Coax Unbalanced by Definition?
Coax Unbalanced by Definition?
Yes—by the conventional geometrical definition. But that does not make its normal conductor currents unequal, put the wanted return on the shield exterior, or suspend Kirchhoff’s current law.
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.
“Coax is unbalanced” is true, but it is often used to justify conclusions that do not follow. The word describes the cable’s asymmetrical relationship to the outside world. It does not mean that ordinary coax transmission requires current on the outside of the shield.
The short answer: coax is geometrically unbalanced, its intended TEM mode has equal-and-opposite conductor currents, and its outer shield surface can carry a separate external mode. All three statements can be true at the same time.
One Word, Three Different Questions
Arguments about coax often become circular because “balanced” is being used for three different properties. Separate them and most of the mystery disappears.
| Question | What it actually asks | Answer for ideal coax |
|---|---|---|
| Is the line geometrically balanced? | Are both conductors physically equivalent and similarly coupled to the environment? | No. One conductor surrounds and shields the other. |
| Are the wanted line currents balanced? | Are the two longitudinal currents of the intended TEM mode equal in magnitude and opposite in direction at a cross-section? | Yes. |
| Is an external mode present? | Is there net longitudinal current on the cable as a whole, with a return through the antenna, station or environment? | Not necessarily. It exists only when that mode is excited. |
Twin lead gives the complementary example. It is geometrically balanced when installed symmetrically, but an asymmetric antenna or nearby conductor can still excite common mode on it. Geometry influences mode conversion; it does not dictate the conductor-current sum in every operating condition.
Why Coax Is Conventionally Called Unbalanced
In a coaxial line, the centre conductor is enclosed by the shield. The two conductors therefore do not have interchangeable relationships to chassis, earth or nearby objects. The shield normally becomes the equipment enclosure reference at a connector, while the centre conductor remains screened from the exterior.
That asymmetry is sufficient for the conventional label unbalanced transmission line. The label remains valid whether the cable is 50 Ω or 75 Ω, matched or mismatched, grounded or floating. Characteristic impedance is set primarily by the conductor geometry and dielectric; it is defined between the centre conductor and the shield, not between the centre conductor and a ground rod.
A floating spool of 50 Ω coax is still 50 Ω coax. Removing its earth connection does not make the two conductors geometrically equivalent, and adding an earth connection does not create its characteristic impedance.
Calling coax unbalanced is therefore correct. Saying “it is unbalanced, so the shield current must be different from the centre-conductor current” is not.
The Intended Coax Mode Is a Two-Conductor TEM Mode
In the normal transverse electromagnetic mode, the electric field lies mainly between the centre conductor and the shield’s inner surface. The associated magnetic field circles the centre conductor within that same region. At any cross-section of an ideal line, the longitudinal conductor currents satisfy:
Icentre(z) + Ishield,inner(z) = 0
This is not merely a low-frequency circuit approximation. It is the modal current relationship associated with the confined TEM fields. The shield’s inner-surface current is the wanted return current.
A forward wave and a reflected wave may both exist. Their voltage and current waves combine to form standing waves, and a reactive load changes the phase between the resulting voltage and current. None of that requires the two conductor currents of the TEM mode to stop being equal and opposite.
For the advanced reader: with a uniform line, V(z) is the sum of forward and reverse voltage waves, while I(z) contains their difference divided by Z0. That scalar I(z) is the current on the centre conductor; the corresponding inner-shield current is −I(z). Reflection changes the spatial voltage-current relationship, not the two-conductor continuity condition.
Reactance Does Not Manufacture a Third Current Path
An inductor or capacitor changes current phase relative to voltage—ELI the ICE man remains safe. But an ideal two-terminal component still has one branch current. Current entering one terminal is matched by current leaving the other when conduction and displacement current are treated consistently.
So a reactive antenna impedance can produce reflected power and standing waves while the system remains purely differential. Reactance alone does not create a relative phase shift between the centre-conductor current and its inner-shield return at the same line cross-section.
If those currents do not sum to zero, another path exists. It may involve the shield exterior, antenna structure, mast, radials, station bonding, mains wiring, capacitance to earth or radiation into the surrounding field. That added path is the physics that a two-terminal spreadsheet omits.
Do not reverse the causality. Tuning an antenna can change measured common-mode current because it changes current distribution and the boundary conditions of the complete structure. That correlation does not prove that reactance created the external mode. A matched but asymmetric antenna can have substantial common mode; a reactive but symmetric two-terminal load can have none.
The Shield Is One Conductor with Two RF Surfaces
A practical coax model needs three conducting surfaces:
- the outside surface of the centre conductor;
- the inside surface of the shield; and
- the outside surface of the shield.
The first two support the intended internal mode. The shield exterior can support an additional mode relative to the surrounding world. At frequencies where the shield is many skin depths thick, inner- and outer-surface currents are strongly separated. They are not the same current somehow being “pushed through” the shield.
Iprobe = Icentre + Ishield,total ≈ Ishield,outer
That is why a clamp current probe placed around the entire coax is useful. The internal centre and inner-shield currents cancel magnetically at the probe, leaving the net longitudinal current associated mainly with the shield exterior. Measuring at several positions is important because the external mode can form its own standing-wave pattern.
