Re: Tuning Experiments

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Steve,

At 10:03 PM 2/21/2003 -0700, you wrote:
>
>
>I made some experiments to try to pin down the effects of streamers on 
>secondary F_res.  I am not sure how to interpret some of the results and 
>solicit comments.
>
>I built a (gridless) dip meter to measure coil resonances.  I added a 
>buffered output to the dip meter to drive a frequency counter for 
>accuracy.  The depth of the dip, all other factors being constant, is an 
>indication of the Q of the tuned circuit.
>
>I wanted to determine the effect a streamer would have on secondary 
>resonance.  I used a 4 x 23 coil with a 20 pf toroid.  Resonant freq was 
>191.4 kHz.  To simulate a streamer I added a wire, suspended by a string 
>horizontally from the outside of the toroid, and got the following 
>measurements (switch to fixed width font):
>Wire length   F res kHz
>  0            191.4
>  6            190.3
>12            187.8
>18            185.2
>24            181.9
>30            179.0
>36            176.2
>
>Assuming the streamer was really low impedance (like a wire), then a 
>typical 2 foot streamer drops the frequency by 10 kHz or about 5%.  This 
>is equivalent of adding about 3 pf to the toroid.  A 3 ft streamer drops 
>the freq by about 8% and acts like an additional 5 pf.

I have used piano wire to make wire streamer simulators too.  It actually 
works pretty well.

>
>Terry Fritz has reported a typical streamer impedance of 220K in series 
>with 5 pf.  So I took another series of measurements, with various value 
>resistors between the toroid and a 22 inch wire, as follows:
>
>resistance      F res kHz
>    0             182.3 deep dip
>    1K            182.2 deep dip
>    10K           182.2 shallow dip
>    100K          no dip!
>    220K          no dip!
>    470K          189.0 barely perceptable dip
>    1 meg         189.7 shallow dip
>    10 meg        189.7 deep dip
>
>As the resistance goes up, the Q goes down then up, and the frequency 
>rises toward the no-wire value.

I am not really sure how grid or dip meters work.  It is interesting that 
the dip is lost at around 220k, but I don't know why.

>
>(I didn't try strings of resistors to simulate a distributed resistance 
>along a streamer path.   If I did try this, how many ohms per inch would 
>you guess a 3 ft streamer would have?  And would the resistance per inch 
>increase by some nonlinear factor with distance from the toroid?  I would 
>guess it would.)

The resistance per length is probably like 220k / 36 = 6k/in.  I would 
"guess" it is linear.

>
>So, it looks like if a streamer impedance gets in the neighborhood of what 
>Terry uses in his simulations, the Q is greatly reduced and I would expect 
>a corresponding reduction in coil performance.  This may account for the 
>amazing performance Richard Hull got with his 10 inch magnifier with the 
>huge toroid.  I would think the larger the toroid, the less effect a 
>streamer will have on Q and detuning.  Does our use of smaller toroids 
>(relative to secondary size) promote excessive Q spoiling?

The system may just be coming close to being critically damped or Rstreamer 
= Rcoil so the power transfer to the streamer my be maximum.  Larger 
toroids seem to work better than smaller ones in general.  But there are 
many factors at work there.

>
>Am I interpreting these results correctly?  Does the same effect happen in 
>your simulations?
>
>My goal was to be able to use the dip meter to match F pri to F sec.  I 
>found that when measuring primary F res I had to either remove the 
>secondary or at least remove the toroid, otherwise the secondary coupled 
>to the primary resulted in misleading measurements.  I figured if I tuned 
>the primary to the secondary resonance, with a wire simulating the 
>streamer, it would be fairly close.  I am not so sure now.  Unknowns are 
>the loading effects of a real streamer and also the extra capacitance from 
>the ion cloud around the toroid.

In general, the primary frequency should be tuned about 7% lower than the 
secondary.  Then when the streamer is at it's longest, the coil will be 
perfectly tuned.  Streamer loading typically drops the secondary frequency 
around 7%.

>
>Guess I think using WinTesla or the like is about as good as using a tuner 
>for an initial rough tuning.  The best way is to patiently try different 
>tap points for max streamer length.  A possibly better way is to be able 
>to vary some off-axis primary inductance in real time at full power to 
>tweak the coil performance.

Some people have tried such things especially with solid state coils.  I am 
not sure the great effort of active tuning is worth it but it is 
interesting.  Since the frequency is changing over a single cycle and the 
streamers are presenting unstable loads, tracking the Fo frequency in real 
time as in a feedback loop is a challenge.  Some tube coils my do this 
automatically depending on how they are setup.

Cheers,

         Terry

>
>Your thoughts?
>
>--Steve Young
>
>
>