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

Coaxial cable is often called “unbalanced” because the shield is usually tied to chassis/ground. But that label mixes two ideas: the intended coax mode keeps fields inside the cable, while the problem mode is common‑mode current on the outside of the shield.

Why coax is “balanced” at 50 Ω

In the differential (TEM) mode that coax is designed to carry, the electromagnetic fields are contained between the inner conductor and the inside of the shield. The current on the center conductor is returned by an equal‑and‑opposite current on the shield’s inner surface, so (ideally) there is no net external field from the cable itself.

When the source and load are both matched to the coax’s characteristic impedance (≈50 Ω for most ham gear, 75 Ω for much of TV/broadcast), there are no reflections, so voltage/current do not form standing‑wave peaks. That “quiet” condition reduces coupling into the surroundings and makes it less likely that the installation will accidentally excite current on the outside of the shield.

When coax becomes “unbalanced”

A mismatch (not 50 Ω at one or both ends) creates reflections and standing waves in the differential mode—but those standing waves are still confined inside an ideal coax. The behavior hams usually call “unbalanced” is different: it’s when a separate common‑mode current is driven on the shield’s outer surface, using the outside of the coax (and nearby objects) as part of the RF circuit.

Common‑mode is typically excited by asymmetry and discontinuities: feeding a balanced antenna directly with coax, unequal coupling of the two antenna halves to the environment, routing the coax close to metal, a station ground/bonding situation that provides an easy return path, or a tuner/ATU arrangement that leaves the feedline “floating” in RF terms. High standing‑wave voltages/currents from a mismatch can worsen the symptoms (more coupling, more RF where you don’t want it), but mismatch is not the root mechanism that “creates” outer shield current.

Coax as an impedance transformer

Any transmission line transforms impedance. The impedance you see at the shack end depends on (1) the load at the far end, (2) the line’s characteristic impedance Z₀, and (3) the line’s electrical length at that frequency. That is why changing coax length can make an analyzer show a different R and X at the rig—even if nothing at the antenna changed.

Important nuance: in an ideal (lossless) uniform line, the SWR magnitude set by the load is the same everywhere on the line, but the local impedance cycles with position. With real coax loss, the SWR you read in the shack can be slightly lower than at the feedpoint—especially on long runs and high SWR.

Multiple SWRs inside a cable

People sometimes say a mismatched coax has “many SWRs,” but what really varies along the line is the instantaneous impedance, not the (lossless) SWR. The magnitude of the reflection coefficient |Γ| is constant along a uniform, lossless line, while its phase rotates with distance—so the impedance repeats every half‑wavelength.

That’s why inserting a device that reports impedance (R/X) at different points can show very different numbers. And it’s why real‑world SWR readings can differ with measurement point when the line is lossy, when the meter itself is frequency/placement sensitive, or when common‑mode currents are contaminating the measurement.

Using coax length as a matching tool

Because coax transforms impedance, line length can be used deliberately as a matching element (quarter‑wave transformers, stubs, and specific electrical lengths). This can be perfectly valid engineering—when you know the load impedance and account for velocity factor and loss.

However, using random coax length to “make the SWR look good” at the rig can hide a severe mismatch at the feedpoint and can increase feedline loss or create high voltage/current points. It’s still not “tuning the antenna”; it’s choosing where the impedance transformation lands.

The need for chokes

If common‑mode currents are present, the coax outer surface becomes part of the antenna system. The most effective fix is usually a common‑mode choke (current balun) where the coax transitions into the antenna system. In many real stations, it is also beneficial to add suppression near the station entry to keep RF off equipment and to reduce noise pickup.

Practical best practice (especially on balanced antennas, end‑fed arrangements, and “RF‑hot” installations) is often:

• One choke at or near the feedpoint (to prevent the feedline from becoming a radiator).
• One choke at the shack entrance (to keep any remaining common‑mode off your gear and house wiring, and to reduce receive noise pickup).

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