Why a Low EFHW Can “Look Good” on SWR

Put an end-fed half-wave close to earth, wet vegetation, a fence or a building and it may still present a tidy SWR curve. That trace is real—but it belongs to the complete installed network seen through the transformer and feed line. It cannot, by itself, identify radiation efficiency, pattern, ground loss or outside-coax current.

Start With the Measurement Plane

For a line of reference impedance Z₀, the complex reflection coefficient at its measurement plane is:

SWR retains only the magnitude of Γ. It does not retain impedance phase, locate the mismatch or describe anything after the declared plane. A shack-end sweep can include coax attenuation, electrical-length transformation, connectors, a choke, the EFHW transformer, the radiator and every exterior-current path coupled to that system.

Lossy feed line makes the round-trip reflection smaller before it returns to the instrument. A shack trace may therefore look better than the reflection at the antenna end. Calibrating at the feedpoint, or separately measuring and de-embedding the feed line within the instrument’s limits, changes the question from “what does the shack see?” to “what enters the matching assembly?”

Keysight’s field cable-and-antenna measurement guidance treats return loss, VSWR and insertion loss as separate measurements and explains why cable loss and the calibration plane matter. Its network-analysis fundamentals define Γ and SWR at that plane.

End Feeding Produces High—but Not Fixed—Impedance

A resonant half-wave wire has a current minimum and voltage maximum near an open end. Feeding close to that end can therefore present a high terminal impedance, commonly of kilohm order. The exact value is not an EFHW constant. It changes with:

• wire length, diameter, insulation and the exact feed tap;
• height, slope, bend and orientation;
• soil conductivity and permittivity;
• capacitance to trees, roofs, gutters, fences and people;
• the return path through a counterpoise, coax exterior, equipment, bonding and displacement current; and
• frequency and harmonic current distribution.

An ideal 7:1 turns ratio gives a 49:1 impedance ratio, so it maps 2450 Ω to 50 Ω in the ideal declared case. That arithmetic is a design target, not evidence that every installed EFHW presents 2450 Ω or that 49:1 is always the best ratio. Measure the complex impedance at the antenna-side plane under the intended installation conditions.

Lawrence Livermore National Laboratory’s NEC models wire antennas with ground and nearby-structure effects. A useful model records the exact geometry, feed, conductor and ground parameters and is checked against an installed impedance or current measurement; “one metre high” alone is not a complete model.

Height Is a Coordinate, Not a Pass–Fail Limit

Height changes image-current interaction, ground loss, terminal impedance, current distribution and elevation pattern. The effect depends on the entire wire geometry and ground model, not just matchbox height. The often-quoted 0.1λ and 6 m values are useful only after they are normalized to frequency:

Nothing discontinuous happens at 0.1λ, and 6 m is not the same electrical height on 40, 20 and 10 metres. Use h/λ as one input to a geometry-and-ground study, then verify impedance, outside-coax current, field or gain evidence and the pattern required for the intended path. A low installation may be useful for a portable or high-angle objective; it simply cannot inherit an efficiency or DX verdict from height alone.

Nearby Objects Alter Both Reactance and Loss

At a high-voltage end, even small capacitance to the surroundings can materially change terminal reactance and current distribution. For a declared 10 pF capacitance:

That arithmetic does not claim that a particular end has 10 pF or that all coupled power is lost. An ideal capacitance stores and returns energy. Real soil, vegetation, building materials and conductive structures add frequency-dependent resistance, dielectric loss and alternate current paths. Some nearby conductors can also reradiate. Only a declared electromagnetic model or measurement can divide the changed input resistance into radiation and loss.

A good-looking resistive input can therefore correspond to several very different systems: an efficient radiator, a lossy ground-coupled radiator, a transformer with appreciable loss, a feed line that attenuates the reflection, or a larger antenna system that includes the coax exterior and station wiring. Added loss does not always lower SWR; it moves the complex input according to where and how that loss enters the network.

The Shunt Capacitor Changes the Network, Not the Evidence Standard

A capacitor placed across the transformer primary is part of the complete matching network. Depending on transformer magnetizing inductance, leakage inductance, interwinding capacitance, load impedance and frequency, it can compensate reactance and change the location or depth of an SWR minimum. Its useful value is specific to the completed assembly and load.

The capacitor does not, by itself, create real-power loss or prove that loss has occurred. Its finite equivalent series resistance and dielectric loss can dissipate some power, while the transformer core and windings have their own loss mechanisms. The only defensible result comes from measuring the completed network with representative complex loads, declared fixtures and reference planes—not from the capacitor value or the depth of one S11 dip.

