Notice: This page was initially generated with the assistance of AI and is pending human review. The information may contain errors or omissions. Amateur radio operators are encouraged to verify all technical details independently. Help improve this page by submitting corrections and additions. Learn how to contribute Remove this banner after human review is complete.
If you have ever connected a coaxial cable directly to a dipole, you have unknowingly created a problem that baluns are designed to solve. Baluns and ununs are devices used in antenna systems to manage the transition between different types of transmission lines and antennas, and to transform impedances. They are among the most misunderstood components in amateur radio, but understanding what they do and when you need one will improve your station's performance and help you avoid frustrating problems.
The terms "balanced" and "unbalanced" refer to how current flows in a transmission line or antenna relative to ground:
An unbalanced line has one conductor at ground potential (or connected to a shield) and the other carrying the signal. Coaxial cable is the most common unbalanced transmission line — the outer shield is typically grounded, and the centre conductor carries the signal.
A balanced line has two conductors that carry equal but opposite currents, with neither conductor at ground potential. Ladder line (open-wire line) is a balanced transmission line. A dipole is a balanced antenna — the two halves are mirror images, and current flows equally in each half.
The issue arises when you connect an unbalanced line (coax) directly to a balanced antenna (dipole). The connection creates an imbalance that can cause current to flow on the outside of the coax shield. This common-mode current radiates from the feedline, distorting the antenna's radiation pattern, causing RF to flow back into the shack (leading to RF interference with other equipment), and making the SWR reading unreliable.
A balun (from "balanced to unbalanced") is a device that provides a proper transition between a balanced system and an unbalanced system. When placed at the feed point of a dipole that is fed with coax, a balun prevents common-mode current from flowing on the outside of the coax shield.
Baluns can also provide impedance transformation. A 1:1 balun provides a balanced-to-unbalanced transition without changing the impedance. A 4:1 balun transforms the impedance by a factor of four (for example, converting 200 ohms balanced to 50 ohms unbalanced) while also providing the balanced/unbalanced transition.
An unun (from "unbalanced to unbalanced") transforms impedance between two unbalanced systems without changing the balance mode. For example, a 9:1 unun can transform a high-impedance unbalanced feed point (such as an end-fed wire antenna) down to approximately 50 ohms for connection to standard coax.
The key distinction: a balun changes the balance mode (and may also transform impedance), while an unun changes impedance only without altering the balance mode.
Common-mode current is current that flows on the outside of the coax shield (or equally on both conductors of a balanced line, rather than differentially). It is the primary issue that choke baluns are designed to suppress.
Common-mode current causes several problems:
A good choke balun (also called a common-mode choke or current balun) presents high impedance to common-mode current while having minimal effect on the desired differential-mode signal. This forces the current to flow only in the intended paths — inside the coax and in the antenna elements.
The choke balun works by presenting high impedance to common-mode current. It does not use transformer action and therefore does not saturate under high power. This is the most recommended type for most amateur applications.
Ferrite choke balun: Coax is wound through or around ferrite cores (toroids or clamp-on cores). The ferrite provides impedance to common-mode current flowing on the outer shield while the signal inside the coax passes through unaffected. Mix 31 ferrite is popular for HF use, while Mix 43 works well at higher frequencies.
Air-wound choke balun (ugly balun): Several turns of coax are wound into a coil, typically 15–20 cm (6–8 inches) in diameter. The inductance of the coil presents impedance to common-mode current. This is the simplest and cheapest balun to build — it works, though not as effectively as ferrite designs, especially at lower HF frequencies. It is sometimes called an "ugly balun" because it looks like a coil of leftover coax.
Stacked ferrite bead choke: Multiple ferrite beads (or snap-on chokes) are placed over the coax near the feed point. Effective and simple, though it requires enough beads to develop sufficient choking impedance.
The voltage balun uses transformer action to force equal and opposite voltages at the balanced output. Traditional designs wind separate windings on a ferrite core. While voltage baluns provide the balanced-to-unbalanced transition, they are less effective at suppressing common-mode current than choke baluns and can saturate at high power levels when the load is reactive (high SWR).
For most amateur applications, a choke balun is the better choice because it handles high SWR gracefully and directly addresses the common-mode current problem.
For impedance-transforming baluns and ununs, two design approaches are widely used:
Guanella balun: Uses parallel transmission-line segments wound on ferrite cores. These segments are connected in parallel on one side and in series on the other to achieve the desired impedance ratio. Guanella designs have wider bandwidth and handle higher SWR better than Ruthroff designs. A Guanella 4:1 balun is an excellent choice for feeding a folded dipole or a doublet antenna system.
Ruthroff unun/balun: Uses a single transmission-line winding on a ferrite core with a specific connection arrangement to achieve impedance transformation. Ruthroff designs are simpler to build but have narrower bandwidth and are more sensitive to reactive loads. A Ruthroff 9:1 unun is commonly used with end-fed half-wave antennas.
| Ratio | Type | Typical application |
|---|---|---|
| 1:1 | Balun (choke) | Dipole fed with coax — the most common use. Suppresses common-mode current without impedance transformation. |
| 1:1 | Balun (choke) | At the point where ladder line transitions to coax entering the shack. |
| 4:1 | Balun | Folded dipole (300 Ω) to coax (75 Ω). Doublet antenna with ladder line to a tuner with balanced input. |
| 9:1 | Unun | End-fed half-wave or random wire antenna to coax. Transforms roughly 450 Ω (or higher) to 50 Ω. |
| 49:1 | Unun | End-fed half-wave antenna (high impedance feed point of ~2500 Ω) to 50 Ω coax. |
| 6:1 | Unun | Off-centre fed dipole (OCF) to coax, where the feed point impedance is roughly 300 Ω. |
Both commercial and homebrew baluns and ununs can work well. For hams who enjoy building, these are satisfying projects that require only ferrite cores, wire or coax, and an enclosure. Key considerations:
For purchased baluns, look for units that specify the ferrite material used, the choking impedance across the frequency range, and the power rating under mismatched (high SWR) conditions — not just matched-load ratings.
At the feed point of any balanced antenna fed with coax. This means any dipole, doublet, loop, or Yagi fed with coaxial cable should have a 1:1 choke balun at the feed point. This is the single most impactful place to use a balun and addresses the most common source of common-mode problems.
At the transition from ladder line to coax. If you use ladder line from the antenna and then transition to a short run of coax into the shack, place a 1:1 choke balun at (or near) that transition point.
At the entry to the shack. An additional choke on the coax where it enters the building can further reduce common-mode current and help with RF interference inside the station.
On receive antennas. Common-mode current on receive feedlines picks up noise from electrical sources near the cable run. A choke balun can significantly improve the signal-to-noise ratio on receive-only antennas.