Why the PA0FRI “Coax Balun 1:4 and 1:1” Document Keeps Coming Back
Why the PA0FRI “Coax Balun 1:4 and 1:1” Document Keeps Coming Back
The construction is real, the reputation is understandable and the circuit deserves a fair test. The essential question is whether each measurement proves the function assigned to it.
Readers regularly send me PA0FRI’s page about a coaxial 1:4 and 1:1 balun. That is no surprise: PA0FRI was a respected figure in the Dutch amateur-radio community, the construction is practical and the page contains unusually candid measurements. Respect makes the circuit worth studying closely; it does not allow transformation, balance, choking, loss and power handling to collapse into one result.
Joeri’s position: the parallel/series coax circuit is a legitimate transmission-line-transformer topology. Its impedance transformation, differential transmission, output balance, common-mode impedance, loss and thermal limit are separate properties and must be measured separately.
The PA0FRI Page Is the Starting Point
The discussion concerns PA0FRI’s English “Coax Balun 1:4 and 1:1” page, with the Dutch page and a preserved PDF copy available for the drawings and tables.
The page proposes two equal coaxial sections connected in parallel at one side and in series at the other. It discusses an air-wound implementation, compares conductor-current readings with another balun, gives SWR data into a resistor and describes operation between an unbalanced tuner and open-wire line. Those are several different experiments. The useful work is to map each one to the exact proposition it can support.
| Proposition | Required evidence | What does not establish it alone |
|---|---|---|
| Impedance transformation | Complex input and output impedance or calibrated mixed-mode S-parameters at declared reference impedances. | A nominal turns ratio or low SWR at one frequency. |
| Differential transmission | Insertion loss, return loss, phase and load over the intended band. | Equal RF-current magnitudes in two conductors. |
| Balance and mode conversion | Magnitude and phase of both conductor currents, their vector sum, or mixed-mode conversion terms. | Two separate diode meters that show similar numbers. |
| Common-mode suppression | Complex common-mode impedance or common-mode transmission with a characterized fixture. | Differential SWR into a resistor. |
| Power rating | RF voltage, current, loss and temperature under a declared load, frequency, duty cycle, enclosure and ambient condition. | The absence of a ferrite core or one short on-air trial. |
Differential and Common Mode Must Stay Separate
Choose the current reference directions consistently. For a two-conductor line, the two measured currents can be resolved into differential and common-mode components. One common convention is:
ID = (I1 − I2)/2
IC = (I1 + I2)/2
The factor convention may vary; the vector sum and the stated current directions may not.
For the intended differential mode, the currents are equal in magnitude and opposite in direction. For common mode, they have an in-phase component with a return through the surrounding structure, equipment or displacement-current path. On coax, the intended TEM return is on the inner shield surface; current on the outer shield surface belongs to the exterior/common-mode circuit.
ITU-T K.37 explicitly separates differential disturbance from common-mode voltage/current and explains that symmetry, screening and common-mode impedance control their conversion. The terminology is not cosmetic: a component can transmit the desired differential mode well while offering too little impedance to the unwanted common-mode circuit.
Equal Meter Readings Do Not Prove Balance
The PA0FRI comparison places current indicators on the two conductors of the open line. Similar magnitudes can be useful, but magnitude-only readings do not reveal relative phase. Equal and opposite currents and equal in-phase currents can therefore produce the same two meter indications while representing entirely different modes.
A clamp probe around both conductors responds to their vector sum. With consistent orientation, the intended differential components cancel and the residual exposes the common-mode component. A probe on the outside of the coax shield measures the exterior path directly. Neither test should be made at only one convenient point: common-mode current can form a standing-wave pattern, so a local minimum is not proof of low current along the complete line.
The ARRL’s common-mode-current measurement article illustrates the clamp-current approach on coax. For a balanced component, a calibrated multiport measurement can go further: IEC 62783-1-1 treats differential and common-mode transmission as distinct parameters with defined fixtures, while the Rohde & Schwarz balanced-component note shows how mixed-mode S-parameters expose differential transmission, common-mode transmission and conversion between them.
