Common-mode currents are a well-known enemy of clean RF communication. When RF travels along the outside of your coaxial cable, it causes feedline radiation, noise pickup, RFI in the shack, audio distortion, and unpredictable SWR variations. To solve this, radio amateurs typically install a CMC choke (also called a current balun or common-mode filter) at the antenna feed point. The design shown in the image is a simple yet powerful DIY CMC choke using steel wool, RG-213 coax, and a 500 × 50 mm PVC tube. This type of choke uses a resistive-absorptive method to kill unwanted common-mode energy and offers excellent wideband performance across the HF spectrum.
In this article, we break down how this choke works, why steel wool is used, how to construct it, and how different tube lengths affect performance.Although simple, it provides surprisingly wideband suppression and works well on a variety of HF antennas, including EFHWs, long wires, verticals, and dipoles.
A choke of this type typically provides:
- 10–25 dB of common-mode current attenuation across HF
- More attenuation at higher frequencies
- Strong suppression noise and feedline coupling
For casual HF operation, this is excellent performance for the cost.
How This CMC Choke Works
To understand the purpose of this device, it is important to know the difference between differential-mode and common-mode signals. Differential-mode RF flows inside the coaxial cable, between the center conductor and the inner surface of the shield. This is the desired signal, and nothing in the choke interferes with it. Common-mode currents, however, travel on the outer surface of the coax shield. These currents radiate, introduce noise, and often cause unwanted RF feedback.
The steel-wool CMC choke addresses this by absorbing and dissipating the unwanted energy. Steel wool contains thousands of thin conductive fibers. These fibers have electrical resistance and mild magnetic properties, and when packed around the coaxial cable, they create a lossy environment. As common-mode currents propagate along the outside of the coax shield, they interact with the steel wool and lose energy in the form of heat. This attenuation reduces feedline radiation significantly, while the desired differential-mode signal continues unaffected inside the coax.
The result is a broadband choke that works on many HF bands without requiring precise tuning or expensive ferrite materials.
Construction Overview
The construction is straightforward. A PVC tube of 500 × 50 mm is prepared as the outer housing. RG-213 coax is passed through the tube, and the remaining space is loosely packed with steel wool. The steel wool must not be compressed tightly; slight airflow spaces improve the absorption behavior and avoid damaging the coax. Once filled, the ends of the tube are sealed with silicone or glue to produce a weatherproof assembly. Hooks at the top allow the choke to be hung at the antenna feed point, and an SO-239 or similar connector is installed at the bottom for the coax interface.
This creates a compact, rugged choke suitable for permanent outdoor installations. Because it is sealed, the steel wool does not corrode significantly and retains stable performance for a long period.
Steel wool may seem like an unusual material for RF suppression, but its electromagnetic properties make it extremely useful. Each fine strand presents a resistive and inductive path to high-frequency currents. When thousands of strands are grouped together, they behave similarly to a distributed lossy ferrite. Instead of producing a large inductance at a specific frequency, steel wool offers broad absorption over a wide range of HF frequencies. This makes it ideal for multiband antennas where a single ferrite mix might not cover everything effectively.
The CMC choke does not rely on resonance. Instead, it continuously drains the unwanted current as it attempts to move along the coax shield. The more steel wool the current encounters, the more attenuation occurs.
Choosing the Length of the Choke
The length of the PVC tube determines how effective the choke is, especially on the lower HF bands. Shorter tubes provide moderate suppression and are usually sufficient for the higher HF bands like 20 to 10 meters. A medium length of around 500 mm, as shown in the diagram, provides a good compromise for broad HF use including 80, 40, and 20 meters. Longer tubes, in the range of 600 to 800 mm or more, can improve suppression on lower frequencies and are preferred in high-power or low-band installations.
The 500 mm design is popular because it delivers wideband results while remaining lightweight and mechanically manageable.