How Long Is Too Long for an End-Fed Counterpoise?

This question looks simple until an end-fed wire leaves the drawing and meets a real installation. The transformer has two RF terminals. If one connects to the long wire, current at the other must close through some combination of a counterpoise, radial system, feed-line exterior, mast, equipment, building wiring, nearby metal and displacement current through the surrounding electric field. The system chooses the complete impedance network—not the label we give one wire.

The Antenna Is the Complete RF Circuit

An end-fed system is not a solitary wire with current vanishing at the transformer. It includes the radiator, matching network, return conductor or capacitance, feed line, common-mode path and coupled surroundings. Current can close conductively through metal and capacitively through the electric field; both paths are frequency-dependent.

The matching ratio transforms the impedance presented between its ports. It does not assign which external conductor must carry the return current. That distribution follows the impedance of every available path, including mutual coupling and the common-mode impedance of the installed feed line.

On coax, keep two current systems separate:

• Intended differential mode: current on the centre conductor is paired with equal-and-opposite current on the shield’s inner surface.
• Exterior common mode: current on the shield’s outside closes through the antenna, station and environment as a separate path.

A clamp around the whole coax ideally rejects the equal-and-opposite differential currents and responds to net exterior current. It does not measure delivered differential power. Conversely, a good SWR at the transmitter does not prove that exterior-shield current is small.

“Counterpoise” Can Mean Several Different Jobs

Much of the confusion comes from using one word for different engineering functions:

• A deliberate return conductor: one or more wires connected to the matching-network return terminal.
• A low-loss radial system: conductors arranged to reduce current through lossy soil and to shape the current distribution of a monopole-like system.
• A radiating second arm: a conductor carrying material current and contributing to the installed field pattern.
• A boundary before a choke: a defined section of conductor or feed-line exterior intentionally left on the antenna side of the common-mode impedance.

Those jobs can overlap. A wire can provide return current, radiate and alter loss at the same time. Calling it a counterpoise does not make its radiation disappear.

Why a Fixed Percentage Cannot Be a Law

The familiar suggestion of roughly 5–10% of the lowest-band wavelength can be a first experiment, but it is not a universal design boundary. The same physical wire has a different electrical length on every band. Its installed impedance also changes with height, insulation, bends, proximity to soil and metal, transformer behaviour and coupling to the radiator and feed line.

A free-space wavelength estimate is useful bookkeeping:

At 14 MHz, one quarter of the free-space wavelength is about 5.36 m. That number is not a prediction that any 5.36 m installed wire must resonate or carry a particular current. End effects, conductor insulation, folding, loading, coupling and its termination all move the result.

Likewise, being near a quarter wavelength does not automatically make a conductor bad. It means the conductor may present a relatively low or high impedance at a particular plane, depending on how it is terminated and coupled, and may therefore carry substantial current. That can be deliberate—as with a tuned radial—or unwanted when the wire runs through the shack.

“Too Long” Depends on the Intended Boundary

If the counterpoise is meant to be a radiating part of the antenna, route it as an antenna: keep it away from people, wiring and uncontrolled metalwork, and include it in the model and exposure assessment. If it is intended only to keep current off the feed line, the question is whether the measured current actually transfers into the chosen return system.

A Choke Defines a Boundary; It Does Not Erase the Circuit

A common-mode choke inserts a complex impedance into the exterior-current path while ideally leaving the intended coaxial mode largely unchanged. Its effect depends on placement because it changes the electrical length and impedance of the conductors on each side.

Placing a choke immediately at the matching network can be appropriate when a separate return system is intended. Leaving a deliberate section of feed-line exterior on the antenna side can be appropriate when that section is part of the designed return. A second choke closer to the station may reduce current entering equipment or wiring, but it also creates another bounded section whose length and coupling matter.

