Ground-Mounted Vertical Lengths: Quarter-Wave, Half-Wave and Five-Eighths-Wave
Ground-Mounted Vertical Lengths: Quarter-Wave, Half-Wave and Five-Eighths-Wave
Put three vertical radiators near the ground and the brochure labels stop being enough. Their current distributions, feed impedances, return systems and elevation patterns differ—and the installed ground can dominate the result.
A five-eighths-wave radiator is often sold as an automatic upgrade from a quarter-wave. A half-wave vertical is sometimes placed in the same ranking. That comparison is incomplete: the three are not the same antenna made progressively longer. They present different current and voltage distributions, require different feed and return arrangements, and can produce different patterns.
Joeri’s practical point: raising more of the current-rich conductor can help, but radiator length alone does not establish performance. Compare complete installed systems at equal accepted power—including radials or counterpoise, matching loss, coax-exterior current, soil and the directions that matter.
The Three Labels Hide Different Circuits
A ground-mounted quarter-wave is normally a monopole. Its image in an ideal conducting plane completes the familiar half-wave current solution, while the real installation depends on a radial or counterpoise system and lossy earth. Current is high near the feedpoint and tapers toward the open top.
A half-wave vertical can be a centre-fed vertical dipole, an off-centre-fed wire or an end-fed half-wave. Those are different feed circuits. The centre-fed version has two intentional arms; the end-fed version operates near a high-voltage, low-current point and still needs a second current path. Saying only “half-wave vertical” does not identify its return or common-mode boundary.
A five-eighths-wave radiator supports a longer standing-wave distribution. Its feedpoint usually needs a reactive matching arrangement, and its free-space or ideal-ground pattern cannot simply be pasted onto a short real radial field. Depending on geometry and frequency, extra electrical length can lower the main lobe, create a high-angle lobe, or do both.
| Radiator description | Return that must be declared | What the length does not prove |
|---|---|---|
| Quarter-wave monopole | Ground plane, elevated radials, on-ground radials or another explicit counterpoise | Low ground loss, low-angle gain or low coax-exterior current |
| Half-wave vertical | Opposite dipole arm, deliberate short branch or another defined end-fed return | Balance, easy matching, freedom from common mode or superior efficiency |
| Five-eighths-wave vertical | Ground or counterpoise system plus the matching-network reference conductor | A universal gain advantage, one feed impedance or one elevation pattern |
Current Distribution Matters More Than Overall Length
Radiation comes from the complete time-varying current distribution, including magnitude and phase. A current-rich section does not radiate independently of the rest of the conductor; fields from all current elements combine. Bends, loading, feed geometry, return conductors, ground and nearby structures reshape that solution.
Keeping the feedpoint at ground level does not keep every current maximum at ground level. A half-wave or five-eighths-wave radiator extends farther upward, and its current-rich regions can sit higher than those of a quarter-wave. That can change ground coupling and pattern. It still does not guarantee a better wanted signal, because the phase distribution and return loss change at the same time.
The useful question is therefore not “Which one is longest?” It is “Where does current flow in this complete installation, with what phase, and what field results in the required elevation and azimuth directions?”
Ground Loss Is a Return-Current Problem
Loss near a ground-mounted vertical is not simply radiation having to pass through a lossy blanket of soil. The ground and radial system participate in the source circuit and in the near field. Current flowing through resistive earth and imperfect connections dissipates power; the surrounding ground also changes the reflected field that forms the elevation pattern.
Rudy Severns, N6LF, demonstrated why the installation must be measured. His experimental summary of HF vertical ground systems compares actual signal and impedance changes as radial systems change. The result is not one universal radial count: soil, radial height, length, number, symmetry and test geometry all matter.
A half-wave end-fed vertical may need much less conventional ground-current return than a quarter-wave monopole, but it does not work without a second source terminal. If the coax exterior or station wiring supplies that terminal, the installation can acquire an unintended radiator and noise path. A defined counterpoise and measured choke boundary can make that current path deliberate.
