Re: THOR resonance freq. measurement results

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Marco,

	It is wonderful seeing your big project finally getting to the point of
making arcs!!

At 09:07 AM 7/14/00 +0300, you wrote:
>Hello all (again).
>
>I have also been measuring the primary and secondary resonance frequencies 
>and I
>would like somebody to comment my results.
>
>Secondary measurement
>--------------------------------------
>I disconnected the secondary bottom from ground and connected the function
>generator hot lead to it. Func. generator cold lead heartened. Measuring the
>electric field with a scope probe. Setup using long shielded cable to saty far
>from the coil. Top toroid included in the measurement..
>
>I measured 3 resonance frequencies:
>
>65.46 kHz
>222.75 kHz
>346.3 kHz
>
>Can somebody explain the error (these are not exactly 1st, 3th and 5th
>harmonics, why?) ?

The resonances are much like a vibrating wire pictures as shown in my
Malcolm's ruler experiment of late may:

http://users.better-dot-org/tfritz/site/misc/MalRulMa/

you can see the 1st, 3rd, 5th harmonics clearly.  However, I now think that
unlike this wire, the Tesla coil has a fairly non-uniform capacitance along
it's length.  This would be like adding weights to the wire in various
places.  the harmonics are still there but the frequencies are different
than expected.  Robert Jones has also discussed dispersion which is the
change in the basic characteristics with the frequency.  All this is a
little "bleeding edge" but I think we are on the right track.  So, if the
coil were perfectly uniform, the harmonics should work out.  But the large
bottom capacitance and the large top capacitance do to the end of the coil
make the coil have "heavy" areas that give it rather strange harmonic
frequencies.  This can also be seen in my harmonic graphs at:

http://users.better-dot-org/tfritz/VoltDist.jpg

http://users.better-dot-org/tfritz/VoltDistBare.jpg

Robert Jones and I have been trying to find ways to study this with
computer simulations and such but "I" have not gotten it to work yet...


>
>I also measured the frequencies were the measured field amplitude was halved:
>
>65.72 kHz halved
>65.46 kHz full
>65.21 kHz halved
>
>But I don't recall how to calculate Q from it. Could anybody help, please?

Q = Fo/BW  Fo is 65.46 and the bandwidth is the difference of the 1/SQRT(2)
levels which is 0.7071.  Since you measured at 0.5 the frequencies are a
little off.  Maybe you can do them again and use the 0.7071 level instead
of 0.5 .

The Q would be  65.46 / (65.72 - 65.21) = 128 but since the levels are low
the actual BW would be less and the Q would be higher.  Marco says to
multiply by SQRT(3) to correct it (he knows things like that ;-)) so the Q
is 222.3

Note that SQRT(65.72 x 65.21) = 65.46 so your measurements are very good!

>
>
>Primary measurement
>---------------------------------
>
>First I tried to measure primary inductor and capacitor connected as a series
>resonator, but I could only and always spot the secondary res. freq. Then I
>connected them as a parallel resonator, fed them with the function generator
>through a 0.1 ohm resistor and measured the resistor voltage (i.e. the current
>sourced). I wrote down the points were the current was MINIMUM.
>
>Primary turns            Freq.              Wintesla predicted
>   used                   (kHz)                        (kHz)
>     9                      54                           54
>     8                      60                           61
>     7                      66     (78)                  70
>     6                      70      81                   81
>(there was little difference between 70 kHz and 81 kHz minimum levels)
>     5                     (72)     96                   96
>
>The numbers in brackets are a second minimum that was not so low in value as 
>the
>first one. Below 5 turns it was very hard to detect the minimums, so I
gave up.
>I am anyway interested only in frequencies near 65 kHz. It seems to me that 
>with
>the aid of the Wintesla prediction it is easier to read the measurement
results
>and to conclude that the prediction was correct. Any comments anybody?
>

Measuring primary inductance is a little hard since the stray capacitance
has a big effect.  This is solved by placing a large known capacitance
across the coil that will swamp out the stray capacitance and allow
accurate inductances to be found.  You then can resonant the LC circuit to
find the numbers.

Wintesla uses the same formulas all the rest of us use and they should be
very close.  With good data, such formulas can usually find the tap point
to with 1/4 turn.  When you start to get long streamers, you will have to
add a little (~7%) primary inductance to allow for streamer loading.

I will be looking forward to your experiments!!

Cheers,

	Terry