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bi-polar (center-fed) TC



Hi all,
I have not seen any mention on the list of a center-fed TC, so I want to try 
to make one. This may provide considerable entertainment for all. I have 
wound 23 inches, 1480 turns, of #28 on a piece of 3-inch pvc. I used a piece 
of schedule-40 pvc; is that my first mistake? The sked-40 pvc has 1/4-inch 
walls so the diameter of the coil is 3.5 inches.

The resistance of the coil is 80 Ohms.  The calculated inductance of the 
solenoid using
(rN)^sq/9r+10 x length and the classical equation using the permeability of 
free-space,
(mu -naught) give a value for L of 30 mH within 0.1 mH of each other. The 
Wavetek 27XT splits the difference so I have considerable confidence in the 
value of L. The sig-gen/counter/scope measurements show a resonant frequency 
of 642 kHz with harmonics within a few kHz. Now, I get into trouble because 
when I try to use the resonant frequency
with the value for L to determine the distributed (self) capacity of the coil 
nothing works. I
expect just a few picoF; the calculated value is 0.5 picoF. My hand and body 
capacity and
the coil itself with its connections offer so much stray capacitance as to 
make any measurement unreliable. Can I assume that the self capacitance of 
the coil is
just a few picoF? 

My next question is how do I determine the ratio of the primary and secondary 
diameters?
Will the equations for the coefficient of coupling (k) work? What should the 
value be? I plan on forming 1/4-inch copper pipe so the copper pipe primary 
will be freestanding on insulators. I assume the primary has to resonate with 
the secondary. What range of Q
values should I expect for these coils? 

Other puzzlement's: Is there an electrical difference between white and green 
pvc?
Will 3/4-inch stainless steel balls make a good SG? Can I use two highly 
polished brass doorknobs for a SG? The doorknobs are a little more than 
2-inches in diameter.

Thank you.

Ralph Zekelman