Currents on the Coaxial Cable: A Multi-Lane Highway of RF Behavior
Currents on the Coaxial Cable: A Multi-Lane Highway of RF Behavior
A coaxial cable can carry the wanted signal, a reflected wave and an exterior current at the same time. The lanes are defined by fields and conductor surfaces—not by folklore about which way current “must” flow.
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.
Think of coax as a multi-lane RF highway. Two surfaces carry the intended transmission-line mode. The outside of the shield can carry another mode with another return path. Reflections, skin effect and shielding describe different parts of that traffic; mixing them together is how a simple cable becomes a difficult station problem.
Begin with the Cable, the Fields and the Complete Path
A coaxial cable has a centre conductor, dielectric insulation and a tubular outer conductor called the shield. In its intended transverse electromagnetic mode—usually shortened to TEM mode—the electric and magnetic fields are mainly in the dielectric between the two conductors and are transverse to the direction of travel. The moving energy is in those fields; the conductor currents and charges establish the boundary conditions that guide them.
Differential-mode current means the two-conductor current of that wanted coaxial mode. At any cross-section of an ideal line, the longitudinal current on the outer surface of the centre conductor is equal in magnitude and opposite in direction to the current on the inner surface of the shield. “Longitudinal” simply means along the cable axis. It is a direction, not a separate third mode.
Every current needs a complete path, but that does not require electrons to cross the insulating dielectric. A changing electric field contributes displacement current: Maxwell's term for the time-varying electric flux between conductors and through surrounding space. Conduction current in metal and displacement current in dielectric regions are parts of one continuous electromagnetic solution. At connectors, loads, antennas and the surrounding environment, the complete geometry decides how the path closes.
The Intended Inside Lane
For a forward TEM wave, voltage and current travel together. The line's characteristic impedance, Z0, is the ratio of forward-wave voltage to forward-wave current for that mode. A 50 Ω label describes this voltage-to-current relationship for the cable's intended mode; it does not say that every antenna connected to the cable is 50 Ω.
When the load impedance differs from Z0, part of the incident wave reflects. Before using an equation, define the quantities: ZL is the load impedance at the selected reference plane, Z0 is the line's reference impedance, V+ is incident-wave voltage and V− is reflected-wave voltage. The complex reflection coefficient is:
Γ = V− / V+ = (ZL − Z0) / (ZL + Z0)
The incident and reflected components add to form position-dependent voltage and current along the line: a standing-wave pattern. That reflected component still belongs to the inside TEM mode. Mismatch does not, by itself, force current onto the shield's outer surface. A discontinuity, asymmetry or external return connection can convert energy between modes, but reflection and mode conversion are separate events.
The Exterior Lane Uses Another Reference
The shield's outer surface can carry a net longitudinal current. In antenna work this is commonly called common-mode current or outside-shield current. Its equal-and-opposite return is not on the shield's inner surface at the same cross-section. The return may be distributed through a mast, equipment chassis, protective bonding, another cable, soil, nearby structures and displacement current through the surrounding electric field.
That exterior current can be driven by an asymmetric antenna feed, an intentional end-fed return arrangement, a connector or enclosure transition, coupling from another conductor, or a local noise field. It is not automatically unwanted: some antennas deliberately include the feedline exterior in their installed geometry. But if it was not included in the design, it can alter feedpoint impedance and pattern, radiate transmitter energy, pick up local noise or bring RF to equipment.
| Traffic | Where it flows | What closes the path | How to observe it |
|---|---|---|---|
| Intended TEM mode | Centre-conductor outer surface and shield inner surface | The two coax conductors and their terminations | Calibrated differential port measurement |
| Reflected TEM component | The same two internal surfaces | The same modal circuit, travelling back from a discontinuity | Complex reflection coefficient at a declared plane |
| Exterior/common mode | Shield outer surface and any conductors joined to it | The installation and its distributed environmental return | Current probe, field or pattern test along the installed path |
| Shield coupling | Between exterior and interior field regions through a real shield | Braid apertures, finite conductivity, seams and terminations | Transfer-impedance or screening test with a specified fixture |
This is the highway idea in its useful form: the lanes share one physical cable, but they are defined by electromagnetic modes and reference conductors. A current arrow drawn along the cable does not identify its lane. You must ask which surfaces carry the equal-and-opposite currents and where the fields close.
Skin Effect Shapes the Surfaces but Does Not Create the Modes
Skin effect is the tendency for alternating current density to concentrate near a conductor surface as frequency rises. For a good, uniform conductor with conductivity σ in siemens per metre, permeability μ in henries per metre and angular frequency ω in radians per second, the plane-conductor approximation for skin depth δ in metres is:
δ = √(2 / ωμσ)
The equation is a useful scale, not a complete cable model. Surface roughness, plating, braid geometry, magnetic material, shield apertures and connector transitions all matter. Skin effect raises conductor loss and helps currents concentrate on the relevant boundaries. It is not caused by SWR, and it is not a magic wall that makes the shield's two surfaces perfectly independent.
A thick, continuous, highly conductive shield can make inside-to-outside coupling very small over a useful band. A braid or foil-plus-braid shield is finite, has apertures and has terminations. Its performance must be described and measured, not inferred from “double shielded” alone.