Near Ground Does Not Make the Patterns Equal
Low mounting and poor ground can erase an expected advantage, but they do not force quarter-, half- and five-eighths-wave systems to the same pattern. Their current distributions remain different, and so do their interactions with the return system and ground reflection.
ITU-R BS.705-2 treats vertical-monopole patterns with explicit earth systems and separates theoretical diagrams from practical variations caused by topography, conductivity and surrounding structures. A statement such as “mostly NVIS” or “always low angle” needs an operating frequency, complete geometry, ground model and polar plot.
A vertical radiator also does not become a horizontal near-vertical-incidence antenna merely because it is low. NVIS suitability depends on the elevation field, polarization after propagation, frequency relative to ionospheric conditions and the required path. The installed elevation pattern must answer that question.
Matching Can Conceal the Comparison
Each length can present a very different R + jX at its feed terminals. A loading or matching network can make the transmitter see an acceptable impedance, but it cannot make the three current distributions equivalent. Its own conductor, dielectric and magnetic losses must be included in the accepted-power budget.
Measure at a declared reference plane. Record complex impedance at the radiator terminals, then characterise the matching network and feedline separately. A low SWR at the radio does not distinguish radiated power from loss in soil, radials, coils, transformers, feedline or unintended common-mode paths.
Model Every Conductor That Carries Current
The Numerical Electromagnetics Code can model wires, surfaces, homogeneous ground, networks and transmission lines and report current, near fields and radiation patterns. A useful model includes the radial or counterpoise system, feed structure, mast and any coax-exterior section that materially carries current.
Run the three radiators over the same declared ground, but do not force them to share an unrealistic feed or return. Compare realised gain or field at equal accepted power, not source voltage. Sweep frequency and inspect the full elevation pattern; a single peak-gain number can hide an unwanted lobe.
A Fair Field Comparison
- Fix the objective. Name the band, directions, elevation region, bandwidth and power envelope that matter.
- Document each complete circuit. Record radiator, radials or counterpoise, matching network, choke, feedline, mast and bonds.
- Measure the terminal load. Save R + jX at the same calibrated plane across the intended bandwidth.
- Account for loss. Measure or bound matching, feedline, conductor, connection and ground-system loss under representative power.
- Map exterior current. Probe the coax, mast and station cables before and after the intended common-mode boundary.
- Compare the wanted result. Use calibrated field data or rapid A/B/A tests at equal accepted power, with propagation and uncertainty logged.
Joeri’s Bottom Line
A ground-mounted quarter-wave, half-wave and five-eighths-wave vertical do not automatically perform the same—and the longer radiator does not automatically win. Low height, loss and an inadequate return system can compress the difference between them, while current phase and geometry can create genuinely different patterns.
Raise the useful current when the site allows it, but measure the complete current path. The best choice is the installation that puts accepted power into the wanted field, with controlled return current, acceptable bandwidth and mechanical survival—not the one with the longest label.
Mini-FAQ
- Do all three vertical lengths perform the same near ground? No. Loss can reduce an expected advantage, but their current distributions, feed circuits and patterns remain different.
- Is a five-eighths-wave vertical always better than a quarter-wave? No. Its result depends on matching loss, return system, current phase, ground and whether its elevation pattern serves the required path.
- Does a half-wave vertical need no return path? It still needs a second source terminal. That may be an opposite arm, deliberate counterpoise, defined coax-exterior section or another measured branch.
- Does low SWR identify the most efficient vertical? No. SWR at one plane does not separate radiation from matching, feedline, conductor, connection or ground loss.
- Does a low vertical automatically produce NVIS? No. NVIS suitability depends on the installed elevation pattern, operating frequency, ionosphere and path—not feedpoint height alone.
- How should the three systems be compared? Hold frequency, site and accepted power constant; document each complete return system; then compare loss, current maps and calibrated field or rapid A/B/A results.