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Resonator Measurements For Robert Jones (in particular).
Sent to the list for the benefit of all.
I measured three resonators last night in less than ideal
circumstances. I am confident that the measurements are close
enough to be useful. I have tried to estimate the number of turns as
accurately as possible. All three coils were closewound with the
same gauge of wire (0.56mm Cu diameter not incl. insulation) and I
measured a mean TPI = 40. The capacitance figures which dropped
out from Bob's equation are quite believable in my opinion.
Test conditions: Coils were stood upright on a sawhorse
approximately 24" off a concrete floor. They were approximately
centred as far as clearances go from anything else including the
supporting (wooden) beams. The bracketed figures for coil (1) were
taken with the resonator standing on the concrete floor. Coils were
measured with base feed from a 7 Ohm resistive source to get the
1/4 wave frequencies and connected directly across the source to
get the 1/2 wave frequencies. As expected, symmetry meant that it
didn't matter which way round the generator was connected for the
1/2 wave readings. For these readings, the connecting wires formed
a square loop 2' per side roughly so that nothing was draping
against the side of the coil.
(1) Hs = 21.75", Ds = 4.75", N approx 870t, L(Wheeler) = 17.87mH
1/4 wave = 394.5kHz ( ? - sorry )
1/2 wave = 825kHz (800kHz)
(2) Hs = 22.75", Ds = 5.95", N approx 910t, L(Wheeler) = 29.3mH
1/4 wave = 276.5kHz (269.0kHz)
1/2 wave = 591.6kHz (575.7kHz)
(3) Hs = 16.2", Ds = 9.05", N approx 650t, L(Wheeler) = 42.25mH
1/4 wave = 219.0kHz (212.7kHz)
1/2 wave = 476.0kHz (458.6kHz)
Finally I should note that measured L will be somewhat different
in all cases, and had I been able to achieve better isolation, I know
that the frequencies would have been higher. These things are
incredibly sensitive to their surroundings.
Regards,
Malcolm