Balun Ratio Calculator

A balun (BALanced to UNbalanced transformer) connects a balanced transmission line or antenna feed to an unbalanced coaxial feedline, and in most practical amateur radio installations it also transforms impedance. The word "balanced" describes a two-conductor system where both conductors carry equal and opposite currents with respect to ground, like a dipole antenna or open-wire ladder line. "Unbalanced" describes a system where one conductor (the coax shield) is at ground potential, carrying return current on its outside surface. Without a balun at the transition, common-mode current flows down the outside of the coax shield, causing RF in the shack, pattern distortion, and potential interference.

The impedance transformation ratio of a balun is determined by its turns ratio squared. If the primary (coax side) winding has n₁ turns and the secondary (antenna side) has n₂ turns, the impedance transformation ratio is (n₂/n₁)². A 1:1 turns ratio transforms no impedance and serves purely as a common-mode choke. A 2:1 turns ratio transforms impedance by a factor of 4 (4:1 balun), and a 3:1 turns ratio gives a 9:1 ratio. Standard baluns are built with integer turns ratios (1, 2, 3, 4) yielding ratios of 1:1, 4:1, 9:1, and 16:1. Non-integer ratios like 6.25:1 (2.5:1 turns ratio) are also wound for specific applications.

The most important distinction in balun design is current balun versus voltage balun. A current balun (also called a choke balun) forces equal and opposite currents in the two conductors regardless of load symmetry. Because it responds to currents rather than voltages, it maintains balance even when the antenna is slightly asymmetric due to nearby objects, asymmetric feed point geometry, or off-resonance operation. A voltage balun forces equal and opposite voltages, which works perfectly only when the load is exactly balanced. For all practical dipole and doublet antenna work, a current balun is strongly preferred.

Selecting the right balun ratio is a matter of matching the antenna's feedpoint impedance to the 50-ohm coaxial feedline used in most amateur stations. A resonant half-wave dipole at its feedpoint typically presents 72 ohms — close enough to 50 ohms that a 1:1 choke balun works well. A folded dipole presents approximately 288 ohms, so a 4:1 balun steps that down to 72 ohms. An EFHW (end-fed half-wave) antenna presents 2,700 to 5,000 ohms at its feedpoint, requiring a 49:1 or similar transformation. Open-wire ladder line typically operates with a characteristic impedance of 300 to 600 ohms, making 4:1, 6.25:1, or 9:1 baluns the appropriate choices depending on the actual feedpoint impedance at the operating frequency.

When building a toroidal current balun, the number of turns matters as much as the ratio. Too few turns produces insufficient choking reactance at low frequencies — the choke's inductive reactance should be at least five to ten times the source impedance at the lowest operating frequency. For a 1:1 choke balun on 160m (1.8 MHz) feeding a 50-ohm dipole, you need at least 250 ohms of choking reactance, which means the bifilar winding inductance must be at least 22 µH. Use a type 31 or type 43 ferrite for maximum permeability at HF frequencies.

The SWR and mismatch loss values shown for the nearest standard balun assume a purely resistive load. Real antenna feedpoints have a reactive component (X ohms of inductance or capacitance) in addition to the resistive component. This reactance increases the actual SWR at any given frequency, which is why operating a multi-band doublet with a tuner and 9:1 balun at frequencies away from resonance may show higher SWR than calculated here. An antenna analyzer measuring the actual feedpoint impedance will give you the real impedance transformation required.

For transmitting baluns at legal-limit power (1500 W in the US), core saturation and thermal dissipation become critical. Verify that the core material maintains its permeability at the operating frequency and that the power dissipation per watt of reflected/common-mode current does not overheat the core. The ARRL Antenna Book tables provide Amidon/Fair-Rite core data with power handling guidelines. For receive-only applications such as SDR-connected loop antennas, small Type 43 toroids with just 10–15 turns are more than adequate.