Feedpoint Geometry for Quarter-Wave and 5/8-Wave Verticals
Feedpoint Geometry for Quarter-Wave and 5/8-Wave Verticals
A vertical feedpoint is the two-terminal boundary between the radiator and its return structure. Its position matters, but moving it upward by a fixed distance does not automatically raise efficiency or lower the elevation pattern.
A small upward feedpoint offset is tempting to read as useful current lifted away from the ground. That shortcut confuses mechanical height, the feed port, the radial plane and the height of the complete RF current distribution. Separate those boundaries and the design decision becomes much more useful.
A Monopole Feedpoint Has Two Terminals
A vertical monopole is not fed at an isolated point on one conductor. One port terminal connects to the radiator. The other connects to the intended return structure: a radial field, conductive vehicle body, metal roof, ground screen or another explicitly modelled counterpoise.
The feedpoint is that two-terminal electrical port. The radial plane is the physical region in which intended return current divides among the radials or conducting surface. They are often close together, but the names do not mean the same thing.
If a coax connector is placed above the radial hub, the conductors between connector and hub do not disappear electromagnetically. A radiator-side lead, support tube, bonding strap, coax exterior or matching assembly can carry RF current, store energy, dissipate power and change the input impedance or pattern. A useful model includes every conductor on both sides of the port.
Keep the Five Planes Separate
| Boundary | What it means | What it does not prove |
|---|---|---|
| Mechanical base | The physical bottom of the support or antenna assembly | Where RF current begins or ends |
| Feedpoint | The two-terminal radiator/return port | That everything below it is RF-inactive |
| Radial plane | The conducting surface or wires carrying the intended return current | An infinite perfect ground or loss-free earth |
| Measurement plane | The calibrated plane at which impedance, S11 or power is stated | The same impedance that exists at a connector reached through coax |
| Local earth and terrain | The lossy near field and far-field reflection environment | A terminal that can replace a deliberate return conductor |
The measurement plane deserves special attention. Coax and a matching network transform complex impedance between the antenna terminal and the radio. Calibrate or de-embed to the named plane before comparing two feed arrangements. An SWR value without its reference plane is incomplete.
The Quarter-Wave Base Current Is Not Wasted
In the ideal thin quarter-wave monopole over a perfect conducting plane, current magnitude is highest near the feedpoint and falls toward zero at the open tip. That high-current base region is an essential part of the radiator. It is not a useless length that can be removed merely because it is close to the radial plane.
Radiation comes from the magnitude and phase of current over the complete conductor, combined with its image or real return environment. A short differential contribution near the plane may contribute less to a chosen far-field direction than an equal current section higher up, but deleting or bypassing it changes the electrical length, port impedance and current everywhere else.
Ground loss is primarily a return-current problem. Current entering lossy soil, poor radial junctions or resistive conductors converts accepted power into heat. Raising the centre connector while leaving the same loss current in the return network does not cure that. Improve the radial or conducting return system and verify where the current flows.
Raising the Whole Antenna Is a Different Experiment
Elevating the radiator together with its radial or counterpoise system changes the height of the entire current distribution. It also changes radial current, earth coupling, feed-line geometry, safety clearances and the ground-reflected field. That can change efficiency and the elevation pattern, but the result depends on electrical height, radial tuning and slope, soil, terrain and nearby structures.
Moving only the feed connector while the radiator top and radial plane remain fixed is not the same experiment. Depending on the conductor arrangement, it may:
- shorten or lengthen the active radiator;
- create a series lead or tapped matching section;
- move the return-current connection away from the radiator terminal;
- put RF current onto a mast, support or coax exterior; or
- change the measured impedance without producing the intended pattern.
No fixed physical offset can represent the same electrical distance on 160 metres, 20 metres and 10 metres. A distance should be stated as part of one declared geometry and frequency range, not promoted as a universal performance improvement.
A 5/8-Wave Is Not a Lifted Quarter-Wave
A base-fed 5/8-wave monopole has a different signed current distribution from a quarter-wave monopole. In the ideal sinusoidal model it contains an internal current node, a strong upper current region and a lower section carrying opposite-phase current. That complete distribution produces its ideal pattern—not the vertical position of its connector alone.
The 5/8-wave input is normally reactive and requires a matching arrangement. The network can make the port acceptable to the feed line, but it does not create directivity. Its loss, voltage, current and thermal behaviour reduce realised performance if they are not controlled.
Over an infinite perfect ground plane, the ideal 5/8-wave can concentrate more radiation near the horizon than an ideal quarter-wave at the same base height. A finite radial field, real soil, matching loss, unequal overall height and surrounding conductors can narrow, move or erase that advantage. Compare installed realised gain at a declared elevation angle and accepted power.
The Low-Elevation Field Belongs to the Complete Site
A useful elevation pattern is the result of the current on every conductor and its interaction with the environment. The controlling variables include:
- radiator electrical length, diameter and taper;
- height of the full current distribution;
- radial count, length, position, resistance and symmetry;
- soil conductivity and permittivity in the near field and reflection region;
- terrain slope and obstructions;
- matching-network and conductor loss;
- mast, guy, fence, roof and other coupled structures; and
- unintended current on the feed-line exterior.
