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Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Is this what you are looking for? Z=2*pi*F*L and Q=Z/R where R is the DC resistance and Z is the ideal reactance at a given freq. As Q goes up, the inductor approaches an ideal inductor.
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
No I'm afraid not. Loaded Q from what I understand is the Q of an inductor (or network?) during operation.(?) I'm wondering if it is possible to calculate it? Or even how to measure it?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
This gets somewhat complicated since you need a scope or something that will show you your amplitude. Basically tune for maximum current like shown in the picture, which for a Tesla coil as a quarter wave transmission line goes, you will see a huge attenuation in signal strength, this is Fo. Then, since this is a little backward to the graph, you deviate from your Fo until your amplitude increases by 0.30 on either side of Fo. Subtract the higher F2 from lower F1 and you have your Bandwidth.
As mentioned before, R is DC resistance (although I'm not really happy with that approximation, Rac plays an important role here) And you have reactance, XL = 2piFL
Q= XL/R
Don't be surprised if Bandwidth is very small. When the TC acts as a resonator it is very sensitive. Tune your coil to Fo and get within 4 feet of the coil and you will see how sensitive it can be.
Other measurements like impedance can be performed by a decade resistance in series with the coil, and Zmatch occurs when the signal generator voltage is halved at the point where the decade box connects to the coil. This is the Voltage divider principle applied to HF loads. Since you are performing this at resonance, Z essentially = Rac and reactance cancels or is very small.
Measurement of a driven system is difficult because this would interfere with the working coil, but if you use an SWR bridge insterted to the base of the coil you can perform a relative measurement of the active system. And if you design the bridge carefully it will not impact the coil in operation.
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A Loaded (Q)uestion