Do not qualify a choke by material name, turns count or a single small-signal impedance point. Check complex common-mode impedance over the operating bands, fixture and parasitic effects, expected current, RF voltage, waveform, duty cycle and temperature. The W7EL balun analysis explains the distinct current paths, while the ARRL common-mode current procedure demonstrates installed exterior-current measurement.

Soil, Radials and Symmetry Change the Answer

Ground is not one universal component. Conductivity and relative permittivity vary with material, moisture, frequency and location; Recommendation ITU-R P.527-6 documents that variability. A conductor close to lossy ground can drive displacement and conduction currents whose loss is not visible from SWR alone.

Many on-ground radials can reduce loss by distributing current over more conductor and less soil, but the result depends on radial count, length, layout, frequency and ground. Elevated radials can produce a controlled low-loss return with fewer wires when their lengths, heights and currents are tuned, but they require deliberate symmetry and safe clearance.

Far-field cancellation between radial currents is a symmetry result, not a property of the word radial. Unequal current, sloping conductors, nearby buildings or a single wire can add a directional field component. That may be acceptable; it simply belongs in the installed antenna pattern.

Measure the Path You Are Trying to Control

A VNA establishes complex impedance or reflection at its calibration plane. A current probe establishes current at its probe position. A field-strength receiver or remote station can compare the radiated result only when distance, polarisation, bandwidth, transmitter power and propagation are controlled. WSPR, FT8 and ordinary signal reports can add useful field experience, but propagation and receiver variation prevent a single report from proving efficiency.

Record uncertainty and repeatability. A relative current measurement can be valuable without being traceable to absolute amperes, provided the probe orientation, position, frequency, power and detector response remain the same. For an absolute claim, calibrate the fixture and state its bandwidth, linearity and uncertainty.

SWR Is One Measurement, Not the Verdict

SWR describes reflection at a named reference plane. A lower SWR can result from a better match, but it can also accompany extra conductor, dielectric, ground or matching-network loss. A higher SWR after changing a choke or return path can mean that an unintended radiating path has been removed and the feedpoint impedance has changed.

Evaluate at least three separate outcomes:

• impedance and accepted power at declared planes;
• current on intended and unintended conductors; and
• radiated field or controlled on-air performance.

Add temperature when a transformer, choke or contact may dissipate significant power. Match quality alone establishes none of those other quantities.

Include Every Accessible Conductor in the Safety Assessment

Any conductor carrying RF current can produce electric and magnetic fields or an accessible contact voltage. The radiator, counterpoise, radials and any feed-line section carrying exterior current therefore belong inside the installation’s exposure and contact-current assessment.

The ICNIRP 2020 RF guidelines cover 100 kHz to 300 GHz and distinguish whole-body, local and contact-current considerations across frequency. Compliance cannot be inferred from wire length or a generic far-field calculator when people can approach conductors in the reactive near field. Use the rules applicable to your jurisdiction, declared power and duty cycle, and keep people outside the assessed boundary.

If touching equipment or conductors produces an RF sensation, if a connector arcs, or if insulation or ferrite heats unexpectedly, stop transmitting and diagnose the current path. Those are fault indicators, not measurement techniques.

A Practical Commissioning Sequence

• Choose whether the return conductor, feed-line exterior or radial system is intended to be part of the radiating structure.
• Draw the complete common-mode path, including mast, equipment, protective bonding, wiring and nearby conductors.
• Install the return system and common-mode boundary that match that architecture.
• Measure impedance at a declared plane and exterior current at marked locations on every operating band.
• Check component temperature and accessible RF voltage at the intended power, waveform and duty cycle.
• Compare the radiated result with a repeatable field or A/B/A method.
• Change only one length, route or boundary at a time and keep the configuration that improves the quantities you actually care about.

Engineering References

• ARRL: Common-Mode Current and Common-Mode Chokes
• W7EL: Baluns, What They Do and How They Do It
• Recommendation ITU-R P.527-6: Electrical Characteristics of the Surface of the Earth
• ICNIRP: Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz)

Questions, antenna-factor records or height trials to share? Contact RF.Guru.