Why You Need a 1:1 BALUN or UNUN on Every Antenna: The Forgotten Role of Skin Effect

A quick note before diving in: whether you call it a 1:1 BALUN, a 1:1 UNUN, a line isolator, or a choke, you're referring to the same essential device—a tool designed to block unwanted common-mode currents and ensure that RF stays where it belongs. The naming often depends on whether the system is balanced or unbalanced, but the purpose is identical: to maintain clean RF current flow.

Most radio amateurs understand that a 1:1 current balun is useful when feeding a dipole antenna. The usual reason cited is common-mode current (CMC) prevention—a valid and important concern. However, there’s another critical and often-overlooked reason to use a proper 1:1 balun on every antenna: the skin effect.

The Universal Nature of Skin Effect

The skin effect is a fundamental physical phenomenon where alternating current (AC), especially at RF frequencies, flows mainly on the outer surface of a conductor. The higher the frequency, the thinner this conductive layer becomes. For instance, at 30 MHz in copper, current only flows in the top 12 microns—far thinner than a human hair. This effect isn’t just academic—it directly impacts all parts of your RF system.

Not Just for Dipoles

While dipoles are often discussed in relation to baluns, any antenna fed with coaxial cable can suffer from common-mode currents. That includes:

  • Vertical antennas
  • Off-center-fed dipoles (OCFD/Windom)
  • End-fed half-wave antennas (EFHW)
  • Random wires
  • Loops
  • Multiband fan dipoles

In all these cases, the outer shield of the coax can become an unintentional radiator if common-mode currents are not properly choked. These currents travel on the outside of the shield—governed by the skin effect—and cause unpredictable performance, increased noise pickup, and RFI problems.

How Skin Effect Amplifies the Problem

Without a 1:1 current balun or choke, the RF energy leaks onto the outer surface of the coax. Because of the skin effect, that surface carries the majority of the unwanted current, with minimal penetration into the conductor. This causes:

  • Increased loss and heating on the shield
  • Radiation from the coax altering your antenna pattern
  • RF exposure risks in the shack
  • Coupling to other equipment and noise sources

A proper 1:1 current balun places a high impedance in the path of this unwanted current, blocking it from flowing on the coax shield. In doing so, it ensures that RF stays in the controlled part of the system: the inside of the coax and the antenna elements.

Don’t Let a Low SWR Fool You

One of the biggest misconceptions is that a low SWR equals a healthy antenna system. That’s not always true. You can have a great match at the feedpoint, but still have serious issues if common-mode currents are riding back to your shack. And thanks to the skin effect, those currents stay on the very outermost layer—where they do the most harm.

Conclusion

Every antenna—regardless of type—benefits from a 1:1 current balun when fed with coax. Yes, it stops common-mode currents, but more importantly, it manages how RF flows over conductors governed by the skin effect. Ignoring this leads to noise, inefficiency, RF feedback, and unpredictable radiation behavior.

The next time you install or troubleshoot an antenna, ask not just about impedance and SWR—ask where the current is flowing, and whether you’ve truly controlled it. A good 1:1 balun ensures that your coax stays a feedline, not an antenna.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.