The Fair-Rite technical catalogue shows the broadband-transformer equivalent circuit and separates the low-, mid- and high-frequency limits created by magnetizing impedance, winding/core loss and parasitics. It also makes clear that ferrite data from a simple winding are not a finished transformer’s insertion-loss or power rating.

The Return Path Decides What the Antenna System Includes

Current cannot stop at a one-terminal matchbox. The RF circuit closes through some combination of a dedicated counterpoise, the outside surface of the coax shield, equipment and bonding conductors, nearby structures and displacement current to the environment.

A common-mode choke adds complex impedance to one exterior-current path. Its effect depends on frequency, placement and the impedance of all competing paths. Put a choke at the feedpoint with no adequate local return and the antenna may find another return path, change impedance or place high RF voltage across the choke. Move it along the cable and both the exterior-current distribution and measured SWR may change.

A counterpoise is equally conditional. Its length, route, height and coupling determine its impedance and current; it is not automatically an RF ground and should not be confused with protective earth or lightning bonding. ITU-T K.37 treats common-mode conversion, cable screening and choke effectiveness as properties of the installed system rather than universal component prescriptions.

Build a Power Ledger Instead of Reading Efficiency From SWR

At a declared, calibrated plane, the fraction of incident power not reflected is:

That accepted power can still be divided among feed-line loss, transformer and capacitor loss, conductor loss, ground or nearby-object loss, common-mode paths and radiation. At the antenna terminal, radiation efficiency is radiated power divided by accepted power. Total efficiency combines radiation efficiency with the mismatch factor multiplicatively. Those definitions require the same boundary and reference plane throughout.

NIST’s antenna-efficiency work separates radiation efficiency from total efficiency and includes measurement uncertainty. It is a useful reminder that SWR is only one term in a radiated-power result.

A Measurement Plan That Can Separate the Mechanisms

• Declare the boundary. Sketch radiator, transformer, capacitor, feed line, choke, counterpoise, mast, bonding and station wiring. State which parts count as the antenna system.
• Measure at more than one plane. Calibrate at the shack and, safely with the transmitter disconnected, at the matchbox input. Measure or de-embed the feed line and adapters within the VNA’s fixture limits.
• Record complex impedance. Save R, X and Γ, not only SWR. Repeat after height or surroundings change and keep the same calibration and cable routing.
• Map exterior current. Use a characterized clamp-current probe at several positions and frequencies. A single current minimum cannot describe the whole coax.
• Characterize the matching assembly. Measure insertion loss with representative loads and fixtures; at transmit power, record temperature with a suitable non-contact or instrumented method, declared duty cycle and cooling.
• Model the installed geometry. Enter height, shape, conductor, ground and nearby conductors. Compare the model with measured terminal impedance or current before trusting its loss or pattern result.
• Use a controlled radiated comparison. Prefer simultaneous or rapidly switched measurements against a stable reference antenna, fixed receiver settings and repeated paths/times. Report uncertainty.
• Change one variable at a time. Height, choke position, counterpoise, transformer and coax length can each change several coupled quantities. An A/B/A sequence helps expose drift but still needs current, power or field evidence.

Keep the High-Voltage End Inaccessible

An end-fed voltage maximum, transformer secondary and compensation capacitor can carry substantial RF voltage. Do not touch or manually “feel” any component during or immediately after transmission. De-energize, disconnect and verify the system before moving wire, chokes or fixtures. Keep people and conductive objects outside the installation’s evaluated electrical-clearance and RF-exposure boundary, and account for maximum power, duty cycle, frequency and every conductor that becomes part of the radiating system.

ITU-T K.52 (08/2024) bases exposure assessment on accessibility, antenna properties and emitter power. Height is therefore a safety input, not a universal substitute for an exposure assessment or local installation rules.

Primary Technical References

• Keysight: Network Analysis Fundamentals—Reflection Coefficient and SWR
• Keysight: Precise Cable and Antenna Measurements in the Field
• Lawrence Livermore National Laboratory: Numerical Electromagnetics Code
• Fair-Rite: Ferrites in Broadband Transformers
• ITU-T K.37: Cable Screening, Mode Conversion and EMC Mitigation
• NIST: Radiation and Total Efficiency of Antennas
• ITU-T K.52 (08/2024): RF-EMF Exposure Compliance Guidance

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