Baluns and common-mode chokes

A balun (balanced-to-unbalanced) is the bridge between a balanced antenna and unbalanced coax.

Why you need one

Connect coax directly to a balanced dipole and RF current can flow back down the outside of the shield. This common-mode current makes the feedline radiate, distorts the pattern, and causes RF in the shack. A balun forces the antenna current to stay balanced.

Types

• Current (choke) balun — chokes common-mode current; the best general choice for a dipole feedpoint.
• Voltage balun — enforces equal voltages; used in some tuners.
• Transformer baluns — also change impedance, e.g. 4:1 for an off-center-fed dipole, 49:1 for an end-fed half-wave.

Ratios

A balun’s ratio (1:1, 4:1, 9:1, 49:1) sets how it transforms impedance. Pick the ratio that maps your antenna’s feedpoint impedance toward the 50 ohms your coax and radio want.