50 Ω Coax — Balanced at Its Design Impedance, Unbalanced When It’s Not

Coaxial cable is often called “unbalanced.” But the truth is more nuanced: it’s only unbalanced when it’s not operated at its design impedance.

Why coax is “balanced” at 50 Ω

Inside a coaxial cable, the fields are contained between the inner conductor and the inside of the shield. When the source impedance and the load impedance are both equal to the coax’s characteristic impedance (≈50 Ω for most ham gear), the currents on the inner conductor and the shield’s inner surface are perfectly equal and opposite. The cable is then balanced in the transmission-line sense, with no net external field leakage.

When coax becomes “unbalanced”

If either end of the cable is not 50 Ω, reflections appear. The shield current no longer exactly cancels the center conductor’s return current, and some energy flows on the shield’s outer surface as common-mode current. That’s what we call “unbalanced behavior.”

Coax as an impedance transformer

Any feedline is an impedance transformer. The impedance you measure at one end depends on the length of the line and what’s connected at the far end. That’s why coax length can appear to “shift resonance” of an antenna: it’s not changing the antenna, it’s transforming the impedance the shack sees.

This is why you may measure a dip in SWR at a different frequency at the rig than what exists at the feedpoint. The feedline transforms both the resistive and reactive parts of the load impedance.

Multiple SWRs inside a cable

A mismatched coax doesn’t have “one SWR.” Every point along the line has its own instantaneous impedance, cycling with line length. That’s why you can see different values depending on where you insert a meter. The reflection coefficient is the same, but the local impedance changes with position.

Using coax length as a matching tool

Because coax transforms impedance, deliberate use of line length can act as a simple matching element. A quarter-wave line, for instance, can invert impedances, while other lengths provide intermediate transformations. This is not “tuning the antenna,” but rather exploiting the feedline’s transformer behavior.

The need for chokes

If the system isn’t a perfect 50 Ω ↔ 50 Ω match, the coax outer surface becomes part of the antenna system. To suppress that, you need at least two common-mode chokes:

• One at the feedpoint (to keep RF off the outer braid as it leaves the antenna).
• One at the shack entrance (to keep any remaining common-mode off your gear).

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