EFHW Station Loss Budget: Coax, Transformer and Common Mode
EFHW Station Loss Budget: Coax, Transformer and Common Mode
An EFHW station can lose power in the tuner, feed line, matching transformer, conductors, choke and surrounding ground. A useful budget measures each stage at a defined reference plane and keeps dissipative loss separate from mismatch and radiation-pattern effects.
A short run of low-loss coax may contribute little beside a lossy high-ratio transformer. A long, wet or thin feed line can instead dominate an efficient matching unit. A weak signal in one direction may be a radiation-pattern null rather than dissipated power. A complete loss budget separates these mechanisms before any component is changed.
RF safety: an end-fed half-wave has a high-voltage feedpoint. At 100 W into a purely resistive 2450 Ω, the simple sinusoidal value is about 495 V RMS and 700 V peak; real mismatch and transients can raise local stress. De-energize before changing the transformer, wire, counterpoise, choke or test fixture, and use components, enclosures and clearances rated for the actual voltage, power, duty cycle and environment.
Start With a Power-Flow Diagram
“Antenna-system loss” is not one component. A defensible audit follows power through successive boundaries:
Transmitter → tuner/filters → coax differential mode → matching transformer → antenna terminals → conductor/ground/environment → radiation
A parallel path may also carry common-mode current on the outside of the coax shield, station wiring or another conductor. That path can radiate, dissipate heat, change the intended pattern, couple noise on receive and create RF-safety or EMC problems.
| Quantity | What it means | What does not prove it |
|---|---|---|
| Matched line attenuation | Conductor and dielectric loss of a specified cable, length, frequency and condition when terminated in its nominal impedance. | SWR alone or a generic cable-family name. |
| Additional line loss under mismatch | Extra conductor/dielectric dissipation caused by the higher voltage/current distribution on the lossy line. | The interface mismatch-loss formula by itself. |
| Transformer dissipation | Core, winding, dielectric and connection loss at a stated impedance, frequency, power, waveform and temperature. | A low input SWR or an unloaded VNA sweep. |
| Mismatch loss | The fraction of an incident wave not accepted at a specified reference plane: −10 log10(1 − |Γ|²). | Heat in that interface; reflected energy is not automatically dissipated there. |
| Radiation efficiency | Radiated power divided by power accepted by the antenna structure. | A good SWR, a distant signal report or one field-strength direction. |
| Realized gain | Efficiency, mismatch and radiation pattern combined in a stated direction and polarization. | Transformer temperature alone. |
Do not double-count: if a measured end-to-end insertion result already includes mismatch, fixture and connector effects, those terms cannot simply be added again. Define the input and output reference planes before adding dB.
Coax Loss Depends on Cable, Length, Frequency and Load
Matched attenuation is set by the exact cable construction, length, frequency, temperature and condition. The official Times Microwave LMR-400 data sheet, for example, specifies typical attenuation of 0.7 dB per 100 ft at 30 MHz for a 1.0:1 termination and 25 °C. That leaves about 85% of the input power after 100 ft. It says nothing about RG-58, a 10 ft jumper, a 100 m tower run, water ingress or damaged connectors.
Loss in dB converts to a power-transmission ratio through:
η = 10−LdB/10
Thus 0.1 dB transmits 97.7%, 0.5 dB transmits 89.1%, 1 dB transmits 79.4% and 3 dB transmits about 50.1%. These percentages are useful only after the dB term has been tied to a real component and test condition.
Mismatch changes line dissipation
A lossy line has baseline matched attenuation. With mismatch, forward and reverse waves change the current and voltage distribution along the line, so total conductor and dielectric dissipation generally increases. The added loss depends on the line’s attenuation, electrical length and complex load reflection—not SWR magnitude alone.
That extra line dissipation is distinct from interface mismatch loss. At a load plane, 1 − |Γ|² is the fraction of the incident power accepted on that encounter. What happens to the reflected power depends on the source, tuner and intervening lossy network. “Multiple passes” can be a helpful story, but a steady-state transmission-line calculation is the reliable method.
Loss also makes a shack-end SWR look deceptively better because the reflected wave crosses the feed line twice. Rohde & Schwarz’s antenna-system return-loss note shows that feeder attenuation and cable mismatch can corrupt an antenna return-loss measurement made at the bottom of the line. A low shack reading therefore does not prove low feedpoint reflection or low line loss.
A 49:1 Label Does Not Specify Transformer Efficiency
A nominal 49:1 impedance transformation corresponds to a 7:1 turns ratio in the ideal model. It maps 2450 Ω to 50 Ω. A real EFHW terminal impedance is not fixed at 2450 Ω: wire length and diameter, height, shape, frequency, nearby objects, ground, the opposite-terminal path and the transformer itself all change the complex load.
