Tuning Experiments

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

Original poster: "S & J Young by way of Terry Fritz <teslalist-at-qwest-dot-net>" <youngs-at-konnections-dot-net>



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.

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 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.)

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?

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.

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.

Your thoughts?

--Steve Young