The Ratio Is Real, but the Reference Impedances Matter
In the ideal parallel/series connection, two equal line sections share current at the low-impedance side and add voltage at the high-impedance side. That gives a 1:2 voltage ratio and a 1:4 impedance ratio. The transformation is not restricted to a purely resistive load: an ideal transformer reflects the complete complex impedance.
Zin = Zload / n²
For n = 2, a complex 200 + j80 Ω load reflects ideally as 50 + j20 Ω. A real transformer then adds line mismatch, leakage, parasitic coupling and loss.
The individual coax impedance sets the natural reference condition of the two-line arrangement. The PA0FRI page usefully points out the resulting pairs:
| Each coaxial section | Parallel-side reference | Series-side reference |
|---|---|---|
| 50 Ω | 25 Ω | 100 Ω |
| 75 Ω | 37.5 Ω | 150 Ω |
| 100 Ω | 50 Ω | 200 Ω |
Two 50 Ω sections can still transform impedance, but they are not naturally a broadband 50-to-200 Ω device. PA0FRI’s own resistor test shows strong frequency variation in input SWR. That is valuable evidence: it demonstrates why a nominal 4:1 label does not specify bandwidth or match.
Transformation and Choking Are Different Jobs
Series/parallel connection establishes the intended differential transformation. Preventing the complete assembly, tuner and feed line from carrying common-mode current requires adequate impedance in the unwanted mode. Coiling the coax adds inductance to the exterior-current path, but the resulting impedance is complex and frequency-dependent.
An air-wound coil has turn-to-turn capacitance, capacitance to nearby conductors and enclosure, lead inductance and distributed coupling. It therefore has resonances. Below, near and above those resonances, the common-mode impedance can be inductive, resistive or capacitive. “It has 33 µH” does not describe that full function, and neither does a differential SWR sweep.
A useful choke result gives complex impedance or transmission across the intended band, together with fixture details and self-resonant behaviour. A Y-parameter two-port fixture is one practical method when its series-equivalent model and reference planes are valid; mixed-mode multiport measurement is another. The instrument is not the verdict—the calibrated model is.
No Ferrite Means No Ferrite Limit, Not Unlimited Power
Removing ferrite removes ferrite-specific flux-density, permeability and core-heating limits. It does not create a power rating. The finished air-wound assembly is still bounded by:
- coaxial conductor current and conductor/dielectric loss;
- RF voltage across the load, series connection, connectors and adjacent turns;
- dielectric strength, spacing, contamination, moisture and altitude;
- outer-surface current, common-mode voltage and the resulting external field;
- self-resonance, parasitic current concentration and local heating;
- duty cycle, enclosure ventilation and ambient temperature.
A carrier trial into one antenna is a useful survival observation for that configuration. It is not a transferable rating for every band, load or duty cycle. A power claim belongs to the complete assembly and test condition.
Reactance Changes the Circuit but Does Not Consume Average Power
The PA0FRI page compares a calculated inductive reactance with a feeder impedance and then treats their parallel combination as though a large part of the transmitter power must become heat. An ideal reactance stores and returns energy; its average real power is zero. Heat is set by the real loss component.
Ploss = Irms² Rloss
Reactance changes current, voltage, phase and tuning. Dissipation requires resistance, dielectric loss, radiation or another real-power path.
This does not make low magnetizing or common-mode reactance harmless. It can shunt current away from the intended path, increase circulating current, raise voltage elsewhere and make the tuner work into a difficult load. But the thermal conclusion requires ESR, dielectric loss, radiation loss and temperature—not reactance alone.
A Tuner Can Hide Stress Without Creating Efficiency
Putting 1:1 or 4:1 transformation between an unbalanced tuner and open-wire line changes the impedance presented to the tuner. Sometimes that reduces tuner loss or keeps voltage/current within range. In another installation it can move the load toward a high-current or high-voltage corner and increase stress. The ratio is a design choice, not an automatic efficiency upgrade.
