Baluns and ununs

What it is

A balun (balanced-to-unbalanced) connects a balanced antenna or line to unbalanced coax. An unun (unbalanced-to-unbalanced) connects two unbalanced sides. Either is usually a transformer wound on a ferrite or powdered-iron core, and either can also change impedance. Which one you want depends on the antenna.

How it works

Why balance matters. A dipole is two equal halves, so each half should carry the same current. Coax is unbalanced: its shield and its centre conductor do different jobs. Connect coax straight to a dipole and the shield side of the antenna has another path to follow, the outside of the shield itself, so some current flows on the cable instead of the antenna. That is common-mode current. A 1:1 balun forces the currents to be equal and opposite, which is the same job as a choke at the feed point.

Why impedance matters. A transformer scales impedance by the square of its turns ratio, so a 2:1 turns ratio gives 4:1 and a 7:1 ratio gives 49:1.

A half-wave wire fed at one end has a very high impedance, a few thousand ohms, which a 49:1 unun turns into something near 50 Ω (50 × 49 = 2450 Ω). A 9:1 unun suits loads of a few hundred ohms (9 × 50 = 450 Ω) and is common with random wires. A 4:1 balun suits a balanced load of roughly 200 to 300 Ω. These ratios are rules of thumb; real antennas vary with height, length and band.

Types and uses

Device	Changes Z?	Balanced and unbalanced?	Typical use
1:1 balun / current balun	no	yes	dipoles, Yagis, loops fed with coax
4:1 balun	yes	yes	folded dipoles (about 300 Ω), ladder line to a coax output
9:1 unun	yes	no (both unbalanced)	random wires and long wires
49:1 unun	yes	no	end-fed half-wave wires
1:1 unun / choke	no	no	keeps common-mode current off a vertical's feed line

In practice

• Put the balun at the feed point for a dipole. A long coax with no balun lets current flow on the cable.
• Mind the power rating. Transformers and chokes made of ferrite heat up. Under high power, with a large mismatch ratio, or at a frequency where the core is lossy, they run hot and can fail; use parts rated for the power and frequency.
• Match the ratio to the real load, not just the nominal one. If the antenna isn't close to the assumed impedance, the SWR won't be good and the balun runs hotter.
• A ladder-line tuner's balun is often in the tuner, not the line. See Antenna tuners and Open-wire and ladder line.
• No balun is a bad idea on a dipole with a long run of coax: the cable can radiate, pick up noise and cause interference in the shack.

Related

• Common-mode chokesKeeping RF off the outside of the coax.
• Open-wire and ladder lineLow-loss balanced feed lines.
• Antenna tunersWhat they match and what they don't fix.
• The half-wave dipoleThe reference antenna: pattern, impedance and length.
• End-fed half-wave antennasA half-wave fed through a high-impedance transformer.
• Coaxial cableConstruction, types and choosing a coax.