Coax or Balanced Feed Line? Choose the Complete System
Coax or Balanced Feed Line? Choose the Complete System
Coax, ladder line and open wire can all transfer RF efficiently. The useful comparison starts with the installed complex load, line data, route, tuner, balance, common-mode boundary and operating stress—not with a winner’s badge.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
My practical position is not “ladder line good, coax bad.” Coax makes the wanted field largely internal and tolerates routing that would badly disturb an exposed two-conductor line. Open wire can offer very low attenuation under severe mismatch. Either can become a poor choice when its actual load, loss, current path or installation is ignored.
Compare systems at declared reference planes. Start with antenna-terminal R + jX, the actual line’s complex characteristic impedance and propagation constant, its installed length and route, and the tuner or matching network. Then account for differential loss, common-mode current, voltage/current maxima and the radiation pattern of the complete installation.
Two Structures, More Than Two Current Modes
A coaxial cable has a centre conductor, dielectric and surrounding outer conductor. In its wanted differential mode, current on the centre conductor is returned on the inner surface of the outer conductor, and the principal field is between them. Current on the outer surface is a separate common-mode branch whose amplitude depends on the antenna, feedpoint, choke, mast, station bonds and nearby environment.
Ladder line and open wire use two exposed conductors. Their wanted differential currents are equal and opposite. Closely controlled geometry makes much of their far field cancel, but unequal coupling or termination can add common-mode current. Calling a line “balanced” describes the intended mode and geometry; it does not prove that an installed pair carries perfectly balanced current.
Nominal values such as 50 Ω, 75 Ω, 300 Ω, 450 Ω or 600 Ω are characteristic-impedance labels, not universal input impedances. The impedance seen at the source depends on the terminating load and electrical length. Real lines also have frequency-dependent conductor, dielectric, leakage and radiation loss.
Mismatch Does Not Burn Power by Itself
A reflected wave is not a resistor. In a lossless line, mismatch creates standing voltage and current but dissipates no power in the line. In a real line, those higher local voltages and currents increase dielectric and conductor loss above the matched-line value. The result depends on the line’s attenuation, length, characteristic impedance and complete complex load.
This is why both slogans fail:
- “Coax is low loss when SWR is low” omits cable type, frequency, length, connectors and installation.
- “Ladder line stays lossless at any SWR” omits finite conductor and dielectric loss, leakage, contamination, radiation, joints and extreme voltage/current regions.
Open-wire line often begins with lower matched attenuation than practical coax of similar service, so it can retain an advantage in many mismatched HF systems. That is a calculation to perform with real line data, not a licence to ignore a difficult load. The ARRL TLW examples show that an extremely severe load can produce substantial additional loss even in a nominally low-loss line.
The Line Transforms the Antenna Load
For an ideal lossless uniform line:
Zin = Z0 × (ZL + jZ0 tan(βl)) / (Z0 + jZL tan(βl))
ZL is the antenna-terminal load, Z0 the characteristic impedance and βl the electrical length. A real line uses complex Z0 and propagation constant γ = α + jβ. The tuner therefore sees a transformed complex load—not the number printed on the cable.
In the ideal lossless special case, a half-wave multiple repeats the load and a quarter-wave section inverts it according to Z0² / ZL. Those are relationships, not magic cutting instructions. Loss, velocity factor, multiple bands and the starting R + jX decide whether a particular length makes the tuner’s job easier or harder.
A tuner at the shack establishes a match at its transmitter-side reference plane. It does not remove standing waves or loss from the line between tuner and antenna. A remote matching network at the antenna changes that boundary, but adds its own loss, control, weather and voltage/current constraints.
Routing Is Part of the Electrical Design
Coax is convenient because its wanted field is substantially confined. It can pass near structures with less differential-mode disturbance than an exposed line, but sharp bends, crushed dielectric, water ingress, poor connectors and outer-surface current still change performance.
An exposed two-conductor line needs stable spacing and similar surroundings for both conductors. Running one side closer to a mast, gutter, wet wall, roof, tree or cable bundle changes its capacitance and coupling. The required separation is not a universal 5 or 10 centimetres; it depends on line spacing, wavelength, nearby material and the amount of impedance, loss or imbalance the system can tolerate.
A random metal tube around existing ladder line does not merely “protect” it. The added conductor changes the electromagnetic structure, characteristic impedance, mode coupling and loss. A purpose-designed shielded balanced line can be valid, but it is a different transmission line and must be characterised as such.
Balance, Shielding and Noise Need Separate Evidence
Coax shielding can reduce direct electric-field coupling into its differential mode. It does not guarantee a quiet receiving system when current flows on the outside of the shield or when noise enters through power, control, bonding or antenna paths.
A symmetrical two-conductor line can reject fields that couple equally to both conductors when the termination preserves balance. That is not universal noise immunity. Unequal routing, antenna-arm impedance, tuner capacitance to chassis or a lopsided transition can convert common mode into differential signal.
Measure wanted differential current and unintended common-mode current separately. On coax, scan the outer surface along the route. On a two-conductor line, compare conductor-current magnitude and phase and measure the current common to the pair. Judge a receive change by wanted-signal SNR under a repeatable source or time-controlled A/B/A test—not by noise floor alone.
