Topband: Measuring Inductor Q

[Larry Molitor]

--- On Sun, 7/6/08, Rick Karlquist <richard at karlquist.com> wrote:
> 
> What I like to do is make a parallel resonant tank
> with the inductor under test and a high Q capacitor.
> (Any NP0/C0G ceramic will have Q>1000).  I ground
> one of the terminals of the tank and then very
> loosely couple into the tank with a signal generator
> by connecting a 1 pF capacitor from the signal generator
> to the non-grounded terminal of the tank.  I then
> connect another 1 pF capacitor from the non-grounded
> terminal of the tank to a spectrum analyzer and observe
> the 3 dB BW of the tank.  The Q is then given by dividing
> this into the center frequency.  

The method described by Rick above is what I use with an exception. Instead of the 1 pF coupling capacitors, I use a pair of single turn shielded links. The coil under test can then be physically placed far away from any material that will affect the Q reading. I use Polystyrene supports and place the link coils as far away from the the ends of the inductor under test as possible given the sensitivity of the detection device. One link for input, one for output at the opposite end of the coil under test. I have measured Q's in excess of 600 in this manner.

This works for large coils like tank coils or mobile antenna coils but you need to go back to the 1 pF coupling caps to measure toroid coils.

You can use a MFJ-259B or similar analyzer for the signal generator and almost anything that will detect the signal, the more sensitive the better. Before I had a spectrum analyzer, I usually used a signal generator that had 1 kHz modulation and used an old HP415B SWR meter as the detector.

A word about the resonating capacitor. Vacuum variables are a poor choice simply because they are so physically so large that they cannot be separated far enough away from the inductor under test to prevent degrading Q. If you need a variable cap you can use a small split stator air variable and connect the coil between the two stators allowing the rotor to float. This eliminates the "wiper" from the circuit and the result is a variable cap with a Q as high as a vacuum variable and much less mass. Even so, I prefer to use several small silver mica caps in parallel and connect them to the coil ends with a 1/2 inch copper strap. This produces the highest Q readings for a particular inductor because of the minimal influence of surrounding objects required for test.

Larry - W7IUV
DN07dg - central WA
http://w7iuv.com