When the Feedline “Becomes the Antenna”?

Differential vs Common-Mode on Coax and Open-Wire

(“Common-mode” is often used as a convenient shorthand in ham radio. In strict EMC language it implies a defined reference structure; here we’re describing unwanted same-direction current components and stray return paths through the environment.)

The phrase that’s “sort of true”… and why it can mislead

You’ll often hear: “When ladder line becomes unbalanced, both conductors become part of the antenna.” That statement has a kernel of truth, but it can be misleading if you take it literally.

A two-wire feeder always carries differential-mode current (the wanted “feed the load” current). If the system becomes unbalanced, it also carries a common-mode component that makes the line radiate and look like “it became part of the antenna.” The differential current doesn’t vanish just because common-mode appears.

The clean model: two currents, two modes

On a two-conductor line, the current on each conductor can be decomposed into two modes:

• Differential mode (DM) … equal magnitude, opposite direction
• Common mode (CM) … equal magnitude, same direction

What “both conductors become part of the antenna” really means

When I_CM exists, the line is no longer electromagnetically “self-contained.” For the common-mode component, the pair behaves like a single conductor threading through space, and the return path is the environment. So yes, both wires participate in radiation… but specifically as a pair carrying a same-direction current component.

The “phantom third conductor” is real: it’s the environment and displacement current

It can feel like common-mode current “travels until it finds something,” but at RF the return path does not require a physical wire. The loop can be closed through distributed capacitance and displacement current to nearby structures and to Earth.

• Earth (via capacitance)
• House wiring, gutters, metalwork, masts, towers
• Coax outer surface, rig chassis, desk wiring
• Any conductive object nearby

In other words, the line plus its surroundings form a distributed impedance network everywhere along the run. Near the shack, tuner, or transition to other wiring, the return impedance often becomes lower, so common-mode current can increase there. This mechanism is distributed EM coupling, not “it finally found a wire.”

Why coax more often shows unwanted current than open-wire

Coax has an easy, explicit common-mode path: the outside of the shield

Coax supports the intended transmission mode inside the cable, but it can also carry current on the outside of the shield. Because of skin effect, the inside and outside surfaces of the shield behave like separate RF conductors. That’s why people describe coax as having a “third conductor” behavior: there are distinct current paths for:

• center conductor (intended mode)
• inside of shield as the intended return (intended mode)
• outside of shield (unwanted common-mode path)

Coax will be “unbalanced with respect to the world” in real HF installations

Even if the antenna is symmetric on paper, the environment rarely is: one leg closer to the mast, one side over a roof, feedline leaving at an angle, or feedpoint hardware that isn’t truly symmetric. Those asymmetries create a common-mode voltage that can drive current on the outside of the coax shield unless you stop it with a proper choke/current balun.

Why 600 Ω open-wire tends to have fewer common-mode headaches

Symmetry plus a typically high common-mode impedance to the environment

A well-installed open-wire line uses two similar conductors in a similar environment (air around both conductors) and is kept away from metal. That makes the stray capacitances more equal, which reduces mode conversion in the first place.

More importantly, the common-mode return path is often dominated by small capacitances to distant objects and Earth, which means the common-mode impedance can be high. If the driving common-mode voltage is small and the common-mode impedance is large, the resulting common-mode current tends to stay small.

Higher characteristic impedance shifts the V/I picture (and open-wire stays low loss under mismatch)

For the same power, a higher characteristic impedance implies lower line current (in the matched-wave picture). Lower current generally means less magnetic coupling into nearby conductors and lower conductor loss.

The second piece matters just as much in real multiband systems: doublets and non-resonant loads can produce high SWR on many bands. Coax losses rise significantly with SWR (conductor + dielectric losses with standing-wave peaks), while open-wire lines (mostly air dielectric, larger spacing) often remain very low loss even under severe mismatch. That’s why “tuner in the shack” can be practical with open-wire, but becomes expensive (lossy) with coax.

300 Ω vs 600 Ω: why “higher impedance is better” is not a universal rule

This is where it gets nuanced: higher impedance reduces current, but raises voltage. That shifts which coupling mechanism dominates in your installation.

• Magnetic (H-field) coupling trends with current … higher Z₀ generally means lower current for a given power.
• Electric (E-field) coupling trends with voltage … higher Z₀ generally means higher voltage for a given power.

If you can maintain good clearance and symmetrical routing, 450–600 Ω open-wire is excellent. If you must run close to objects, tighter spacing can reduce field interaction… but many 300 Ω “TV twinlead” styles use solid dielectric and suffer higher loss, especially under high SWR or moisture exposure. That’s a big reason 450 Ω ladder line is popular: it’s a practical compromise.

Practical bottom line

• A two-wire feeder can “become part of the antenna” when it carries significant common-mode current.
• The accurate statement is: imbalance adds a common-mode current component, and that component is what radiates.
• Coax more often shows trouble because the outside of the shield is an easy radiating path, and the shack environment provides an excellent return network.
• Open-wire often behaves better because it can be kept geometrically balanced and its common-mode return is typically high-impedance (when installed with clearance).

How to keep the feedline from becoming the radiator

You don’t get to decide whether the feedline is “part of the system.” You only get to decide whether it carries significant common-mode current. In practice that comes down to symmetry, routing, and where you intentionally block common-mode.

• Keep open-wire symmetric and away from metal … especially near the feedpoint and near the tuner entry.
• Use a real current choke where modes want to convert … commonly at the transition to coax, or at the tuner output where the environment becomes asymmetric.
• Keep coax runs from tuner to antenna as short as practical when the tuning happens in/near the shack.
• Assume the shack is an RF structure … bonding, cable routing, and choke placement matter as much as the “antenna in the air.”

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