The Tuner Interface Is a Network, Not a Label
A genuinely balanced tuner can drive a two-conductor line without a single-ended output transition, but its balance, reachable load domain, loss and stress still depend on topology and load. An unbalanced tuner followed by a current balun can also be valid when the balun tolerates the transformed differential voltage/current and provides useful common-mode impedance.
A two-conductor line can terminate directly at a suitable balanced tuner. That electrical possibility does not waive installation-specific conductor clearance, RF-exposure, lightning, bonding or building-entry requirements; those remain separate from the matching decision and must follow the applicable rules.
Do not select that interface from a fixed 1:1 or 4:1 rule. Impedance transformation and common-mode suppression are different functions. A ratio can move one band into the tuner’s useful domain and another band out of it. A device that provides high common-mode impedance in a fixture may still have unacceptable differential loss or voltage stress under the installed load.
When the installed antenna system is intentionally unbalanced, a measured-load UNUN and a separately characterised choke can divide those jobs clearly. A current balun remains a sound interface for a demonstrably balanced load. In every case, choke placement follows the intended current boundary and measured current path rather than the names “coax” or “ladder line.”
Coax Can Be the Better Choice
Coax is often attractive when the antenna or remote matching network presents a controlled load, the route passes close to conductors, the installation moves frequently, or the feed line must enter through a compact weatherproof path. Its outer conductor also provides a defined mechanical and electrostatic enclosure for the wanted mode.
The decision still uses the exact cable. Calculate matched attenuation and mismatch loss at the operating frequency and length. Check connector, voltage, current, temperature, bending, moisture and power limits. Include any common-mode choke as a separate measured component rather than assuming the braid cannot radiate.
Open Wire Can Be the Better Choice
Open wire or ladder line is often attractive for a multiband doublet whose antenna-terminal impedance changes widely across bands. Low matched attenuation can make it practical to carry a large standing-wave ratio to a tuner without the loss penalty of a particular coax run.
That advantage survives only when the route preserves geometry and balance, the line remains sufficiently clear of disturbing objects, joints and spacers stay fit for the environment, and the tuner can handle the transformed load. High-voltage and high-current points move with frequency and line length; “QRO” alone does not make ladder line the safe or efficient winner.
A Measurement-Led Choice
- Define the operating set. Record frequency, power, waveform, duty cycle, required paths and installation constraints.
-
Measure the antenna terminal. Save calibrated
R + jXat a named plane with the intended return structure installed. - Characterise candidate lines. Use manufacturer or measured complex line data, electrical length and connector/transition loss.
- Transform every required load. Calculate the tuner-side load and voltage/current distribution across the full frequency set.
- Overlay the tuner domain. Check matchability, insertion loss and component stress—not only the transmitter SWR.
- Audit the route. Record spacing, supports, nearby conductors, movement, moisture exposure and transition geometry.
- Map common mode. Measure exterior or pair-common current at several positions before choosing choke locations.
- Verify under power. Increase power progressively while monitoring voltage, current, temperature and connection stability within declared equipment limits.
- Compare at equal conditions. Use A/B/A measurements of accepted power, loss and wanted-signal SNR so a changed band, route or propagation state does not become the apparent feed-line result.
Primary Engineering Sources
- ARRL Antenna Book companion — TLW Transmission-Line Program: complex load transformation, matched and mismatch loss, line voltage/current and tuner-side load examples.
- Roy W. Lewallen, W7EL — Baluns: What They Do and How They Do It: differential and imbalance current on coaxial and two-conductor lines.
- Keysight — Impedance Measurement Handbook: complex impedance, lossy-line relations, electrical-length compensation, fixtures and calibrated reference planes.
- Keysight — Balanced Measurements: differential mode, common mode, mode conversion and balanced-port measurement definitions.
Practical Conclusion
Use coax when its field confinement and routing freedom solve the harder installation problem. Use open wire when its low attenuation under the measured multiband loads is worth the routing and balanced-interface discipline. Neither name proves efficiency, silence or suitability.
The best feed line is the one whose complete loss, transformation, common-mode path, tuner stress and installation remain acceptable on every required band—and whose result survives measurement.
Mini-FAQ
- Is ladder line always lower loss than coax? No. Compare the exact line, frequency, length, complex load, route and transitions. Open wire often starts with low matched attenuation, but extreme loads and poor installation still add loss.
- Does high SWR itself consume power? No. A standing wave is not a dissipative element. In a real line, the associated voltage and current distribution increases conductor and dielectric loss above the matched-line value.
- Can coax carry common-mode current? Yes. Wanted return current flows on the shield’s inner surface; a separate current can flow on the outer surface when the installed return network allows it.
- Is balanced feed line automatically balanced? No. Equal-and-opposite current depends on the antenna, route, tuner and nearby environment as well as the line geometry.
- Should every ladder-line interface use a 1:1 current balun? No. It is a valid candidate when the tuner accepts the measured load and the device meets balance, loss, common-mode and stress requirements. Other topologies can also be valid.
- Can one feed-line length work ideally on every band? Rarely. Electrical length and antenna-terminal impedance change with frequency. Choose a length from the complete tuner-load and stress map, then verify the installed system.