About BALUNs

By Danny Horvat, E73M / N4EXA — antenna engineer and founder of MyAntennas.com.

What Is a BALUN?

The word is a contraction of BALanced-to-UNbalanced. A balun connects a balanced load — a dipole, an off-center-fed dipole, a loop, or open ladder line — to an unbalanced feed such as 50-ohm coax, whose shield is grounded. It belongs to the family of devices called transmission-line transformers, made by winding coax, bifilar, or multifilar conductors on a high-permeability ferrite core. The high winding impedance is what isolates the balanced side from the unbalanced side, and that isolation is the whole point.

Current Balun vs Voltage Balun — and Why It Matters

There are two kinds, and the difference is not academic. A voltage balun forces equal voltage across the two sides of the load. A current balun forces equal current into the two sides. For real antennas, current balance is what you actually want, because a real antenna is never perfectly symmetrical, and it is unequal current — not unequal voltage — that ends up flowing back down the outside of your coax shield and radiating.

A 1:1 current balun is, at heart, a simple transmission line wound on a ferrite core — the same principle as a common mode choke. Voltage baluns are more complex and, in practice, noticeably worse at keeping common-mode current off the feedline. That is why the baluns we build are current baluns: at MyAntennas we design and measure for the choking impedance that keeps your feedline quiet, not for a specification that looks good on paper but leaves current on the shield. Roy Lewallen, W7EL, demonstrated these real-world differences with measurements years ago, and our own bench data agrees with him.

When Do You Need a BALUN?

The two classic uses are feeding a coax-fed dipole at its center, and transitioning from balanced ladder line to the unbalanced input of an antenna tuner. In both cases the job is the same: stop RF current from flowing on the outside of the coax shield.

Why does that current appear at all? Inside the coax, current flows on the center conductor and on the inside of the shield — equal and opposite, contained. Because of skin effect, the current on the inside of the shield is a separate path from the outside. When it reaches a dipole feed point, the center-conductor current flows into one leg of the antenna; the shield current wants to flow into the other leg — but part of it can instead flow back down the outside of the shield. How much takes that wrong path depends on the impedance it sees. A 1:1 current balun at the feed point makes that outside path a high impedance, so nearly all the current goes where it belongs: into the antenna.

The symptoms of skipping it are familiar: a distorted radiation pattern, TVI or telephone interference where the coax passes near other cables, a “hot” microphone that stings your lip, and RFI into computers and USB devices in the shack.

Why It Sometimes “Seems Fine” Without One

Here is a subtlety that fools a lot of hams. The coax shield is usually grounded somewhere near the shack entrance. The length of coax between that ground point and the antenna feed point decides how much shield current you get. If that length is close to a half wavelength (roughly 468 ÷ frequency in MHz, and the coax velocity factor does not enter into it), the low impedance of the ground point is reflected up to the feed point and a large shield current flows. If the length is close to a quarter wavelength, the feed point sees a high impedance and very little current flows on the shield. Lengths in between land somewhere between those extremes.

This is why an antenna can appear to behave on one band and misbehave on another with no balun, and why “it worked fine at my old length” turns into a problem after you re-route the feedline. A proper current balun removes the guesswork by presenting a high impedance to shield current regardless of feedline length.

BALUN or Common Mode Choke?

People often ask what the difference is, and the honest answer is that a 1:1 current balun and a common mode choke (line isolator) are the same class of device used in two different places. At a balanced feed point — a dipole center — it does its job as a balun. On the coax run, at the point where the cable enters your home, the identical kind of device is called a line isolator or CMC and it stops accumulated feedline noise from reaching your shack. Many stations benefit from both: a current balun at the antenna and a choke at the house entrance. For the full explanation of placement, see About CMC.

4:1 Baluns and Higher Ratios

Not every balanced load is near 50 ohms. Off-center-fed dipoles, some folded and loop configurations, and ladder-line-fed multiband antennas present higher balanced impedances, and a 4:1 balun (200:50) matches those to your coax or tuner while still providing current balance. Choosing between a 1:1 and a 4:1 comes down to the feed-point impedance of your particular antenna — when in doubt, ask, and I’ll point you to the right ratio.

Choosing a MyAntennas BALUN

Our 1:1 current baluns are the workhorses for coax-fed dipoles and for taming feedline current; the 4:1 baluns serve OCF dipoles and higher-impedance balanced loads. Every unit is a current-type design, wound and measured in-house for high resistive choking impedance across its rated band. Browse the full line on the BALUNs & UNUNs page. (Looking for an end-fed transformer instead? That is a UNUN, not a balun — see All About EFHW Antennas.)

Further reading and references: Roy Lewallen, W7EL, “BALUNs: What They Do and How They Do It” (ARRL); the ARRL Antenna Book; .

Related: About CMC · About Grounding · All About EFHW Antennas