RF UNUN Loss: From dB Claims to Measured Efficiency

The useful reality check is not that one ratio “usually loses 4%” and another “loses 12%.” Loss belongs to a particular transformer operating under particular electrical and thermal conditions.

State Which Ratio You Mean

For an ideal transformer:

A 1:2 turns ratio therefore gives a 1:4 impedance ratio. Unfortunately, amateur-radio labels often quote impedance ratio while transformer texts often quote turns ratio, and a “49:1 EFHW transformer” commonly means high impedance to 50 Ω even though the source-to-load direction may be described the other way.

This article writes impedance ratios explicitly, such as 50 Ω to 200 Ω or 2450 Ω to 50 Ω. That prevents a label from doing the engineering.

Transformation, Balance and Choking Are Different Specifications

An UNUN transforms one nominal unbalanced port impedance to another. A common-mode choke impedes an unwanted external mode. Port balance, galvanic isolation, impedance ratio and common-mode impedance are different properties.

A separate choke is often convenient because each function can be measured and placed independently. It is not a law that the functions must occupy separate boxes: a deliberately designed transformer can provide useful common-mode impedance too. The requirement is to specify and verify both functions instead of assuming one from the other.

Low SWR proves neither low dissipation nor low common-mode current.

A Ratio Does Not Have a Typical Efficiency

A transformer-efficiency figure is meaningful only when it identifies the ferrite, topology, frequency, load, construction, temperature and measurement uncertainty. Nominal ratios such as 1:2, 1:4, 1:9, 49:1 or 70:1 cannot carry a generic percentage-loss range by themselves.

A well-designed high-ratio transformer at one frequency can outperform a poorly designed low-ratio transformer at another. Higher transformation ratios often make broadband design harder because voltage ratio, impedance span and parasitic sensitivity increase, but the ratio alone cannot supply a percentage loss.

What “Insertion Loss” Actually Means

Mini-Circuits defines transformer insertion loss as the power lost when a real transformer replaces an ideal lossless transformer of the same ratio in an impedance-matched system. Its transformer measurement note also stresses the correct theoretical terminations and impedance transformation required at the measurement ports.

That definition matters. If a 2450 Ω port is connected directly to an uncompensated 50 Ω receiver, the resulting transmission loss is dominated by the test mismatch. It is not transformer heating.

Three quantities must be kept separate:

A scalar S21 trace can combine mismatch, fixture loss and transformer loss. Renormalisation or correct matching networks, complex multiport data and de-embedding are needed before calling the result intrinsic efficiency.

Exact dB-to-Power Conversion

Once insertion loss is validly defined, the conversion is exact:

“Power not delivered” equals transformer heat only after mismatch, fixture loss, unintended radiation and common-mode power have been accounted for.

Where Real Transformer Loss Comes From

• Core loss. The lossy component of permeability dissipates energy. It depends on material, frequency, flux amplitude and temperature.
• Winding loss. DC resistance, skin effect and proximity effect raise conductor loss with frequency and current.
• Leakage flux. Imperfect coupling adds series reactance and can increase current or mismatch.
• Magnetising current. Finite primary inductance loads the low-frequency end and contributes copper and core loss.
• Distributed capacitance. It changes voltage distribution and creates resonances or bypass paths at the high-frequency end.
• Dielectric and contact loss. Insulation, terminals, joints and contaminated surfaces matter under high RF voltage.
• Unintended modes. Feedline and fixture current can carry power outside the nominal two-port transformer path.

TDK's current ferrite design guidance treats loss as a function of frequency, flux density, temperature, material and core geometry, and also includes winding skin and proximity effects. That is why a core part number and turns count are still not a power rating.

Flux Density: Useful Formula, Limited Model

For an approximately sinusoidal voltage across an ideal winding:

Bpk ≈ Vpk / (2π f N Ae)

where Ae is effective magnetic area. The relation explains why low frequency, fewer turns and higher winding voltage increase flux swing.

Do not insert transmitter output voltage blindly. Use the voltage actually appearing across the relevant winding in the real circuit. Autotransformers and transmission-line transformers divide voltage and flux differently, reactive loads alter node voltages, and distributed behaviour limits the lumped model at upper HF.

Also, avoiding saturation is not enough. A ferrite can dissipate unacceptable power well below saturation. Manufacturer loss data and temperature rise remain necessary.

