SWR, Reflected Power and RF Return Paths: Three Different Questions, One Antenna System
SWR, Reflected Power and RF Return Paths: Three Different Questions, One Antenna System
Three measurements and current paths that belong to one RF system, without asking an SWR number to answer every question.
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.
An SWR meter responds to a load. Forward and reflected waves combine on the feed line. Current returns through the coax shield. All three statements can be true, yet none answers the other two questions. I find it more useful to ask separately: How well is the system matched at the meter? What is the net power flow there? Where does current flow in the complete installation?
This distinction matters when a discussion moves from a meter reading to claims about antenna efficiency or the need for a counterpoise. A correct description of one part of the system can become misleading when it is used as the whole explanation.
An SWR reading is not a complex impedance measurement
A directional bridge responds to the voltage and current at its measurement plane. For an ideal bridge referenced to a real 50 Ω system, the samples can be expressed as forward and reverse voltage components:
That is compatible with an impedance-based explanation of the bridge. It does not mean that the number displayed as SWR identifies the load’s resistance and reactance. An ordinary scalar meter discards the relative phase needed to locate a unique impedance on a Smith chart. A calibrated vector measurement retains it.
At a specified frequency and measurement plane, let Z be the impedance looking toward the load and Z_0 the real reference impedance. Then
Recovering the impedance requires the complex reflection coefficient:
The difference is visible in four calculated examples for a 50 Ω reference:
| Impedance at the stated plane | Reflection coefficient | SWR |
|---|---|---|
| 100 + j0 Ω | +1/3 | 2:1 |
| 25 + j0 Ω | −1/3 | 2:1 |
| 40 + j30 Ω | +j/3 | 2:1 |
| 40 − j30 Ω | −j/3 | 2:1 |
These are calculations, not measurements. The last two impedances have a magnitude of 50 Ω, but each still produces 2:1 SWR. Knowing |Z| is no substitute for knowing R+jX. Knowing SWR gives a constant-mismatch circle, not a unique point on it.
There is a useful ideal exception: 1:1 SWR means , hence 50 + j0 Ω at that 50 Ω measurement plane. If a tuner lies between the meter and antenna, the meter sees the tuner’s input. It has not measured the antenna feedpoint directly. Real bridges also have finite directivity and calibration limits.
An ordinary SWR meter indicates the degree of mismatch to its reference impedance. It does not uniquely determine the impedance that produced it.
Waves add; forward and reflected watts are accounted for separately
On a uniform line, incident and reflected voltages superpose. Under one fixed positive-current direction, toward the load, a backward-travelling wave contributes current with the opposite sign:
The relative phase of the two waves changes with position. That makes voltage and current maxima and minima along the line. A high local voltage does not mean that extra average power has appeared there.
With RMS phasors and an ideal lossless line with real Z_0, the time-average power crossing a plane toward the load is
If a directional meter at one plane reads 100 W forward and 25 W reflected, net average power toward the load at that plane is 75 W, not 125 W. On a lossless section between that plane and the load, the load accepts 75 W. With downstream loss, some of that net power heats the line or other components before it reaches the load. These directional readings do not, by themselves, establish radiated power.
What happens when the reverse wave reaches the transmitter end? The answer depends on the source-end boundary. A matched source termination does not generate another reflection of that wave. A mismatched source end can generate a new forward-travelling contribution; repeated reflections are a valid description, especially while a signal is being switched on. In sinusoidal steady state, the directional readings already include the resulting wave components at the measurement plane. Re-reflection does not license adding the two wattmeter readings to calculate delivered power.
Nor should we treat every transmitting amplifier as a fixed 50 Ω resistor that simply absorbs a returning wave. Its active devices, output network, protection behavior and operating point affect the response to mismatch. The safe engineering claim is narrower: superposition describes the combined voltages, currents and fields; forward minus reflected power describes net flow at the stated plane.
The coax return conductor is not the whole antenna-side return system
In the intended differential mode of a coaxial feed line, current on the center conductor is accompanied by opposing current on the shield’s inner surface. The shield really is the line’s return conductor. Calling it something else would obscure how coax works.
