Balun

Overview

A balun connects a balanced load — a dipole, a loop, a ladder-line feeder — to unbalanced coax. Without one, the outside of the coax shield becomes part of the antenna, carrying current that radiates in unwanted directions and brings interference into the shack.

Two jobs get confused under the same name. A 1:1 current balun forces equal and opposite currents in the two feed conductors and stops shield current. A 4:1 impedance balun additionally transforms impedance, which is what a folded dipole or a 300 ohm feeder needs. They are not interchangeable.

Bands and modes

Band coverage

Modes

Safety note

Using it well

• Use a 1:1 current balun for a coax-fed dipole; reserve 4:1 for folded or high-impedance loads.
• Choose the core for your lowest operating band, not the highest.
• Weatherproof the box and the connector below it.
• Check for heating after a long transmission — a warm balun is a saturated core.