High-ratio broadband transformation is demanding, but “49:1” does not determine the loss. Core material and volume, primary inductance, turns, winding resistance, interwinding capacitance, leakage inductance, construction, load, frequency, power, duty cycle and temperature all matter.
Fair-Rite’s official broadband-transformer guidance divides response into low-, mid- and high-frequency regions. Low-frequency roll-off is linked to falling shunt/magnetizing impedance; the high-frequency region is limited by leakage inductance and parasitic capacitance. The cutoff frequencies belong to the individual design—not to every transformer with the same ratio.
Measured designs can differ substantially
Wes Hayward, W7ZOI, published a controlled EFHW transformer measurement that used a tunable L-network to return the high-impedance side to 50 Ω. He measured the L-network component Q, calculated its loss and subtracted it from the cascade. His small-core QRP examples are not ratings for modern commercial units, but they show how strongly loss can depend on a particular construction:
| Measured construction | Load | 7 MHz loss | 14 MHz loss |
|---|---|---|---|
| FT-114-61, 3:27 turns | 4.4 kΩ | 0.10 dB | 0.21 dB |
| FT-114-43, 3:27 turns | 4.4 kΩ | 1.30 dB | 1.30 dB |
At 1.3 dB, about 74% of accepted input power remains; at 0.1 dB, about 98% remains. The comparison does not establish a universal core-material ranking: it applies to Hayward’s turns, cores, load, frequencies, fixture and low-power conditions. Transformer loss therefore needs measurements or specifications for the exact construction and operating point.
The MyAntennas EFHW-8010 product page specifies a three-core transformer for 3.5–30 MHz and claims less than 0.4 dB average loss. Treat this as a design-specific manufacturer claim, not independent validation. It is a testable statement for that model and does not establish the performance of other EFHW matching units.
Compensation Changes Both Match and Component Stress
A shunt or compensation capacitor can resonate part of the transformer’s leakage inductance or adjust the input match over a chosen range. That is legitimate network design. It can also carry substantial RF current, see high voltage, add dielectric/ESR loss and create a narrow or load-sensitive response.
Evaluate the complete network against its insertion-loss, return-loss, voltage, current and temperature limits over the intended loads, bands and power. A better S11 trace after compensation proves a better input match; it does not by itself prove higher efficiency.
Upper-Band Pattern Change Is Not the Same as Lost Power
A wire that is a half wavelength on its lowest band becomes several half wavelengths long on harmonic bands. For an 80 m fundamental, it is approximately 3 wavelengths on 15 m and 4 wavelengths on 10 m; for a 40 m fundamental, approximately 1.5 and 2 wavelengths. End effects and installation shift the exact resonances.
As electrical length increases, the radiation pattern develops more lobes and nulls. That can make one path much stronger and another much weaker without a corresponding change in total radiated power. “Pattern stability collapsed” and “efficiency collapsed” are therefore different findings:
- Efficiency needs accepted versus radiated power, conductor/ground-loss modeling, calorimetry or a validated field method.
- Pattern needs a validated electromagnetic model or controlled angular field measurement.
- One signal report includes propagation, remote antenna, polarization, noise, fading and the selected lobe/null.
Lawrence Livermore National Laboratory’s NEC is an established method-of-moments tool for wire antennas and ground interaction. A useful EFHW model must include the actual wire geometry, conductor, ground, transformer equivalent circuit and common-mode feed structure; a perfect source placed on an isolated wire cannot determine transformer or feed-line loss.
The Return Path and Common-Mode Boundary Must Be Defined
An end-fed radiator is not a one-terminal circuit. Displacement current and current on a counter-conductor complete the electromagnetic system. Depending on the installation, that counter-conductor can include a deliberate wire, transformer capacitance, the coax exterior, equipment bonding and nearby structures.
That does not make all outside-shield current “heat.” Common-mode current can:
- radiate and alter the intended pattern;
- dissipate in a lossy choke, soil, building wiring or resistive conductors;
- change feedpoint impedance and shack-end SWR;
- couple local noise into the receive system; or
- create RF exposure, touch-voltage and interference problems.
A choke changes that current path and itself has a complex common-mode impedance and a power/temperature limit. The 2024 ARRL article “Common-Mode Chokes” recommends measuring feed-line current and retesting after installation; it also shows why choke response is frequency-dependent and can fall above self-resonance.
No universal 0.05λ rule: a fixed counterpoise length or choke distance can become an intentional part of one design, but it is not a general cure. Choose the common-mode boundary from a model or current sweep, install a choke with adequate complex impedance and thermal margin over the required bands, then measure again.
Other Antenna Families Do Not Share One Easy Impedance
Off-center-fed dipoles, loops and verticals do not share one nominal 100–200 Ω feedpoint or one matching topology. Their feedpoint impedances and balance properties depend on geometry, feed position, ground/radial system, height and frequency:
- An off-center-fed dipole may use a current balun and an impedance transformation chosen for its actual feedpoint and common-mode environment.
