Understanding Baluns, Ununs, and Common-mode Chokes

Understanding the distinction between Baluns, Ununs, and Common-Mode Chokes is a rite of passage in RF engineering and homebrewing. These devices all manipulate RF currents and impedance, but they solve very different problems in your feedline and antenna systems. Here is a breakdown of how each works, why they are used, and when you actually need them.

1. Balun (Balanced to Unbalanced)

A Balun connects a Balanced system to an unbalanced system.

• The Unbalanced Side: Coaxial cable. The center conductor is shielded and referenced to ground (the outer shield). There are two currents on the shield, on inner aspect and outer aspect. The last one is an unbalanced common mode current.
• The Balanced Side: A symmetrical antenna, such as a center-fed dipole or a Moxon. Both sides of the driven element are electrically equal and neither is tied to ground.

Why you need it: If you connect a piece of coax directly to a resonant dipole, the system is mismatched. The RF current traveling up the center conductor goes into one leg of the dipole, but the current returning on the inside of the shield splits. Some of it goes to the other leg of the dipole, and the rest travels back down the outside of the coax shield.

Your feedline has just become part of your antenna, which distorts your radiation pattern and changes your resonant frequency. A balun forces equal currents into both halves of the balanced antenna, preventing this issue.

Common Types:

• 1:1 Balun: Used when the impedance of the coax matches the antenna (e.g., 50-ohm coax to a ~73-ohm dipole).
• 4:1 Balun: Used for impedance transformation alongside the balancing act (e.g., 50-ohm coax to a 200-ohm folded dipole or Off-Center Fed Dipole).

2. Unun (Unbalanced to Unbalanced)

An Unun connects an unbalanced feedline to an unbalanced antenna. Its sole purpose is impedance transformation.

• The Scenario: You are feeding an unbalanced antenna, like a quarter-wave vertical or an End-Fed Half-Wave (EFHW) wire.

Why you need it: An EFHW has an extremely high feedpoint impedance, often between 2000 and 3000 ohms. Your transceiver and coax are expecting 50 ohms. Connecting them directly would result in a massive SWR mismatch and very little power radiated.

An unun acts as an RF transformer. For an end-fed wire, homebrewers typically wind a 49:1 or 64:1 Unun on a toroidal core. This steps the ~2500-ohm impedance of the wire down to the ~50 ohms your radio wants to see.

3. Common-Mode Choke

A Common-Mode Choke is a device designed to block RF currents from traveling on the outside of your coax shield (common-mode current) while allowing the desired signal inside the coax (differential-mode current) to pass straight through.

Electrically, a 1:1 Current Balun and a Common-Mode Choke are essentially the exact same thing.

Why you need it: Common-mode current is the leading cause of “RF in the shack.” When you are pushing higher output—like running a 100W linear amplifier—stray RF returning down the coax shield can wreak havoc. It can cause:

• “Mic bites” (getting shocked by your microphone).
• Distorted transmit audio.
• Erratic behavior in sensitive shack equipment, causing microcontrollers, SDRs, or single-board computers (like a Raspberry Pi) to freeze or reboot during transmission.
• An increase in the noise floor on receive, as the coax shield picks up switching noise from household electronics and carries it to the antenna.

Placing a high-quality choke (often coax wrapped through a mix-31 or mix-43 ferrite toroid) at the antenna feedpoint—and sometimes a second one right where the coax enters the shack—acts as a brick wall to these unwanted shield currents.

Summary Cheat Sheet:

• Dipole (Balanced) + Coax: Use a Balun.
• End-Fed Wire (Unbalanced) + Coax: Use an Unun.
• Stray RF interfering with your workbench electronics: Add a Common-mode Choke.