Designing and testing ferrite core common mode chokes can be a challenge. You have lots of things to consider.

Generally, we all know that common mode currents on cables are the major source of RFI. In an ideal world, ac currents flowing along a wire are countered by an equal and opposite phase current flowing along a second wire. You will find this should be true for twisted pair or shielded coaxial cable.

When these current flows become unbalanced, you get radiation. Or, in the case of receiving, RFI ingress into your equipment. You will find tons of information about common mode interference online, so I am just noting the basics here.

You have probably noticed that manufacturers and hams seek to reduce this interference or imbalance by placing common mode chokes on cables. Typically, you find these on USB, VGA, HDMI and power supply cables. Usually in the form of a ferrite bead clamped over the cable, as shown above left. Hams often go a step further and wind cables through larger ferrite cores, as shown above right.

So, why do we use ferrites for common mode chokes? In short, ferrites increase the amount of impedance you can use to block common mode currents. Ferrites are magnetic materials that produce inductive impedance for high frequency signals. Properly designed, you can create an ac signal voltage drop, reducing the unwanted common mode current flowing in your cable.

Also, you can increase the amount of impedance by winding your cable around a core. Impedance increases as N², the square of the number of turns. You can see this effect in the graph above lower left.

Designing Common Mode Chokes

Here are some things you need to know. Different ferrite mixes are more effective at different frequencies. Most work at VHF and higher, but only a few at HF. If you are using clamp on ferrite beads, you can add more beads to get more impedance. But how many do you need?

Similarly, if you are winding cable through a toroid core, how many windings will do the job? And, even more perplexing, if you have pickup up ferrites of an unknown mixture, how do you know if and where they will work?

In my next article, I will tell you about a wonderful common mode current test rig you can use with your NanoVNA to answer all these questions.