Real shields are finite. Braid openings, resistance and inductance give them non-zero transfer impedance, so some coupling can occur between exterior and interior. That is a genuine cable-shielding limitation, but it is different from the much larger outside-surface current commonly created at an antenna feed transition.
Where the External Mode Comes From
The coax does not spontaneously decide to radiate. An asymmetry or transition converts energy from the internal mode into an external one. Typical mechanisms include:
- feeding a nominally balanced dipole directly from coax, where the shield exterior becomes a third conductor connected to one antenna half;
- using an end-fed or off-centre-fed system without controlling its external return path;
- unequal antenna-leg coupling to earth, a tower, gutters, wiring or nearby structures;
- routing the feedline asymmetrically through the antenna’s near field;
- connecting equipment, bonding conductors or control cables that complete an unintended external loop; and
- receiving environmental fields that drive the cable as a structure relative to its surroundings.
The resulting mode belongs to the whole electromagnetic structure, not to an isolated patch of copper. Its return path may be distributed through capacitance, conductors, earth and fields. This is why replacing the complete antenna-and-feedline system with a single lumped impedance can hide the mechanism that matters.
What a Choke Actually Does
A ferrite choke around the complete coax responds to net ampere-turns. For the wanted internal mode, equal-and-opposite conductor currents produce essentially cancelling excitation in the core. For the external mode, that cancellation is absent and the choke inserts a common-mode impedance:
ZCM(f) = RCM(f) + jXCM(f)
The choke is not absorbing a “phase error,” swallowing reflected power, or converting all transmitter power directly into heat. It carries little flux from the wanted differential mode when built and installed correctly. Its resistive component dissipates power only to the extent that common-mode current actually flows; its reactance also contributes to reducing that current.
The same physical component can receive different application names:
- At a coax-to-balanced-antenna transition, it is appropriately called a 1:1 current balun because it suppresses the unwanted third path and helps enforce current balance at the balanced port.
- Installed in a coax run to impede an external mode between two otherwise unbalanced ports, it is often called a line isolator or common-mode choke.
“1:1 unun” is sometimes used commercially for the same hardware, but the name is ambiguous. A choke does not inherently perform an impedance transformation, and its essential specification is common-mode impedance versus frequency—not the label printed on the enclosure.
Placement matters because the choke becomes part of the external-mode boundary conditions. A feedpoint choke helps define where the antenna ends and the feedline begins. A second choke at the station entry may reduce a remaining current or received noise path, but it cannot retroactively make a poor antenna transition symmetrical.
Match and Balance Are Independent Coordinates
SWR describes the differential-mode impedance relationship at the measurement plane. It does not report net current on the outside of the feedline. This gives four possible cases:
| Differential match | External mode | Possible? |
|---|---|---|
| Good | Small | Yes—the desired case. |
| Good | Large | Yes—a 50 + j0 Ω reading does not prove the coax exterior is quiet. |
| Poor or reactive | Small | Yes—an ideal asymmetric-free two-terminal load can be mismatched without mode conversion. |
| Poor or reactive | Large | Yes—the real structure can support both reflected differential waves and an external mode. |
A choke may change the measured feedpoint impedance when the feedline was previously part of the antenna. That is evidence that the boundary conditions changed, not evidence that the choke damaged the wanted coax mode.
A Measurement Plan That Separates the Claims
- Use a compact, shielded reference load. A known 50 Ω termination at the cable end tests ordinary differential transmission with minimal opportunity for external-mode excitation.
- Keep geometry fixed. Compare resistive and reactive loads without moving the cable, probe, bonds or nearby conductors.
- Probe the complete coax. Measure net RF current at several positions, not just one current node.
- Add a characterised choke. Record complex ZCM(f), not merely an unexplained “dB” number, then repeat the current measurements.
- Change one asymmetry deliberately. Move the feedline, add or remove a controlled counterpoise, or alter antenna-leg coupling and observe whether the external mode changes.
- Measure differential performance separately. S11, insertion loss and power handling answer different questions from common-mode impedance and outside-current suppression.
Bottom line: coax is unbalanced by geometry, not by a failure of current continuity. Its normal TEM current goes out on the centre conductor and returns on the shield’s inner surface. Current on the shield exterior is a separately excited external mode—and that is the mode a current choke is intended to impede.
Mini-FAQ
- Is coax unbalanced? Yes, in the standard geometrical sense: its two conductors are not equivalent relative to the environment.
- Does that mean the centre and shield currents are unequal? No. In the intended TEM mode, centre-conductor and inner-shield currents are equal and opposite at each cross-section.
- Is the wanted return current on the outside of the shield? No. It is on the shield’s inner surface. Outside-shield current belongs mainly to an external mode.
- Can reactance create common mode by itself? No. It changes voltage-current phase and reflections. Mode conversion needs another path or asymmetry.
- Can a matched antenna still have common-mode current? Yes. Differential match and external-mode excitation are different properties.
- Does a choke burn the transmitter’s reflected power? No. It presents impedance to the external mode. Heating depends on common-mode current and the resistive part of that impedance.
- Why measure at several cable positions? The external mode can form standing waves, so one point can coincide with a current minimum and hide the problem.