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RF Chokes & Common-Mode Suppression

RF chokes are used in amateur radio antenna systems to suppress unwanted common-mode currents that can flow on the outside of coaxial feedlines. These currents can distort antenna radiation patterns, increase noise, and cause RF interference in the station.

This guide explains what common-mode current is, why it occurs, and how RF chokes are used to improve antenna system performance and operating stability.

What Is Common-Mode Current?

Common-mode current occurs when RF energy flows along the outside of the coaxial cable shield instead of remaining confined within the feedline. This often happens when antennas are unbalanced or improperly matched.

  • Causes feedline radiation
  • Distorts intended radiation patterns
  • Can introduce RF into the operating position

What Is an RF Choke?

An RF choke is a device that presents high impedance to common-mode currents while allowing the desired RF signal to pass normally inside the coax. Chokes are typically installed near the antenna feedpoint or at strategic locations along the feedline.

  • Blocks unwanted RF on the coax shield
  • Improves antenna efficiency and pattern stability
  • Reduces RF interference in the shack

Common RF Choke Designs

Several RF choke designs are commonly used in amateur radio installations:

  • Ferrite Chokes: Use ferrite cores to suppress common-mode currents
  • Coaxial Chokes: Formed by coiling coax into an air-wound choke
  • Bead Chokes: Use ferrite beads placed over the feedline

Where to Install RF Chokes

Placement of RF chokes affects their effectiveness. Chokes are often installed:

  • At the antenna feedpoint
  • Near the antenna tuner
  • Before the feedline enters the building

Benefits of Common-Mode Suppression

Proper use of RF chokes improves overall station performance and operating comfort.

  • Cleaner radiation patterns
  • Lower noise levels
  • Reduced RF feedback issues

Related Antenna & Feedline Guides

→ Antenna Basics & How They Work
→ Dipole & Multi-Band Antennas
→ Vertical Antennas
→ Yagi & Directional Antennas
→ Baluns & Ununs Explained

This page is part of the Elmer Reference Library and explains how RF chokes and common-mode suppression improve antenna system performance and reliability.