K + 3K ohms / 100watts delivered (real resistance) in series with
0.58pF + 0.070 pF / 100 watts delivered (capacitance).  

The actual numbers are as follows:

Delivered power	Resistance	Capacitance
100			22887		0.665pF
200			26122		0.754
300			29409		0.827
400*			30779		0.892

*The 400 watt measurement is messed up because the frequency dropped too
low for the generator to tune properly.

I know very little about CW coils (but am learning fast ;-)).

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...

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 ;-))

Much more to come on all this but I wanted to let everyone know about this
fun project to this point.  Obviously impedance matching, computer models
and all that will be greatly aided by such information.  In many ways, CW
coils are far simpler than disruptive coils.  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)...

Much much fun was had today! ;-)))

Cheers,

	Terry
================================================================



At 05:27 PM 12/17/2000 +0000, you wrote:
>R.E.Burnett <R.E.Burnett-at-newcastle.ac.uk> wrote:
>
>> Anyhow, I am going to wind a new coil which will be short, fat, and
>> use less turns of much thicker wire in order to get the same
>> inductance. Hopefully this will lower the base impedance. I will let
>> you know the result.
>
>I've been looking closely at secondary operation for a while now, but
>still cannot offer a suggestion as to the best secondary design for
>either impulsed coils or CW. 
>
>For CW operation, clearly the Q factor is very important, as it
>determines how much input power is necessary to spin the secondary up
>to a given stored energy. Also, and this applies to impulsed coils
>too, is the requirement to minimise as far as possible the coil's
>capacitance, as this determines the voltage achievable for a given
>stored energy. Thus Q and C are perhaps equally important for a CW
>secondary, but I'm afraid that a design formula which will spit out
>the optimum secondary design for an available supply power and
>frequency is still at the top of my christmas present list. 
>
>If I were to suggest that thick wire, with a modest h/d ratio, say
>around 3, were in any sense the best to use, it would be purely wish-
>ful thinking, since that describes my own secondary coils! However I
>have measured a Q of over 700 on a good day. The equivalent energy
>storage capacitance for these 360 turn 0.6 m x 0.8 m, 150kHz coils is
>around 30pF.
>
>> I am using a primary coil to match to my solid state driver,
>> although the primary-secondary coupling needs to be very tight to
>> get good power transfer at the moment.
>
>Yes. I admit to abandoning attempts to find a satisfactory coupling
>through a direct primary winding. I now drive the bases via a matching
>transformer which is removed from the field of the tesla coils.
>
>This doesn't eliminate the need for the tight coupling, but it does
>ease the breakdown and protection requirements, and it also allows the
>secondaries to be more easily raised high off the ground in order to
>minimise the energy storage capacitance.
>
>> I must decrease the impedance of the bare resonator, so that I can
>> reduce coupling, otherwise insulating becomes impossible :-(  
>
>Fully agree with you there. Its a big step-up in impedance from the
>couple of ohms required by a driver, to the hundred or so seen at the
>coil base. Anything you can do to raise Q will be of benefit in this
>respect. I chose the bipolar configuration mainly to eliminate ground
>current losses to give a substantial increase of Q. Those with bins
>full of dead silicon will probably not agree if I suggest that the
>driver design is not as hard as solving the coupling problems,
>especially if they're not concerned with handling the load impedance
>presented off-resonance - perhaps accounting for some of the dead
>silicon :).
>
>PS, if you've been monitoring this list, you'll know to steer clear
>of sonotube for CW coils.
>
>Cheers,
>--
>Paul Nicholson,
>Manchester, UK.
>--
>