Changing the short coax length can also move the impedance seen by the tuner through transmission-line transformation. A successful match at the transmitter proves only that the tuner reached its input target. It does not prove low line loss, low common-mode current, good balance or a stable broadband result.
Open-wire line can be exceptionally low loss under high SWR, but its input impedance depends on radiator impedance, line length, characteristic impedance and frequency. Compare candidate systems with the same antenna, line geometry, tuner reference plane and measured delivered power. “The tuner found 1:1” is the start of that test, not its conclusion.
A Reproducible Test for This Balun
- Document the DUT. Record coax type and length, coil diameter and spacing, lead geometry, connectors, enclosure and winding orientation.
- Define logical ports. State the unbalanced reference impedance, balanced differential reference impedance and common-mode reference path.
- Measure differential behaviour. Record complex S-parameters or impedance, insertion loss and phase into resistive and representative complex loads.
- Measure balance. Capture both output currents with magnitude and phase, or use mixed-mode conversion parameters. Do not compare magnitudes alone.
- Measure common mode. Sweep complex common-mode impedance or transmission with a characterized fixture; map current at several positions on the installed line.
- Find resonances. Extend the sweep beyond the intended bands and repeat with the real enclosure and nearby conductors.
- Run a thermal test. Declare frequency, waveform, duty cycle, load, mismatch, ambient temperature and duration; record temperature at coax, connections and enclosure.
- Repeat installed A/B/A. Keep antenna, tuner state, line route and measurement planes fixed while swapping the DUT, then return to the first state to expose drift.
What the PA0FRI Work Still Contributes
The page contributes a buildable topology, measured SWR data that reveal its reference-impedance problem, practical tuner observations and enough detail for another operator to repeat the experiment. Its enduring value is not that every conclusion must be accepted; it is that the claims can be separated and tested.
That is the respectful engineering answer to a classic document: preserve the circuit, preserve the measurements, identify the missing mode or reference plane and run the next experiment.
Primary and First-Hand References
- PA0FRI: Coax Balun 1:4 and 1:1 — English Page
- PA0FRI Coax Balun Page — Preserved PDF
- ITU-T K.37: Common/Differential Mode, Cable Balance and Common-Mode Chokes
- IEC 62783-1-1: Differential and Common-Mode Cable Measurements
- Rohde & Schwarz 1EZ53: Measuring Balanced Components and Mode Conversion
- ARRL QST: Common-Mode Current and Common-Mode Chokes
Keep Exploring the Measurement Trail
- RF.Guru Technical Deep Dives
- Baluns and UNUNs — Technical Articles
- Ham-Radio Myth Debunking
- Transmission-Line Fundamentals
Final rule: measure the differential path, the common-mode path and conversion between them. Then add voltage, current, loss and temperature before assigning a bandwidth or power rating.
Mini-FAQ
- Is the PA0FRI parallel/series coax circuit a real transformer? Yes. It is a valid transmission-line-transformer topology, but the nominal 4:1 ratio does not by itself establish its reference impedances, bandwidth, balance or common-mode suppression.
- Do equal current-meter readings prove a balanced output? No. The two readings need relative phase, or their vector sum must be measured. Equal magnitudes can occur with either differential or common-mode current.
- Does a ferrite-free balun avoid every power limit? No. It avoids ferrite-specific limits but remains bounded by coax loss, RF voltage/current, connections, spacing, resonance, enclosure and temperature.
- Does inductive reactance dissipate transmitter power? An ideal reactance consumes no average real power. Real loss comes from resistance, dielectric loss, radiation and other dissipative paths.
- Can a VNA test a balun properly? Yes, when calibration, fixture, logical ports, reference impedances and mode conversion are defined. Differential SWR alone is not a complete balun test.
- Is 1:1 or 4:1 always better before an unbalanced tuner? Neither ratio wins universally. Choose from the impedance range, tuner loss/stress, differential loss, common-mode control and installed measurements.