Reactive Power Is Not Automatically Transformer Heat

A reactive antenna load stores and returns energy each cycle; ideal reactive power is not consumed. The practical problem is that a large mismatch or reactive transformation can raise RMS voltage and current in windings, tuner parts and feedline. Those larger fields multiply real core, copper and dielectric losses.

A tuner can present a 50 Ω input to the transmitter while high voltage, high current and loss remain elsewhere. It does not “hide heat,” but an input SWR measurement cannot locate or quantify that heat.

What Common Ratios Really Tell You

The antenna feedpoint impedance must be measured or modelled with its actual height, geometry, conductor, surroundings and return path. Select the ratio from that impedance and the intended matching range.

Why High-Ratio EFHW Transformers Become Difficult

A 49:1 EFHW transformer is asked to span a large impedance range while tolerating high voltage on its high-impedance end. If 100 W is delivered to a purely resistive 2450 Ω load, the ideal load voltage is about 495 V RMS or 700 V peak. Reactive voltage magnification can raise internal node voltage further.

At low frequency, magnetising inductance and core flux are prominent constraints. At upper HF, leakage inductance, lead length and distributed capacitance become comparable to the desired impedances. The same nominal 49:1 ratio can therefore behave very differently with different winding layouts.

On non-harmonic bands such as 17 and 12 m for many common EFHW lengths, the antenna itself may present a difficult complex impedance. On harmonic bands, a favourable SWR still does not prove low loss. Test the intended wire system, not only a nominal resistor.

Do Not Turn Antenna Geometry into an Efficiency Claim

An inverted-L can provide useful low-angle radiation when its vertical section, height, ground environment and current distribution are favourable. It can also lose power through transformer loss, conductor loss, nearby objects, ground coupling or unintended feedline current.

Its shape alone does not prove that more current is in the radiating wire or less is in soil. Radiation pattern and efficiency require a full model or measurement of the complete antenna and environment.

Four Defensible Ways to Measure

1. Correctly terminated small-signal insertion loss

Measure with the source and load impedances that correspond to the ideal transformation. Use calibrated complex data, appropriate port renormalisation or characterized matching networks, and de-embed fixture loss. This is excellent for frequency response and low-power linear performance.

2. Full two- or three-port characterization

Treat a centre-tapped device as a three-port network and examine amplitude/phase balance as well as transmission and match. Mini-Circuits' RF transformer fundamentals explains why modern multiport methods are preferable to assuming a simple half-loss result from a back-to-back pair.

3. Direct power balance under RF load

At a defined frequency and load, measure accepted input power and load power at calibrated planes:

Ploss = Paccepted,in − Pload,out − Pother paths

Directional-coupler directivity, load accuracy, connector/feedline loss and harmonic power set the uncertainty. A small transformer loss can be the difference between two much larger numbers, so uncertainty matters.

4. Calorimetry or thermal calibration

Temperature rise shows that loss exists but not directly how many watts. Calibrate the assembly with known internal heating under the same enclosure, airflow, orientation and ambient conditions. Then temperature can estimate dissipation and reveal thermal runaway or power dependence.

Why Back-to-Back Division by Two Can Fail

Two nominally identical transformers connected back to back give the loss of the pair plus interconnect and mismatch effects. Dividing dB by two is justified only when the units contribute equal loss under reciprocal electrical stress and the pair is correctly terminated.

For an EFHW autotransformer, one unit may operate low-to-high while the other operates high-to-low, with different common-mode paths, stray capacitance and voltage distributions. Measure each direction or validate the symmetry before halving the result.

A Report That Can Be Reproduced

• schematic, topology, turns and winding geometry;
• core manufacturer, material, dimensions and number of cores;
• source and load impedances, including reactance;
• frequency, input power, waveform and duty cycle;
• calibration planes, fixture compensation and instrument uncertainty;
• complex S-parameters or accepted/delivered power;
• common-mode termination and measured external current;
• ambient, enclosure, airflow, test duration and temperatures; and
• repeatability across units.

Without these conditions, “0.2 dB,” “95% efficient” or “1 kW rated” is not a portable specification.

The Practical Verdict

Use the lowest transformation ratio that fits the measured load only as a starting heuristic. Then choose a topology, material, core volume, winding and insulation system for the actual band, power, duty cycle and environment.

Verify match, insertion loss, external-mode current and temperature separately. A cool enclosure is encouraging, but only a calibrated power or thermal measurement can turn that observation into efficiency.

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