Balun & Unun Guide
Impedance transformers and common-mode chokes are essential for clean, efficient antenna systems. This page explains the most-used ratios, when to choose each type, and practical construction notes.
Common Ratios at a Glance
| Ratio | Type | Typical Impedance | Best Used With |
|---|---|---|---|
| 1:1 | Current balun / choke | 50 ↔ 50 Ω balanced | Center-fed dipoles, Yagis, loops |
| 4:1 | Guanella current balun (or voltage type) | ~200 Ω → 50 Ω | OCF dipoles, folded dipoles |
| 9:1 | Unun (unbalanced-to-unbalanced) | ~450 Ω → 50 Ω | Random / long-wire antennas |
| 49:1 | EFHW transformer (unun) | ~2450 Ω → 50 Ω | End-fed half-wave |
| 64:1 | EFHW transformer (unun) | ~3200 Ω → 50 Ω | Higher-impedance EFHW |
What a Balun Actually Does
A balun (balanced-to-unbalanced) converts between a balanced antenna feed (two equal conductors with equal and opposite currents) and an unbalanced coaxial line. A pure current balun also acts as a common-mode choke, forcing equal currents and keeping RF off the outside of the coax shield.
An unun (unbalanced-to-unbalanced) is used when both the antenna and the feedline are unbalanced — typical for end-fed wires. The high transformation ratios (9:1, 49:1) bring the high feed-point impedance of a half-wave end-fed down to something a modern transceiver can handle.
Why the 49:1 is so popular for EFHW
A half-wave wire fed at the end presents roughly 2 000–3 500 Ω depending on height and surroundings. A 49:1 turns ratio (seven-to-one turns, squared) maps that range into the 40–70 Ω region, close enough for a solid-state radio with a modest internal tuner or no tuner at all on the design band.
Core Selection & Winding Tips
- Ferrite mix 43 (FT240-43 or similar) is the usual choice for HF 1.8–30 MHz. Mix 31 is preferred by many for the lower bands and better choking impedance.
- For a 49:1 EFHW unun a common winding is 2 turns primary and 14 turns secondary on an FT240-43, often with the primary as a bifilar or separate winding.
- Keep the primary winding close to the “cold” end of the secondary for best voltage distribution.
- Use PTFE or high-temperature insulated wire; the core can get warm at 100 W continuous.
- A good 1:1 current balun for a dipole is typically 8–12 turns of RG-58 or similar coax through a stack of 31 or 43 beads, or a multi-turn winding on a large toroid.
Common Mistakes
- Using a voltage balun where a current balun is needed — residual common-mode current radiates from the coax and can cause RFI in the shack.
- Omitting a counterpoise or adequate ground with a high-ratio unun; the coax shield then becomes part of the antenna.
- Winding too few turns on a 1:1 choke, resulting in inadequate common-mode impedance on the lower bands.
- Placing the unun right at the operating position instead of at the feed point — the high-impedance line then radiates.
Related calculators: EFHW length · OCF dipole · Random wire lengths
Choosing the transformer for the job
A balun and an unun solve different problems, even though both can change the impedance presented to a feedline. A 1:1 current balun is primarily a common-mode control device: it helps keep unwanted RF off the outside of the coax shield when a balanced antenna is fed from an unbalanced line. A 4:1 transformer is commonly used where the antenna system presents a higher impedance than the coax can conveniently match, such as some off-centre-fed designs. A 9:1 unun is often used as an impedance transformer for a random-wire system, while an EFHW transformer commonly uses a much higher turns ratio to transform a high end-fed impedance toward 50 Ω.
The turns ratio matters because impedance changes approximately with the square of the turns ratio. Core material, winding arrangement, wire insulation, frequency range and physical construction all affect the real result. At HF, a transformer that looks fine on a low-power analyser can still become lossy or hot at higher power if the core, winding spacing or common-mode suppression is inadequate. Treat published ratios as starting points, not guarantees.
For a practical build, specify the intended frequency range and power first, select an appropriate ferrite or powdered-iron core, keep the winding geometry repeatable, and provide a separate common-mode choke when the antenna/feed arrangement needs it. Verify the finished assembly with an analyser at the intended operating power where possible. A good transformer should make the desired impedance transformation while remaining cool and mechanically secure.
How to choose the ratio
A transformer ratio changes impedance according to the square of the turns ratio. A 2:1 turns ratio therefore produces a 4:1 impedance transformation, while a 3:1 ratio produces 9:1. That relationship is easy to remember and easy to misuse: the ratio refers to turns, not directly to impedance. In an EFHW transformer, the winding arrangement, core material, leakage inductance and operating frequency all affect how closely the real transformer follows the ideal calculation.
A current balun and an impedance transformer solve different problems. A current balun is intended to impede unwanted common-mode current on the outside of a coax shield, while an impedance transformer is used to transform the load presented to the feedline. Some practical builds combine both functions, but they should not be treated as interchangeable. If a wire antenna already presents a difficult common-mode environment, simply choosing a different turns ratio may not solve the feedline-current problem.
For construction, leave enough physical room for insulation and keep the winding arrangement consistent from one build to the next. Core heating, wire size, duty cycle and peak voltage can matter more than the nominal turns ratio at higher power. Treat calculated turns as a starting point, then verify the finished transformer with an analyser and a suitable dummy load before connecting it to the antenna.
Assumptions & limitations
- Ratio tables assume ideal transformation; real cores have loss, limited bandwidth and power-handling limits that depend on mix, size and winding.
- No quantitative core-loss or temperature-rise calculation is performed here.
- Construction notes are starting guidance — verify with an analyser and under power.