What is a Choke Balun and Do I Need One?

What is a Choke Balun?

A choke balun (also called a current balun or 1:1 choke) is a device that prevents RF current from flowing on the outside of your coaxial cable. This unwanted current is called "common mode current."

The Problem: Common Mode Current

Your coax cable has three possible current paths:

1. Center conductor (signal going to antenna)
2. Inside of the shield (signal returning from antenna)
3. Outside of the shield (this is the problem)

Current on the outside of the shield causes:

• RFI in your shack: Your coax acts as an antenna, bringing RF back to your radio, computer, and other equipment
• Distorted radiation pattern: The feedline becomes part of the antenna, changing how your signal radiates
• Receive noise: The feedline picks up local noise and adds it to your received signal
• Unpredictable SWR: Changes when you touch the coax or move things around

How a Choke Balun Works

A choke balun presents high impedance (resistance) to common mode currents while allowing normal differential signals to pass freely. It's like a one-way valve that blocks the unwanted current on the outside of the coax.

The most effective choke baluns for HF use ferrite cores. Winding coax or running it through ferrite creates inductance that "chokes off" common mode current.

Types of Choke Baluns

Coax Wound on Ferrite Toroid

Wind several turns of coax through a ferrite toroid core. This is the most common homebrew approach and very effective.

• Use Type 43 ferrite for general HF (80m-10m)
• Use Type 31 for low bands (160m-40m)
• 10-12 turns typically

Ferrite Beads on Coax

Slip ferrite beads over your coax. Less effective than a wound choke but easy and helpful.

Commercial Choke Baluns

Many companies sell ready-made choke baluns. DX Engineering, Balun Designs, and Palomar Engineers are popular choices.

Air-Core Coax Choke

Simply coiling your coax (like making a coil of rope) creates some choking. This is called an "ugly balun." It helps but is less effective than ferrite chokes.

When Do You Need a Choke Balun?

Definitely Use One

• At the feedpoint of any balanced antenna (dipole, doublet, Yagi)
• If you have RFI problems (RF getting into your microphone, computer, etc.)
• If touching your coax changes SWR

Good Idea to Use One

• At the entrance to your shack (as a "line isolator")
• Any HF installation for best performance

Maybe Not Critical

• Very short feedlines
• Temporary field operations (but it still helps!)

Where to Place a Choke Balun

The best locations are:

1. At the antenna feedpoint: Prevents common mode current from even starting
2. At the shack entrance: Blocks any remaining common mode current from entering your operating position

Many hams use both: a choke at the antenna AND a line isolator at the shack.

Building vs Buying

Build Your Own (Cost: $15-30)

• Buy a ferrite toroid (FT-240-43 is popular)
• Wind 10-12 turns of RG-8X or RG-142 through it
• Add SO-239 connectors on each end
• House in a weatherproof enclosure for outdoor use

Buy Commercial (Cost: $40-100+)

• Ready to use
• Professionally built
• Often better weatherproofing
• Good for those who don't want to build

Both approaches work well. Building your own is satisfying and educational, but commercial units are convenient and reliable.

Testing Your Choke

To see if you have common mode problems:

1. Key your transmitter at low power
2. Touch the outside of the coax near your radio
3. If you feel RF warmth or see your power meter/SWR change, you have common mode current

A good choke should eliminate or greatly reduce these symptoms.