Feedline Choke

Overview

A feedline choke is a common-mode suppressor placed around the coax, usually a ferrite sleeve or a set of clamped cores near the feed point. It presents a high impedance to current flowing on the outside of the shield while passing the wanted differential signal untouched.

This matters because shield current is the most common cause of RF getting into audio equipment, computer interfaces and telephones. It also distorts the antenna pattern, because the shield is radiating as an unintended part of the antenna. A choke is one of the cheapest and most effective fixes in amateur radio.

Bands and modes

Band coverage

Modes

Safety note

Using it well

• Fit a choke at the antenna feed point first, before adding one in the shack.
• Choose the ferrite mix for the band you are trying to clean up.
• Wind several turns through a large core for the best common-mode impedance.
• Test the effect by watching whether the interference changes when you touch the coax.