Raised Vertical Height for Upper-HF DX: Model the Complete Installation
Raised Vertical Height for Upper-HF DX: Model the Complete Installation
How high should the feedpoint be on 20–10 m? There is no useful one-number answer. Radiator length, base height, return geometry, ground, matching and the installed elevation pattern have to be evaluated together.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
The practical question is not “What fraction of a wavelength should I raise every vertical?” It is “Which complete installation gives useful gain over the elevation angles and azimuths I need, without unacceptable loss, stress, common-mode current or safety problems?” That keeps height in its proper place: one design variable among several.
Short answer: start with a base height that makes the return system safe, symmetrical and buildable. Then model several nearby heights with the actual radiator, radials, ground and feed structure. Pick from the complete pattern and loss budget, and verify the installed result. Do not start from a universal “DX height.”
Keep Six Variables Separate
Several dimensions are often compressed into the word height. Record each one explicitly:
- Radiator electrical length: approximately one quarter, one half or five eighths of a wavelength is a current-distribution choice, not a mounting-height instruction.
- Base or feedpoint height: the vertical distance from the local ground reference to the radiator’s lower terminal.
- Return-system geometry: radial number, length, height, slope, azimuth and the currents they actually carry.
- Ground and site: soil conductivity and permittivity, moisture, slope, terrain, roofs, masts, cables, fences and vegetation.
- Feed and match: feedpoint impedance, matching-network loss and stress, feed-line attenuation, exterior-coax current and choke placement.
- Installed pattern: gain versus both elevation and azimuth over the angles relevant to the intended paths—not merely the angle of one calculated peak.
ITU-R BS.705-2 treats HF antenna characteristics as functions of antenna geometry, ground and practical surroundings. ITU-R P.527-6 likewise describes the Earth through frequency-dependent electrical properties rather than a single generic “average ground.” A height result from one soil and site is therefore not a transferable specification.
Quarter-, Half- and Five-Eighth-Wave Verticals Solve Different Problems
| Radiator family | What changes electrically | What must be supplied | What decides the installed result |
|---|---|---|---|
| Approximately λ/4 monopole | A current maximum lies near the base in the idealized case | A defined return or radial system and any needed impedance transformation | Return loss, radial-current balance, ground, base height, feed structure and surroundings |
| Approximately λ/2 end-fed vertical | The end terminal is near a current minimum and voltage maximum | A high-ratio or tuned match plus a complete, controlled RF return path | Feed position, end effects, matching loss and voltage, counterpoise or exterior-current boundary, height and site |
| Approximately 5λ/8 monopole | The longer current distribution can reshape the elevation pattern | A return system and usually reactance compensation or matching | Base height, radial geometry, ground, the appearance of additional lobes, match loss and stress |
A five-eighth-wave monopole can provide more gain at low elevation than a quarter-wave monopole in a declared low-base model. It does not win by a fixed number of decibels and does not keep the same advantage when base height, ground or radial geometry changes. Likewise, a half-wave end-fed vertical does not automatically occupy a predictable position between the other two. Its pattern and efficiency belong to the complete installed two-terminal system.
L. B. Cebik’s upper-HF monopole model study is a useful bounded example. It compares quarter-wave and five-eighth-wave monopoles with four quarter-wave radials at 14.15 and 28.5 MHz over several soils and base heights. In those models, the five-eighth-wave advantage at the lowest lobe was generally modest, and raising the base could make a higher lobe the strongest one. Those results explain why height must be swept in the actual model; they are not a 20–10 m height table.
“Takeoff Angle” Is Not an Antenna-Family Constant
The elevation of maximum gain is one coordinate on one pattern. For DX planning, compare gain across a useful elevation-angle range and in the required azimuths. A design with a slightly stronger peak can still be worse over most of the angles the paths use, and a high secondary lobe can make the maximum-angle statistic misleading.
Useful ionospheric path angles vary with frequency, distance, ionospheric state, time and route. ITU-R P.533-14 predicts HF circuit performance from the path and operating conditions; it does not assign one universal DX angle to a band. Decide the path-angle region first, then evaluate the antenna over that region.