Shielding and Exterior Current Are Different Questions
A shield can carry exterior current even when it provides excellent screening of the internal signal. Conversely, exterior fields can couple a small disturbance into the internal circuit even when no antenna imbalance is present. These are related through the real shield, but they are not one quantity.
Surface transfer impedance, ZT, is a standardized way to characterize magnetic coupling through a cable shield. In a specified triaxial fixture, an exterior shield current Iext produces a longitudinal voltage per unit length Vint/ℓ in the internal circuit. After those quantities and the fixture have been defined, the relationship is:
ZT = (Vint/ℓ) / Iext in Ω/m
Lower transfer impedance is generally better for that coupling mechanism, but the value varies with frequency. Screening attenuation, connector bonding, braid coverage, cable length and the impedances terminating both the internal and exterior circuits also matter. “Better shield” therefore does not mean “no outside current,” and “outside current” does not prove that the wanted signal is leaking directly through the shield.
Reference Plane Decides What an Instrument Reports
A reference plane is the electrical location to which a calibrated measurement applies. A one-port vector network analyser normally launches and receives the connector's intended coaxial mode. Its S11 result is the complex reflection coefficient of that port mode at the calibration plane.
That result can show mismatch and the phase of a reflection. It does not directly measure the current on the cable exterior. A good SWR can coexist with large outside-shield current because the complete installation happens to present a convenient input impedance. A high SWR can exist while the exterior remains quiet because the reflected wave stays inside the coaxial mode.
To investigate the outside lane, use a calibrated RF current probe around the whole coax. The equal-and-opposite internal TEM currents ideally cancel in the probe aperture, leaving sensitivity to net enclosed current. Move the probe along the cable because the exterior path can have standing-wave maxima and minima of its own. Keep transmit power, frequency, cable route and surroundings fixed, then use an A/B/A sequence when changing one choke or route. Repeatability matters more than a single convenient reading.
What a Common-Mode Choke Actually Changes
A common-mode choke passes the complete coax through ferrite or forms a controlled coaxial coil. The wanted centre and shield-inner currents ideally make equal and opposite magnetomotive force through the aperture. Exterior shield current produces a net excitation, so the choke inserts impedance into that exterior path.
Choke impedance is complex. RCM is its resistive part, which dissipates energy as heat; XCM is its reactive part, which stores and returns energy. Only after defining those parts is it useful to write:
ZCM(f) = RCM(f) + jXCM(f)
That impedance is frequency-dependent and belongs in the complete exterior-mode circuit. Cable length on either side, choke placement, self-capacitance, connector geometry and nearby conductors can change the result. More turns do not produce unlimited N² improvement: winding capacitance and resonance eventually intervene. At transmitter power, common-mode voltage, ferrite loss, core temperature, coax bend radius, dielectric stress and duty cycle all need margins.
A choke can reduce exterior current when its impedance is significant compared with the rest of that path. It cannot:
- remove a differential-mode mismatch merely because it is fitted around the cable;
- create a missing, safe and intentional return conductor;
- guarantee a lower noise floor when the dominant noise path is elsewhere;
- guarantee an unchanged wanted-mode insertion loss in a real assembly; or
- replace measurement of current, voltage, temperature, pattern and station safety.
Use the Highway Model Without Letting It Drive the Car
The highway metaphor is memorable if every lane has a defined mode and return. It fails when “forward,” “reflected,” “longitudinal,” “shield” and “common mode” are treated as interchangeable current names.
Primary and authoritative references
- NIST Technical Note 2255 — coaxial TEM fields, characteristic impedance and higher-mode cutoff
- NIST — Exact Principal Mode Field for a Lossy Coaxial Line
- IEC 62153-4-3:2013+AMD1:2024 — surface transfer impedance by the triaxial method
- Keysight — network-analysis definitions for reference plane, reflection and reflection coefficient
- Fair-Rite — ferrite suppression-core impedance and common-mode application guidance
- ARRL / Roy Lewallen, W7EL — Baluns: What They Do and How They Do It
Mini-FAQ
- Which current carries the wanted signal in coax? In the intended TEM mode, the centre conductor and shield inner surface carry equal and opposite longitudinal currents, with fields mainly in the dielectric.
- Is every current on the shield common mode? No. Current on the shield's inner surface is the intended TEM return. Net current on the outer surface belongs to an exterior or common-mode path.
- Does a reflected wave put current on the shield exterior? Not by itself. A differential reflection remains on the centre conductor and shield inner surface unless a discontinuity or asymmetry converts energy into the exterior mode.
- Does skin effect create perfectly separate shield surfaces? No. Skin effect concentrates current near surfaces, but coupling still depends on shield material, thickness, geometry, apertures, terminations and frequency.
- Does a one-port VNA measure outside-shield current? No. It normally measures reflection of the intended port mode at its calibrated reference plane. Use a suitable current probe or field test for the exterior path.
- What can a common-mode choke fix? It can add complex impedance to an exterior current path. It cannot by itself cure differential mismatch, provide a missing return or guarantee lower noise.