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I have some snap-on ferrite chokes that are supposed to cut down on EMI, and I was wondering if there was any relatively direct way to measure — on the bench — whether they are effective or not. Is those something where you kinda just "try them on a noisy appliance and see what it does on the S-meter" or is there more of a "test fixture for the VNA" sort of approach available?

I found How to meassure a ferrite using a VNA? here but I think the OP was after more a direct measurement of the material's permeability? And then there's my own How to measure a balun for effectiveness? which I'm trying to figure out if a balun is really trying to accomplish the same think as a choke.

In either case I'm not really following the answers. I think part of the issue is that EMI stuff always kind of throws me. Like, on a piece of coax the ferrite really wouldn't mess with the normal signal, right? Just somehow a signal on the "outside of the shield"…? Or on just a simple pair of power supply wires or motor lines or whatever I would clamp these onto. Is there a way to measure the "noise" aspect of the wire before/after adding the ferrite?

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    $\begingroup$ Complex permeability is the direct and right measure of the ferrite core performance. You can use a VNA to measure the impedance of a one-turn inductor over the frequency range of interest. However, when you mention EMI or power supply wires or motor lines, the way to think about the "choke" is completely different depending on your aim. Whether you are trying to stop the common mode RF or the wideband noise or something else. You want to be specific about the scenario of the problem you try to solve for now. $\endgroup$ Commented Oct 26 at 1:38

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To test the impact of a ferrite choke on an actual line, there are several methods: 1) See if the SWR differs depending on whether the choke is present or not: If it does, then the choke is successfully blocking RF current on the coax shield; the greater the difference, the more necessary the choke is. Keep adding chokes until additional ones have little effct (or run more turns through each choke). 2) As well as the coax, run a small piece of wire through the choke and terminate it with a small-value resistor (10 ohms will do); Then take an RF voltmeter (or O'Scope) and see how much voltage appears across the extra wire inserted within the choke: this will give you an estimate of current flowing on the outside of the coax. Note that these are practical methods of determining whether the choke is successfully reducing unwanted common-mode currents. With a dual-channel O'scope, a signal generator, a resistor, and a piece of wire, you can get a very good estimate of the choke's complex impedance at frequencies of interest. Most modern high-end digital O'Scopes allow you to precisely read out amplitudes and phase shifts; you do not really need a dedicated VNA in many cases. That's why I invested in a Good digital O'Scope to compliment an olde analogue scope.

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