I want to measure a ferrite toroid using a VNA. I would like to classify it for a whole range of frequencies but if it's too complicated I'm OK to just pick one frequency. I have 3 questions actually: 1: Is there a preferred test jig? My plan is to open a coax cable and route the center of the coax through the ferrite while the ground mesh is bunched together and goes along the outside of the ferrite. 2: How do I deal with the effects created by the test setup, I'm afraid it will throw my data out of whack. 3: Do I need to move the ferrite along the length of 1/2 or full wavelength or what happens if the ferrite is placed at a current null?

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    $\begingroup$ Does this answer your question? ham.stackexchange.com/a/18449/8717 $\endgroup$ Commented Oct 28, 2021 at 15:51
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    $\begingroup$ How big is the toroid and what's your maximum frequency of interest? This determines if the jig has realistic inductance or not. $\endgroup$
    – tomnexus
    Commented Oct 28, 2021 at 16:04
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    $\begingroup$ What parameter do you want to measure, and with what precision? $\endgroup$
    – hotpaw2
    Commented Oct 28, 2021 at 16:24
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    $\begingroup$ @tomnexus @ hotpaw2 toroid size up to 1/2", frequency I'd say 300 MHz or some range around that. Trying to measure reflection coefficient or one of the related parameters. tomnexus thanks for your comment about using a cage around the center conductor, I think I will try that. $\endgroup$
    – Jack0220
    Commented Oct 28, 2021 at 18:21

1 Answer 1


Practical use of ferrite is possible without knowing all the details.

  1. Measure the inductance (connector to CH0) for one single winding (N=1) through the ferrite. Do this at a low frequency, f.e. 50 kHz (*). Then you can calculate what more windings will do by taking this value and multiply that with N^2

  2. Make a transformer with N to N windings, primary-secundary, and measure the inductance when the other side is open and also in case of a secondary short-circuit. The coupling factor k kan be calculated (**).

  3. Measure the transfer from that transformer by using both ports, CH0 transfer to CH1, of the VNA. The frequency transfer and the losses are important: the blue line (**).

Of course after calibration.

(*): not the high precision that the purists want.

(**): frequency dependent; measure this for the frequency that you intend to use.

This is the absolute minimum.


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