Balun and unun selection guide
Which balun ratio to use for your antenna. 1:1, 4:1, 9:1, and 49:1 explained with antenna matching guidance.
Select the right balun or unun ratio for your antenna design.
What a balun does
A balun (balanced-to-unbalanced) converts between balanced antenna systems (dipoles, loops) and unbalanced feedlines (coax). Without a balun, current flows on the outside of the coax shield, the feedline radiates, and RF ends up in the shack causing interference, hot microphones, and computer problems.
An unun (unbalanced-to-unbalanced) provides impedance transformation without the balanced/unbalanced conversion. End-fed antennas use ununs because both the antenna and the feedline are unbalanced.
Which ratio to use
1:1 current balun (choke balun)
Use for: Center-fed dipoles. Yagi beam feedpoints. Any antenna with approximately 50-ohm feedpoint impedance.
What it does: Chokes common-mode current off the feedline without changing the impedance. Does not transform impedance — just stops the feedline from radiating.
When you need it: Every coax-fed dipole and Yagi should have a 1:1 choke balun at the feedpoint. This is not optional; it is how you prevent RF problems. A stack of ferrite toroids on the coax near the feedpoint serves the same function.
4:1 voltage balun
Use for: Off-center-fed dipoles (Windom, OCF). Antennas with approximately 200-ohm feedpoint impedance. Full-wave loop antennas fed with coax.
What it does: Transforms 200 ohms to 50 ohms. Also provides balanced-to-unbalanced conversion.
Common applications: Off-center-fed dipoles. Full-wave horizontal loops. Carolina Windom antennas.
9:1 unun
Use for: Random wire antennas. End-fed wire antennas that are NOT a half wavelength. Vertical antennas with high feedpoint impedance.
What it does: Transforms approximately 450 ohms to 50 ohms. Unbalanced to unbalanced (no balanced conversion needed for end-fed wires).
Common applications: Random long wire antennas. Non-resonant end-fed wires. Homebrew verticals with high feedpoint impedance.
49:1 unun
Use for: End-fed half-wave (EFHW) antennas. The most common portable HF antenna design.
What it does: Transforms the very high impedance (~2500-5000 ohms) at the end of a half-wave wire to approximately 50 ohms.
Common applications: End-fed half-wave (EFHW) antennas for POTA, SOTA, and portable operations. The 49:1 unun is the matching transformer at the heart of every EFHW antenna.
6:1 unun
Use for: Off-center-fed dipoles where the feedpoint impedance is approximately 300 ohms.
What it does: Transforms 300 ohms to 50 ohms. Some OCF dipole designs use a 6:1 rather than a 4:1 depending on the feedpoint position.
Common mistakes
Using a voltage balun where a current balun is needed. A voltage balun at a dipole feedpoint does not choke common-mode current. Use a current balun (choke balun) for dipoles and Yagis.
Omitting the balun entirely. "It works without a balun" is not the same as "it works correctly without a balun." The antenna may tune and make contacts without a balun, but the feedline is radiating, the pattern is distorted, and RF is getting into the shack.
Wrong ratio for the antenna. A 49:1 unun on a random wire is wrong (too high a ratio). A 9:1 unun on an EFHW is wrong (too low a ratio). Match the ratio to the antenna's feedpoint impedance.