Dimensionless height: h/λ = h f/c
Stating a base height in wavelengths is useful for comparing bands, but it does not make one fraction optimal. The current distribution, return system, soil, terrain and nearby structures still have to match the model.
The Radials Are Part of the Antenna
A raised monopole and its elevated radials form one radiating and returning structure. Radial current balance matters. A sparse elevated system can work efficiently when its geometry is controlled, but one fixed radial count, length or slope is not a universal recipe.
Rudy Severns, N6LF, reported a careful 7.2 MHz field experiment in which four elevated, approximately quarter-wave radials at a 48-inch base height produced nearly the same measured transmission result as 64 surface radials in that installation. The experiment also measured feed impedance and radial-current division and examined asymmetry. It supports testing a small controlled elevated system; it does not prove that four radials of any length, slope or height work identically on 20–10 m.
For every radial candidate, declare:
- wire length, height profile, slope and azimuth;
- soil model, nearby conductors and feed-line route;
- feedpoint resistance and reactance at a named reference plane;
- current magnitude and phase in each radial; and
- installed pattern, loss and accessible-conductor safety.
Use enough mechanical clearance that elevated wires cannot be contacted, tripped over or pulled into reach. That is a site-safety requirement, not an RF-performance number.
An End-Fed Half-Wave Still Needs a Return Path
High terminal impedance does not remove the second terminal. Displacement current and conduction current complete a loop through some combination of deliberate counterpoise, matching structure, coax exterior, mast, station wiring and surrounding capacitance. If that path is not defined, the installation defines it for you.
Do not prescribe a fixed number of short spokes, a single counterpoise length or one choke location from the label “EFHW.” Model the intended return conductor and the feed-line exterior. Then measure exterior current along the cable and mast. Choose choke location from the boundary you intend to create and choose its complex impedance from the bands and current at that location.
A choke’s impedance is complex and frequency dependent. Fair-Rite’s impedance-measurement note shows why fixture calibration and the real and imaginary parts of impedance matter. At transmit power, also verify voltage, real-power dissipation and temperature in the completed winding. A low-power impedance plot alone is not a power rating.
Vegetation Needs a Boundary, Not a Magic Clearance
Keep foliage from physically touching the radiator, elevated radials, feedpoint and high-voltage hardware. Contact can move in wind, trap water, abrade insulation and create an unsafe or unstable condition. Beyond that physical boundary, there is no defensible universal 30 cm, 50 cm or “two metres from every tree” RF rule.
Vegetation coupling depends on frequency, geometry, species, density and water content. Soil properties also change with composition, moisture and temperature. ITU-R P.833-10 documents the variability of vegetation attenuation, while P.527-6 describes variable Earth-surface electrical characteristics. Neither recommendation supplies a universal near-field clearance for an amateur vertical.
Model major trunks, wet foliage zones, supports and nearby conductors when they are close enough to matter. More importantly, record the installed impedance, exterior current and relative field response in representative dry, wet and seasonal states. If a result moves, the measurement identifies the real sensitivity better than a generic clearance table.
Choose Height With a Reproducible Workflow
- Define the job. List bands, operating segments, target paths and azimuths, likely path-angle ranges, power, duty cycle and mechanical limits.
- Survey the site. Record ground slope and electrical model, available base heights, radial corridors, vegetation, roof and mast geometry, cables, fences and accessible areas.
- Build complete candidate models. Include the actual quarter-, half- or five-eighth-wave radiator, return conductors, feed-line exterior, matcher, mast and major nearby conductors.
- Sweep base height. Compare several buildable heights on every band. Plot the full azimuth/elevation pattern and gain over the useful angular region, not only the strongest lobe.
- Audit loss and stress. Calculate ground/return loss, feed-line and match loss, feedpoint impedance, standing voltage and current, choke dissipation and component margins.
- Select a safe geometry. Treat elevated wires, high-voltage terminals, supports, wind loading, access, RF exposure, protective bonding and lightning protection as separate requirements.
