UNUN Types: Ratio, Topology and Common-Mode Control

The useful question is not simply “Which UNUN do I need?” Start with the complex load at a declared reference plane, identify the intended return conductor, decide whether common-mode current must be stopped at that point, and then qualify the transformer under the voltage, current, mismatch and duty cycle it will actually see.

What the Word UNUN Actually Specifies

UNUN is shorthand for unbalanced-to-unbalanced. It describes the intended relationship of the two ports to their reference conductors; it is not a complete circuit description. A device sold as a 4:1 or 9:1 UNUN might be a tapped autotransformer, a Ruthroff transmission-line transformer, a series/parallel Guanella network or another coupled network.

Those circuits do not behave identically. Some share a conductive path between input and output. Some use transmission-line sections arranged so differential currents combine in series at one port and in parallel at another. Their winding capacitance, leakage inductance, line impedance, ferrite properties and termination all affect the useful frequency and load range.

Ratio Is a Starting Point, Not a Load Guarantee

For an ideal transformer, an impedance ratio is the square of the voltage or turns ratio:

A nominal 4:1 impedance transformer therefore corresponds to a two-to-one voltage ratio under its stated port convention. A nominal 9:1 corresponds to three-to-one. Ratios such as 49:1, 64:1 or 81:1 describe increasingly large ideal voltage ratios, not a promise that every end-fed wire will present the corresponding resistance.

Real antenna loads contain reactance and vary with frequency, height, conductor geometry, ground, coupling and the chosen return path. Transformer loss and parasitic effects also change the impedance presented to the feed line. Measure the antenna system at the transformer load plane before selecting a ratio, then verify the transformed impedance and loss across the entire operating range.

Common UNUN Families

The useful topology follows the actual ports and modes. “Voltage” and “current” are convenient family names, but every passive transformer has voltage and current. The engineering distinction is how the windings and transmission-line sections constrain terminal voltages, terminal currents and common-mode current over a stated load and frequency range.

A 9:1 UNUN for a Non-Resonant Wire

A 9:1 UNUN is often tried between coax and a non-resonant end-fed wire because a three-to-one ideal voltage ratio can bring some high impedances closer to a tuner's range. It does not turn every “random wire” into 450 Ω, and it does not make the wire resonant. Depending on wire length, frequency and surroundings, the load may be low, high or strongly reactive.

The return current still needs a conductor. It may use a deliberate counterpoise, a selected length of coax exterior, a ground or radial structure, capacitance to the surroundings, or several of those paths. Define that current path before placing the choke. If a section of coax exterior is intentionally part of the antenna, choking directly at the UNUN changes the antenna.

High-Ratio UNUNs for End-Fed Half-Wave Systems

Ratios such as 49:1, 64:1 and nearby values are associated with end-fed half-wave systems because the feedpoint resistance can be much higher than 50 Ω near a half-wave current minimum. But feedpoint impedance is not a fixed property of the name EFHW. It changes with conductor length and diameter, transformer location, return conductor, installation geometry and frequency.

High ratio increases the voltage transformation and can expose the winding, compensation capacitance, enclosure and connectors to substantial electric-field stress. Core loss also depends on flux, material, frequency, waveform, duty cycle and mismatch. A low-power SWR sweep cannot establish transmit power capability, efficiency or thermal margin.

“Minimal counterpoise” is not a complete design instruction. There must be an equal-and-opposite current somewhere in the complete system. Deliberately identify it, keep it out of unintended station wiring, and measure shield-exterior current at several positions rather than inferring the return path from SWR.

A 4:1 UNUN for a Measured Unbalanced Load

A 4:1 UNUN can be appropriate when a measured unbalanced load occupies a range that a nominal four-to-one impedance transformation usefully maps toward the feed system or tuner. That may occur with some vertical, off-centre-fed, loop or wire installations, but none of those antenna names guarantees 200 Ω or an unbalanced installed port.

Where the antenna geometry is nominally balanced but the complete installation is not, a measured-load UNUN plus a separate choke often makes the two jobs clearer. Where the installed load is genuinely balanced, a suitable transforming current balun may be the more direct solution. The decision follows measured port behaviour, not the drawing alone.

Why an UNUN Does Not Replace a Choke

Impedance transformation concerns the wanted differential path between the two conductors at each port. Common-mode suppression concerns an additional path, such as current on the outside of a coax shield, a mast or station wiring. A transformer can influence both modes, but a nominal ratio says nothing about its common-mode impedance.

A separate choke lets you specify complex common-mode impedance at the relevant boundary without forcing the transformer to do an undocumented second job. The choke belongs where the intentional radiator and return structure should end. On a vertical that may be the feedpoint. On an end-fed wire using a deliberate coax-exterior counterpoise, it may be farther down the line. Probe the completed installation to confirm the boundary.

What to Measure Before Calling It Suitable

• Complex load: record R+jX at the transformer's antenna-side plane across every intended band.
• Differential transfer: measure transformed impedance, insertion loss and usable bandwidth with representative complex terminations.
• Common mode: measure complex common-mode impedance or mixed-mode behaviour separately, then map exterior current on the installed conductors.
• Stress: verify winding current, terminal voltage, insulation, connector current and temperature at the intended accepted power, waveform, mismatch and duty cycle.
• System result: compare accepted power, current distribution, loss and field behaviour with an A/B/A sequence. Do not use SWR as the only acceptance test.

Primary and Authoritative Sources

• Gustav Guanella, High-Frequency Matching Transformer, US2470307A—primary transmission-line-transformer arrangements and series/parallel interconnection.
• C. L. Ruthroff, “Some Broad-Band Transformers”—primary broadband transformer circuits and transmission-line action.
• Roy Lewallen, W7EL, balun and current-probe appendix—current balance, transformer tests and installed-current measurement.
• Andrew Roos, ZS1AN, “A Better Antenna-Tuner Balun”—load imbalance and separate transformer/choke functions.
• Tom Rauch, W8JI, Common Mode Current—coax inner-return and shield-exterior current paths.
• Fair-Rite technical papers—complex permeability, impedance, frequency and temperature limits of ferrite parts.

Choose the Electrical Job, Then the Hardware

An UNUN is useful when the real source and load ports are unbalanced and an impedance transformation is required. Its nominal ratio is only the first line of the specification. Topology, complex load, differential loss, common-mode behaviour, voltage, current and temperature determine whether it belongs in the station.

For the imperfect balance found in many practical amateur installations, my default remains a suitable UNUN plus a separate measured choke. The important word is suitable: both stages must be selected for their actual loads and verified together in the final geometry.

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