RF Transformer Types and Baluns

Radio-frequency transformers couple signals between circuits at high frequencies, where ordinary mains-frequency transformer designs no longer work well. They are used mainly for impedance matching, for coupling stages while blocking direct current, and for converting between balanced and unbalanced signal paths. To operate efficiently at radio frequencies they use small ferrite or powdered-iron cores, or sometimes air cores, and short windings to keep stray capacitance and losses low.

A common form is the balun, a contraction of balanced-to-unbalanced. A balun connects a balanced line or device, such as a dipole antenna or a differential circuit, to an unbalanced one such as a coaxial cable, whose two conductors are not symmetric with respect to ground. Without a balun, current can flow on the outside of a coaxial shield and disturb the radiation pattern or cause interference. Baluns can also transform impedance, with a 4:1 balun being a familiar example that matches a balanced antenna feed to a coaxial line.

Related devices include ununs, which match between two unbalanced impedances, and transmission-line transformers, which use carefully arranged lengths of line wound on a core to achieve broadband matching with low loss. RF transformers appear throughout radio equipment, in antenna systems, in the front ends of receivers and transmitters, and in test and measurement gear, wherever signals must move between circuits of different impedance or different grounding arrangement.

Frequently asked questions

What does a balun do?

A balun connects a balanced circuit or antenna to an unbalanced line such as coaxial cable. It can also change impedance and helps prevent unwanted current on the outside of a cable shield.

Why are RF transformers built differently from power transformers?

At high frequencies, stray capacitance and core losses become important, so RF transformers use small ferrite or air cores and few turns rather than the bulky laminated cores of mains transformers.

What is the difference between a balun and an unun?

A balun matches between balanced and unbalanced circuits, while an unun matches between two unbalanced circuits of different impedance.