Re: Lumped vs. Helical Resonator

>From hullr-at-whitlock-dot-comThu May 23 21:15:53 1996
>Date: Thu, 23 May 1996 11:45:46 -0700
>From: Richard Hull <hullr-at-whitlock-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: Lumped vs. Helical Resonator

>Tesla List wrote:
> >From jim.fosse-at-bdt-dot-comTue May 21 13:29:23 1996

Jim Fosse wrote:
>> Minds eye experiment: visualize the helical coil as a straight open
>> air transmission line, at the far end place a "guard cylinder" around
>> it. Connect this to the end of the transmission line. This represents
>> the electrostatic shielding effects of the toroid. The mutual (turn to
>> turn)  capacitance has been canceled out and the capacitance to ground
>> has been canceled out by virtue of the "equal potential" (to a first
>> order minds eye rigor) environment.  My only question is: what happens
>> to a transmission line when the C portion of it is eliminated? Do we
>> eliminate the series LC voltage rise on that section and turn it into
>> a pure inductor? A simple delay function?
> >        Anyone have a good method for measuring high RF voltages?
>> Last year Fred Bach posted on the construction of a Field Mill,
>> perhaps Fred could repost the construction and calibration info?
>>         Regards,
>>         jim

Richard Hull replied:


>Voltage at RF frequencies on the order of what comes from a big TC would 
>not be a matter of mere resistive drops.  A capacitive divider might be a 
>better choice for a TC coil voltage measurement device.

>Ricahrd Hull, TCBOR

Hi Guys,

I am in the process of designing/building a high voltage divider 
probe to measure the CW output of a vacuum tube powered T.C. at
up to 250 kilovolts RMS or so.  This is for a commercial client.  I 
am using a string of small value, high voltage vacuum caps in series. 
With ten in series for example and the voltage to be measured across 
the entire string, and sampling at the hot end of the cap closest to 
ground you will end up with a divide by ten probe.  This is not as 
easy a project as it sounds.  You have to electrostatically shield the entire
divider string so that only the high voltage input terminal is 
exposed to the Tesla coil field.  You also have to take into account 
the distributed shunt capacitance that this shield will have on your 
divider string, and  you have to play with your bottom 
capacitor value a bit to compensate for your measuring load once 
connected.  A calibrated length of coaxial cable and VTVM or O-scope. 
You also have to make the total capacitance small enough so that its 
own reactance at the intended operating frequency does not over 
current the divider caps used, or put too high a load on the Tesla 
coil being measured.

I had wanted to comment on Jim's idea of testing a T.C. with the 
secondary strung out as a long wire.  Jim, that's very interesting, I 
have thought about this too!  You will have much higher radiative losses
which will load your Tesla generator and make a direct comparison to 
the standard helical solenoid resonator more difficult.  I thought of 
suspending the wire parallel to the ground and supported maybe ten 
feet up with sections of plastic pipe like telephone poles, but this 
is not the best setup  However,  we can modify a  setup used by some crazy
people  here to advantage.  One of those bridges that span a canyon that is 
used by people to bungee jump from would be perfect!  You drop your 
secondary wire, all 1000 -2000 feet of it with a big aluminum ball on 
the end to act as a topload termination (bottomload?) and to keep the 
wire straight.  Pick a day when it's not windy and preferably when 
there are no bungee jumpers using the same bridge!

The problem with reality (I fought this for  ten hard years trying to 
do radio astronomy and finally had to give up before I died from Brick Wall 
syndrome) is that for dropping a big elastic band from the bridge so you can
scare thie heck out of people you will attract more than enough money to fund
the exersize and pad your wallet, however to conduct a meaningful scientific
experiment, you will never find any money.  The moral here
is,  "REAL scientists fund their own research, then they die".
Sorry, I've become a little case hardened.

Happy (unfunded) Coiling!, rwstephens