Using NanoVNA To Measure Inductor Q

How good are those inductors you just wound? What about those inductors you just bought from your favorite online store? Don’t know? Well knowing the Q of the part is going to help you know whether they are going to be useful in that bandpass filter or oscillator and it could not be simpler if you have a nanoVNA.

So what I have here is a T68-6 toriod with 19 turns of enamel wire on it. 19 turns gives a theoretical inductance of 1.7uH, the actual measured inductance with my cheap LCR meter is 1.747uh yeah that is surprisingly close and not something that I typically find that happens. Usually 1 or 2 turns either way is required to get to the value I want.

For this measurement you want the inductor to be in series. So you can see from the picture above my test jig the inductor goes from the center conductor to the shield of the coax.

So the most important part is how to make the measurement. First is to re-calibrate the nanoVNA for the frequency range of interest for the part. For me, Captain HF, 3 to 30Mhz is where I keep a saved calibration for. Next, the measurement is an S11 1 port measurement and the plot you require is R+jX.

So from here the math is rather simple, Q = X / Rseries for the frequency of interest. So for the above at 14Mhz, for simplicity sake lets call it Q = 200 / 1 || Q = 200. In reality the Rs is less than 1 and the X is less than 200, but thankfully NanoVnaSaver does the actual calculations for you and displays them in this case the actual Q was 150. Which is pretty damn good for a hand wound toriod inductor. Now the good thing is, you know how to measure Q and you can start checking all sorts of parts you might have sitting around.

Take these cheap ebay variable inductors as an example. I built a filter with them for 40m and it was crap. Q at 7Mhz was about 20. But at 14Mhz though to 150Mhz the Q was about 50 to 75 on average. So they would be useful to use at higher frequencies and certainly not useful at lower ones. Unless you have a datasheet that tells you what the Q is for a given frequency range is, you will never know unless you measure them.

And lets face it, measurement is king. Even a non perfect measurement with a non perfect instument like the nanVNA is vastly superior to having no measurement at all. Oh and one last nugget, now that you know the Q of your actual parts, you can then use that value in Eslie when designing filters. Being able to measure things can really improve everything you do.