What is a balun and why do antennas need one?
A balun (balanced-to-unbalanced transformer) connects a balanced antenna, such as a dipole, to an unbalanced feed, usually coaxial cable, forcing equal and opposite currents at the antenna terminals and suppressing common-mode current on the outside of the shield. Without one, the coax becomes part of the antenna, distorting the pattern and making measurements irreproducible.[1][2]
What types of balun exist?
- Current (Guanella) balun: transmission-line windings on ferrite force equal currents; 1:1 and, with two lines, 4:1.[3]
- Voltage (Ruthroff) balun: forces equal voltages; e.g., the Ruthroff 4:1.[4]
- Choke balun: coax wound on ferrite or through ferrite sleeves, presenting high common-mode impedance.
- Sleeve (bazooka) balun: a λ/4 conductive sleeve over the coax, shorted at the far end, presenting high impedance at the feed; narrowband.[1]
- Printed baluns: Marchand, tapered microstrip, and microstrip-to-slotline transitions for printed dipoles and Vivaldis.
What ratio is needed?
The ratio refers to impedance: 1:1 for a dipole near 50–75 Ω, 4:1 for a folded dipole (≈ 300 Ω) to 75 Ω, and higher-ratio ununs (unbalanced-to-unbalanced) such as 49:1 for end-fed half-waves. Impedance ratio equals turns ratio squared.[2]
How do you verify a balun?
Measure common-mode impedance with a VNA (often several hundred to thousands of ohms is desired across the band), check that the antenna’s impedance does not change when the feed cable is moved or touched, and measure differential insertion loss.[2]
Frequently asked questions
Does a monopole need a balun?
It is unbalanced by nature, but a choke can still help if the ground plane is small and the cable carries current.
Can a balun fix a bad match?
No; it handles balance and impedance ratio, not arbitrary mismatch.
Why do ferrite baluns get hot?
Excessive common-mode current or core saturation at high power.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
- “Novel Matching Systems for High Frequencies”. G. Guanella, Brown Boveri Review, vol. 31, pp. 327-329. 1944.
- “Some Broad-Band Transformers”. C. L. Ruthroff, Proceedings of the IRE, vol. 47, no. 8, pp. 1337-1342. August 1959.