- A full-wave loop’s feedpoint impedance changes with shape, height, polarization and feed position.
- A quarter-wave vertical over an adequate radial system is normally below 50 Ω before matching; other verticals can be much higher or reactive.
- A 9:1 random-wire transformer does not create resonance. If a shack tuner matches through a high-SWR coax run, that line can add substantial loss; whether moving the tuner helps depends on the actual line and remote matching conditions.
Lower transformation ratio can simplify one transformer design, but it does not guarantee a lower-loss antenna system. Ground loss, loading, feed-line length, tuner topology and radiation pattern can outweigh the ratio.
How to Measure an EFHW Transformer
- Define the ports and efficiency: specify whether efficiency means output power divided by accepted input power or transducer gain including mismatch.
- Use the intended complex load: a 2450 Ω resistor is one reference, not every EFHW. Characterize the resistor’s RF resistance and parasitics over frequency.
- Calibrate to the real reference planes: adapters, lead-ins, PCB traces and high-voltage fixtures must be characterized or de-embedded. Rohde & Schwarz’s fixture guidance explains why a coaxial VNA calibration stops at its calibration plane.
- Measure S11 and transmission: return loss alone cannot separate transformation from dissipation. Correct raw transmission for fixture, load and mismatch contributions.
- Treat back-to-back tests carefully: halving cascade loss in dB assumes comparable, reciprocal transformers and a controlled intermediate interface. It is a useful cross-check, not an automatic proof.
- Verify at operating power: low-level VNA data do not establish heating, voltage breakdown, core nonlinearity or duty-cycle rating. Use calibrated forward/output power or calorimetry with a controlled load and temperature.
- Report uncertainty: include source and sensor calibration, directivity/dynamic range, load tolerance and parasitics, fixture repeatability, mismatch, connector loss, temperature and stabilization time.
Dan Koellen, AI6XG, demonstrates the accounting problem in his EFHW transformer measurement study: raw S21 through a transformer-plus-load fixture includes transformer, load and mismatch terms, and assuming an ideal high-value resistor can even produce impossible efficiency results.
A Station-Level Test Plan
| Question | Measurement | Useful result |
|---|---|---|
| Is the coax dominant? | Measure or calculate loss for the exact cable and connectors at frequency, then include the actual load mismatch. | Matched and total line loss with reference planes. |
| Is the transformer dominant? | Calibrated/de-embedded transmission and return loss into representative loads; confirm thermally at power. | Loss versus frequency, load, power, duty cycle and temperature. |
| Is common-mode current significant? | Clamp-on RF current sweep along the feed line before and after the proposed choke/counterpoise change. | Current magnitude/distribution and change—not an assumed watt value. |
| Is the antenna dissipative? | Validated model including real ground/conductor loading, or controlled efficiency/field measurement. | Accepted-to-radiated efficiency with uncertainty. |
| Is the path sitting in a null? | Pattern model or controlled multi-angle field comparison using unchanged power and propagation conditions. | Gain by direction and polarization. |
Build the Final Loss Budget
Account separately for the coax, high-ratio transformer, matching network and any common-mode path. Include cable mismatch in the transmission-line calculation, define the common-mode boundary with current measurements, and keep directional pattern nulls separate from dissipated power.
Specify the cable, transformer, load, frequency, power, waveform, installation and reference planes. Then build a non-overlapping dB budget and prioritize the largest measured term. This connects each change to a defined improvement in delivered power, radiation efficiency or realized gain.
Mini-FAQ
- Is coax loss affected by SWR? Yes. A lossy coax has baseline matched attenuation, and mismatch generally adds conductor and dielectric loss by changing the voltage/current distribution. The increase depends on line loss, electrical length and complex load—not SWR alone.
- Does low SWR prove an EFHW transformer is efficient? No. Return loss says how much wave is reflected at one plane. A lossy transformer can show a good input match, so transmission or calorimetric evidence is also required.
- What sets EFHW transformer loss on the upper HF bands? Core material and volume, winding geometry, leakage inductance, parasitic capacitance, compensation, load and construction all matter. Measure the exact design under stated conditions.
- Is common-mode current on the coax automatically wasted power? No. It may radiate, dissipate, alter the intended pattern, couple noise or create EMC and RF-safety problems. Current and field measurements are needed to classify its effect.
- Where should an EFHW common-mode choke go? There is no universal distance. Choose a boundary using the intended antenna model or a feed-line current sweep, use adequate complex choke impedance and thermal margin, then verify the installed current.
- How should coax and transformer loss be compared? Put both in the same calibrated power budget at defined reference planes, frequency, load, power and temperature. Keep matched line loss, extra mismatch-related line loss and transformer dissipation separate.