At the antenna end, however, those two conductors meet the antenna’s two terminals. In a directly fed dipole, the center conductor connects to one arm and the shield to the other. The second arm is an intended branch of the radiating system. For a monopole, radials, a counterpoise or another conducting ground structure form part of the antenna-side system. Identifying the inner shield current does not make that antenna-side structure electrically irrelevant.
The phrase “return path” can refer to two related but different things: the return conductor along the feed line, or the complete antenna-side current and field system connected at its far end. Specify which one you mean. A counterpoise can be the antenna’s other terminal while the inner surface of the coax shield is the feed line’s return conductor. Those statements do not conflict.
An open-ended dipole does not require electrons to fly through the air from one tip to the other. Alternating charge accumulates and decreases along the metal, and the changing electric field participates in current continuity. The familiar capacitor is a first illustration: conduction current ends at a plate, while displacement current accounts for the changing field between plates. An antenna is a distributed structure with fields and currents that vary along its length; it is not merely a lumped capacitor with long leads. In field notation,
where J is conduction-current density and D/t is displacement-current density. Requiring an unbroken metal loop is therefore the wrong test for whether an RF antenna has a complete electromagnetic current system.
Return current is not reflected power
A perfectly matched coaxial line still carries return current on the inner shield surface, even though it has no reverse wave. The forward wave itself requires both conductors. A reflected wave also involves both conductors. Wave direction and the direction of current on a particular conductor are different descriptions.
Consequently, “current comes back on the shield” does not show that the antenna rejected power. It describes part of the intended circuit even under a perfect match.
Current on the outside of the shield is a separate issue. Feed asymmetry, nearby conductors, routing and coupling can excite that exterior path. A nominally balanced drawing or a pleasing SWR does not prove the installed cable exterior is quiet. A current probe around the whole coax responds to the non-cancelling current through its aperture; the intended internal differential currents ideally cancel in that measurement. Probe several positions, since one low reading may be a current minimum.
Three demonstrations worth making
Compare equal-SWR loads. At one stated frequency, place suitable 25 Ω and 100 Ω RF loads directly at the same calibrated 50 Ω measurement plane. Ideally both yield 2:1 SWR. A vector measurement gives their different impedances. Use appropriate power ratings and account for fixture parasitics. The 40 ± j30 Ω rows above extend the point mathematically; they are not claimed as constructed loads.
Plot standing waves and power at the same plane. Calculate the voltage and current envelopes from V^+ and V^-, then show 100 W forward, 25 W reverse and 75 W net at one plane. Keep a low-level switched-pulse demonstration separate from a sinusoidal steady-state plot: the pulse can reveal whether the source end reflects a returning wave, while the steady-state readings summarize the eventual wave components.
Measure cable current rather than infer it from SWR. Draw the two intended antenna branches, the differential current inside the coax and any current on its exterior. Measure around the complete coax at several positions, with a fixed cable route and reference excitation. Compare a balanced termination or choking change at the same frequency and power. A change in exterior current is evidence about that path; an SWR change alone is not.
The practical conclusion is simple: state the measurement plane when quoting SWR; distinguish field superposition from net power; and trace both the feed-line conductors and the antenna-side structure. Then the apparently competing explanations fit into one physical system.
Sources
- Gary Bold, The Bruene Directional Coupler and Transmission Lines
- Keysight, FieldFox display formats
- Bird, Model 4314B wattmeter instructions
- MIT, transmission-line reflections
- W8JI, common-mode current
Mini-FAQ
- Does 2:1 SWR identify the antenna impedance? No. Several different complex impedances can produce 2:1 SWR at the same measurement plane. A vector measurement is needed to distinguish them.
- Do 100 W forward and 25 W reflected mean 125 W reaches the antenna? No. Net average power toward the load at that plane is 75 W. Downstream losses may reduce the power accepted by the antenna further.
- Is current on the coax shield necessarily reflected power? No. The intended forward wave has return current on the inner shield surface even when the line is perfectly matched.
- Does a 1:1 reading prove the antenna feedpoint is 50 ohms? Only if the meter measures at that feedpoint with a 50-ohm reference and the ideal assumptions hold. A tuner or feed line can change what the meter sees.
- Can SWR prove that the outside of the coax is quiet? No. Measure common-mode current around the complete coax at several positions under controlled conditions.