A lower SWR, a connector placed higher or a current maximum that appears higher in a sketch cannot substitute for a pattern or controlled field measurement. Input impedance and radiation pattern are separate model outputs and separate measurements.
The Current VertX Feedpoint Choice
In the current VertX, RF.Guru places the feedpoint as low as mechanically practical. This keeps the intended mechanical layout and current-path boundary explicit. It is not presented as an automatic gain, efficiency or takeoff-angle improvement.
That statement is deliberately narrow. It does not publish a dimension, material, power rating, feed impedance or proprietary construction detail. VertX performance still belongs to the complete installed radiator set, radial field, feed line, matching state, ground and surroundings.
How to Compare Feedpoint Geometries
- Draw both port terminals. Identify the radiator path and the intended return path all the way to the point where common-mode current is controlled.
- Record the complete conductors. Include leads, matching parts, support tubes, mast, radials, coax exterior and nearby metal.
- Declare the comparison. State whether base height, top height, radial height, overall conductor length or current-distribution height is being held constant.
- Calibrate at the antenna terminal. Save complex impedance and S11 at the feedpoint as well as the value seen at the radio.
- Measure return current. Map radial currents and coax-exterior current at repeatable positions. A good match does not prove the intended return boundary.
- Account for loss. Measure or bound the matching network, feed line, conductors and radial system at the installed load and power.
- Compare accepted power. Normalise field or gain results to power accepted at the declared antenna port, not just the transmitter setting.
- Measure the pattern that matters. Use adequate far-field distance, stable polarization, controlled switching, several elevation angles and an uncertainty budget.
- Restore the starting state. An A/B/A sequence helps expose drift caused by weather, soil, connections or the measuring system.
NEC-class modelling is useful for this comparison because it can show the input impedance, conductor currents and pattern separately. Model the physical lower conductors and return structure rather than replacing them with an ideal feed symbol. Then test sensitivity to ground, radial loss, feed-line current and uncertain dimensions.
Mechanical and Safety Boundaries Still Win
A low feedpoint can be easier to inspect, weatherproof and support. An elevated feed system can reduce access to high-current and high-voltage parts, but it can also create exposed radials, trip hazards and greater structural loads. The safer choice depends on the site.
Keep conductors, radials and matching components outside required touch and RF-exposure limits. Maintain statutory clearances from overhead lines and other services. Bond and protect the installation according to the applicable jurisdiction; an RF return path does not replace protective earthing or lightning bonding.
Primary and Authoritative Technical Sources
- ITU-R Recommendation BS.705-2—HF vertical-monopole patterns, radial-wire earth systems and practical environmental effects.
- IEEE Std 149-2021, Recommended Practice for Antenna Measurements—impedance, pattern, gain, efficiency, range and uncertainty practice.
- IEEE Std 145-2025, Definitions of Terms for Antennas—consistent antenna, gain, efficiency and pattern terminology.
- Maley and King, NBS Journal of Research: Monopole Impedance with a Radial-Wire Ground System—feedpoint impedance over radial wires and imperfect ground.
- NIST Technical Note 1347—monopole calibration, reference geometry and measurement boundaries.
- Lawrence Livermore National Laboratory, Numerical Electromagnetic Code v5—wire current, loads, ground, impedance and pattern modelling.
Joeri's Bottom Line
Do not decide a vertical by measuring from its physical base to its connector. Decide it by tracing the complete radiator and return currents. A quarter-wave base-current maximum is not wasted, and a 5/8-wave does not earn its pattern by lifting the feedpoint.
Place the port where the mechanical assembly and intended current boundary make sense. Then verify impedance at the correct plane, control loss and common mode, and measure the installed field. A feedpoint position can be a good design choice without pretending that its height alone creates performance.
Mini-FAQ
- Is current near the base of a quarter-wave vertical wasted? No. The high-current lower section is part of the complete radiator. Return-system loss can be severe near the base, but that is not proof that the radiator section itself contributes nothing.
- Does raising a vertical feedpoint automatically lower the takeoff angle? No. The installed pattern depends on the full current distribution, radial or ground system, electrical height, terrain, matching network and unintended feed-line current.
- Are the feedpoint and radial plane the same thing? They can be physically close, but they are different definitions. The feedpoint is a two-terminal electrical port; the radial plane is the return-current structure connected to one side of that port.
- Is a 5/8-wave vertical just a quarter-wave with its current maximum raised? No. It has a different signed current distribution, including an internal node and an opposite-phase lower section, and it normally requires matching.
- Why is the current VertX feedpoint low? RF.Guru places it as low as mechanically practical to define the mechanical and current-path geometry. That position is not claimed to create automatic gain, efficiency or a lower elevation pattern.
- How should two feedpoint arrangements be compared? Freeze the complete geometry, calibrate at the antenna port, map radiator and return currents, account for every loss, equalise accepted power and compare the installed pattern with stated uncertainty.