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Re: Measuring secondary Q Re: Success Re[TCML] Measuring secondary current



On Wed, 5 Sep 2012, paul wrote:
So, you can get two Q measurements here: one by the ratio V_top
/ V_base and another from the bandwidth.   However, measuring
the top voltage is tricky because directly attaching a 10Meg
scope probe will also spoil the Q.  Some ingenuity may be
required here.

The only way I see to accomplish this directly would be a capacitative divider; unfortunately, for a calculated DC capacitance (per JavaTC) of 11pF, I doubt I can make one that won't disturb the Q -- much less produce reliable measurements.

On the other hand, I may use this as an excuse to finally make some Terry Fritz noncontact voltage / plane-wave antenna probes:
http://www.pupman.com/listarchives/1998/August/msg00357.html

This is very similar to the method that JavaTC recommends:
http://www.classictesla.com/java/script/outQ.html

If you have a calibrated CT on the base current, then you can
measure the base input impedance at the resonant frequency,

 Z_in =  V_base / I_base

and this is equal to the total loss resistance, from which
you can calculate Q.   This may disturb the resonator less
than trying to get directly at the top volts.

When you say V_base, are you talking about the voltage from the coil base to RF ground? I have a feeling it will be immeasurably low, assuming an I_base of ~140mA during operation and 1 ohm of resistance in the lead from base to the cold-water pipe that serves as ground.

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