How to Use Ferrite Beads and Toroid Cores to Reduce Common Mode Current

Ferrite Beads and Toroid Cores

Ferrite beads and toroid cores come in different sizes and material mixtures for most applications. These mixtures have different frequency ranges where they are most effective. They are widely used in electronics, and many hams use them to reduce radio frequency interference (RFI).

You may have already seen them on phone chargers or other wall warts, and they are often built inside electronics that are susceptible to RFI.

Toroid cores can be used for RFI suppression and are an integral part of transformers like bal-uns and un-uns. These are highly used in amateur radio antenna systems.

Common Mode Current

Common mode current runs on the outside of a coax or wire. This current should not be there and is the cause of most radio interference. Any wire can transmit or receive this current, and it is a noticeable problem in amateur radio.

It will affect citizens band radio and even stereo speakers. A known solution for common mode current are ferrite beads and toroid cores. They impede the current from traveling along the outside of the wire without affecting the current inside the wire.

The largest impact of common mode current in amateur radio is on the antenna coax cable. However, it can cause interference on any wire connected to the radio.

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