- Measure de-energized. With transmission inhibited, measure impedance at a named reference plane and map common-mode current using a calibrated method. Never connect an analyzer to an energized antenna.
- Verify the installed pattern. Use controlled relative-field or rapid A/B measurements with unchanged reference power, geometry and instrumentation. Record calibration, repeatability and uncertainty.
- Repeat under real conditions. Recheck after changing radial height, feed-line route or matcher, and after representative wet, dry and seasonal changes.
IEEE 149-2021 treats antenna measurement as a facility, instrumentation, calibration and uncertainty problem. A field comparison needs the same discipline: name the measurand and reference plane, hold the rest of the system constant, and report uncertainty before assigning a small gain difference.
Engineering Decision Table
| If the constraint is… | Investigate first | Do not assume | Verification that decides |
|---|---|---|---|
| Minimum structure height | Quarter-wave radiator with a controlled return system | That shorter always means less efficient | Installed loss, impedance, pattern and required angular coverage |
| Potentially stronger low-elevation gain | Five-eighth-wave cases at several buildable base heights | That 5λ/8 always wins or retains one lobe | Full elevation pattern, match loss and radial-system loss |
| End-fed mechanical convenience | Half-wave radiator with an explicit matching and return structure | That high feed impedance eliminates the return current | Matcher loss/stress, exterior-current map and installed pattern |
| Few elevated radials | Symmetrical geometry and measured current division | That four wires reproduce every published experiment | Radial-current balance, loss, impedance and azimuth pattern |
| Vegetation close to the base | Safe physical separation plus wet/dry measurements | That one clearance distance prevents detuning or loss | Seasonal impedance, current and relative-field stability |
| Roof or mast installation | Complete model including structure and cable route | That more base height always lowers the useful lobe | Pattern over the target angles, common mode and site safety |
Engineering References
- ITU-R BS.705-2: HF Transmitting and Receiving Antenna Characteristics and Diagrams
- ITU-R P.527-6: Electrical Characteristics of the Surface of the Earth
- ITU-R P.533-14: Method for the Prediction of the Performance of HF Circuits
- ITU-R P.833-10: Attenuation in Vegetation
- Rudy Severns, N6LF: Experimental Determination of Ground System Performance for HF Verticals, Part 3
- L. B. Cebik, W4RNL: The 5/8-Wavelength Mystique, Part 3
- IEEE 149-2021: Recommended Practice for Antenna Measurements
- Fair-Rite: Notes on Impedance Measurement
Selection rule: choose the lowest-complexity installation that produces adequate gain over the required angles with verified return-system loss, matching loss, common-mode control, stress margin and safety. Base height is the result of that comparison—not the starting verdict.
Mini-FAQ
- Is there one ideal base height for a 20–10 m DX vertical? No. Sweep buildable heights in a complete model containing the radiator, return system, ground, feed structure and surroundings, then compare gain over the path-angle range and verify the installation.
- Does a five-eighth-wave vertical always beat a quarter-wave vertical? No. A 5λ/8 monopole can have a modest low-elevation advantage in a declared geometry, but base height, ground, radials, matching and additional lobes can reduce or redirect it.
- Is a half-wave end-fed vertical independent of radials or a counterpoise? No. Its terminal current is small, not zero. The complete RF current path can include a deliberate return conductor, matching structure, coax exterior, mast and surrounding capacitance.
- Are four elevated radials always enough? No. Four worked well in a controlled 7.2 MHz N6LF experiment, but radial length, height, current balance, ground and nearby conductors determine whether another installation behaves similarly.
- How far must a vertical be from bushes and trees? Prevent contact and provide the mechanical, electrical and RF-safety clearances required by the installation. RF detuning and loss have no universal distance; verify them in representative wet, dry and seasonal conditions.
- Where should the feed-line choke go? At a boundary justified by the intended current path and installed current measurements. Verify its complex impedance, dissipation, temperature and voltage for every operating band and power condition.