Regional HF Antennas: Pattern, Height and Short-Skip Paths
Regional HF Antennas: Pattern, Height and Short-Skip Paths
DX is only one use of HF. Nets, field operations and nearby stations often need energy at medium or high elevation angles—the part of the pattern a low-angle DX installation may deliberately suppress.
“Local,” “regional,” “short skip” and NVIS are not interchangeable distance labels. Start with the path, the usable frequency and the required arrival angle. Then choose an antenna height and geometry that put useful energy there.
Separate the Paths Before Choosing the Antenna
| Path | Dominant question | Antenna implication |
|---|---|---|
| Line of sight | Are both stations within radio horizon with adequate clearance? | Polarisation alignment and obstruction often dominate |
| Ground wave | How do frequency, ground conductivity and terrain affect attenuation? | Low-angle vertically polarised fields may be useful |
| Oblique skywave | Which elevation angles return the signal at the wanted distance? | Match the elevation pattern to the ionospheric path |
| NVIS | Is the operating frequency below the usable near-vertical limit? | Strong high-angle radiation and low local loss are important |
The distance at which one mechanism overtakes another is not fixed. Frequency, ionosphere, terrain, ground conductivity, time, season and solar conditions all move the boundary.
Why a Horizontal Antenna Often Fits Regional HF
A horizontal dipole at a modest height in wavelengths can produce substantial radiation at high elevation angles. Lowering it generally pushes more energy upward, but moving it extremely close to lossy ground can reduce efficiency. There is therefore an engineering optimum, not a command to hang every dipole as low as possible.
Ben Witvliet's measured and modelled NVIS research found transmit-height optima around 0.18–0.22 wavelength for many of the soil cases studied, with a less pronounced receive optimum around 0.16 wavelength. Those are valuable experimental results, not universal installation dimensions. Soil, antenna type, desired angular coverage and local noise still matter.
Flat Dipole, Inverted V and V Dipole
Bending a dipole changes its feedpoint impedance, polarisation mix and azimuth pattern. An inverted V may need only one tall support and often fills the deep broadside/null contrast of a straight dipole, but it is not perfectly omnidirectional. The included angle that produces a convenient impedance is installation-dependent; 120 degrees is not a universal 50 Ω recipe.
A V or inverted V can be a practical regional antenna when its height and orientation support the required elevation angles. The name of the geometry does not guarantee coverage.
NVIS Is a Propagation Condition
Near Vertical Incidence Skywave propagation uses steep upward radiation that the ionosphere returns to Earth. Whether it exists at a particular frequency depends on the current ionosphere. A good high-angle antenna cannot make a frequency above the usable near-vertical limit return from the F region.
On European paths, 80 m is often useful at night and 40 m often during daylight or stronger ionospheric conditions, but that is a planning tendency—not a timetable. Current ionograms, critical-frequency observations and on-air tests are better guides than a static band rule.
“Omnidirectional” Needs a Tolerance
No finite installed antenna has perfectly equal response in every azimuth and elevation direction. A low horizontal antenna can produce broad regional coverage, while an inverted V can reduce some azimuth variation. Supports, sloping ground, feedline current and nearby conductors still create asymmetry.
State the useful coverage requirement instead: for example, “no azimuth null deeper than the link margin can tolerate over the required high-angle sector.” That can be modelled or measured.
A Better Regional-HF Workflow
- Define the service area. Mark minimum and maximum range, terrain and required availability.
- Select usable frequencies. Use current ionospheric information and allow for day/night changes.
- Estimate path angles. Do not choose antenna height from distance alone.
- Model height in wavelengths. Compare the full elevation pattern over realistic ground.
- Control the feedline. Prevent unintended current from reshaping the pattern.
- Measure signal and noise. Transmit field strength and receive signal-to-noise ratio answer different questions.
- Repeat over time. Propagation variability can overwhelm a single comparison.
The practical lesson remains simple: do not ask a low-angle DX antenna to solve every regional path. Put the energy at the angles the link needs, on a frequency the ionosphere can return.
Primary and authoritative references
- Ben A. Witvliet — Near Vertical Incidence Skywave: Interaction of Antenna and Propagation Mechanism
- ITU-R M.1795 — characteristics of HF systems using NVIS
- U.S. Army ATP 6-02.53 — high-frequency radio communication
- IEEE 145-2025 — definitions of terms for antennas
Mini-FAQ
- Is NVIS always available on 80 or 40 m? No. The usable frequency changes with ionospheric conditions, location, season and time.
- Is an inverted V omnidirectional? Not perfectly. It can broaden azimuth coverage, but height, angle and surroundings shape the installed pattern.
- Is there one correct NVIS dipole height? No. Measured research provides useful ranges, while soil, geometry and the required angle determine the local optimum.
- Why can a DX vertical miss regional stations? A pattern concentrated at low elevation angles may place less energy into the steep paths needed for regional skywave coverage.