Wave Propagation

Part of Radio

Wave propagation describes how radio signals travel from transmitter to receiver — through the ground, atmosphere, and ionosphere — and why some frequencies cover different distances under different conditions.

Why This Matters

Understanding wave propagation is the difference between a radio operator who wonders why contacts work sometimes and fail others, and one who predicts with confidence which frequencies will reach which destinations at what times of day and season. Propagation knowledge turns the unpredictable seeming behavior of the HF radio spectrum into a manageable engineering problem.

For a rebuilding civilization, propagation knowledge enables efficient use of scarce spectrum and transmitter power. Choosing the right frequency for the right path at the right time can mean the difference between reliable communication and complete failure. A 10-watt transmitter on the right frequency at the right time may reach thousands of kilometers; the same transmitter on the wrong frequency may fail to cover 100 kilometers. This factor-of-100 difference in effective range is not mysterious — it is predictable from propagation physics.

Propagation also reveals the natural communication infrastructure available without any construction: the ionosphere, which exists and operates regardless of whether humanity uses it, provides free long-distance relay service for the right HF frequencies. Learning to work with this natural infrastructure is a foundational skill for any communication system planner.

Ground Wave Propagation

At frequencies below about 2 MHz, the primary propagation mode is ground wave — the signal follows the Earth’s curved surface by diffraction. The ground acts as one conductor of a waveguide, and the wave creeps forward along it.

Ground wave range depends strongly on frequency and ground conductivity. Lower frequencies (longer wavelengths) diffract better around the Earth’s curvature and are less absorbed by the ground. Higher frequencies diffract less and lose more energy to ground absorption. Salt water is an excellent conductor (σ ≈ 4 S/m) and supports efficient ground wave propagation; dry desert soil is very poor (σ ≈ 0.001 S/m) and limits range severely.

Approximate ground wave ranges for 100W transmitters:

• 500 kHz over seawater: 500–800 km
• 500 kHz over average soil: 200–300 km
• 1.5 MHz over seawater: 200–400 km
• 1.5 MHz over average soil: 100–200 km
• 3.5 MHz over average soil: 50–100 km
• 7 MHz over average soil: 20–50 km

For a coastal community with ships at sea, AM frequencies (500–1500 kHz) provide reliable ground wave coverage across the near-coastal waters. Inland, HF ground wave at 3.5–7 MHz covers the local region reliably regardless of ionospheric conditions — day, night, through storms, at any season.

Sky Wave Propagation

At HF frequencies (3–30 MHz), the ionosphere — layers of ionized gas at 60–400 km altitude — can bend radio waves back toward Earth. A signal transmitted at an upward angle may be refracted completely back down, landing hundreds or thousands of kilometers from the transmitter. This “skip” propagation enables global communication with modest transmitter power.

The ionosphere is not a sharp mirror — it is a gradient of increasing electron density with altitude. As a radio wave enters the ionosphere, it bends progressively. Whether it bends enough to return to Earth depends on the frequency, the angle of incidence, and the electron density (which varies with solar illumination, time of day, season, and solar cycle).

The maximum usable frequency (MUF) for a given path is the highest frequency that will be refracted back to Earth for that path geometry. Above the MUF, signals pass through the ionosphere into space. The MUF varies with solar activity (high solar activity = more ionization = higher MUF), time of day (daytime = higher MUF), and season (summer = higher MUF in general, but with regional variations).

A practical guide to HF sky-wave frequency selection:

• 3.5–7 MHz: works well at night; absorbed by the D layer during daytime
• 7–14 MHz: transition range; works day and night under varying conditions
• 14–21 MHz: best for long-distance daytime communication
• 21–30 MHz: requires high solar activity (solar maximum); dramatic performance swings with solar cycle

Skip Zone and Coverage Gaps

Between the ground wave coverage area and the sky wave landing zone, there is a “skip zone” — a region that receives neither mode. Signals seem to disappear somewhere between 100 and 500 km, then reappear at greater distances. This is the skip zone.

For a frequency of 7 MHz, typical skip zone extends from about 50 km (where ground wave fades) to 600–1,000 km (where the first sky wave skip lands). Anyone in between hears nothing from the transmitter. This matters for regional planning: a community network that relies on 7 MHz will have reliable local coverage (ground wave) and long-distance coverage (skip), but a “donut hole” in between.

The solution is to use multiple frequencies: a lower frequency for ground wave coverage of the skip zone region, and the HF frequency for long-distance. Alternatively, intermediate relay stations positioned outside the skip zone can cover the gap.

Skip distance changes with ionospheric conditions: at night, when the ionosphere drops lower, the same frequency skips a shorter distance. Operating just above the MUF at certain times creates shorter-than-usual skip — operators learn to exploit this for regional coverage when standard frequencies fail.

Near-Vertical Incidence Skywave (NVIS)

NVIS is a deliberately arranged propagation mode that eliminates the skip zone. By using frequencies slightly below the critical frequency (the frequency that penetrates the ionosphere straight up) and launching signals nearly vertically, the signal bounces almost straight back down and covers a circular area from the transmitter out to about 0–300 km.

Key requirements for NVIS:

• Frequency below the foF2 critical frequency (typically 3–7 MHz during the day, 2–5 MHz at night)
• A horizontally polarized antenna at low height (λ/10 to λ/4 above ground) — this concentrates radiation toward the zenith
• Ideally avoid late-afternoon when foF2 is lowest and NVIS becomes unreliable

NVIS provides coverage that cannot be served by either ground wave (too much loss on long paths) or normal sky wave (skip zone). For mountainous terrain, it is transformative: mountains that block all direct radio paths are irrelevant when signals travel vertically. A NVIS network in the Caucasus or Himalayas works as reliably as on flat terrain.

Unusual Propagation Modes

Sporadic-E: randomly occurring intense ionization in the E layer (90–120 km) reflects VHF signals (30–200 MHz) back to Earth at distances of 1,000–2,000 km. Unpredictable but dramatic — VHF stations suddenly hear each other across Europe or the Americas. Cannot be relied upon for scheduled communication but provides bonuses when it occurs.

Troposcatter: VHF and UHF signals scatter off turbulent layers in the lower atmosphere (troposphere), covering 200–500 km beyond line-of-sight. Requires high transmitter power and large antennas but provides reliable communication at VHF and UHF over distances impossible by line-of-sight. Used by military and maritime services.

Transequatorial propagation: unusual long-distance VHF propagation crossing the magnetic equator, occurring in late afternoon and evening. Covers 5,000–10,000 km on VHF with no special equipment.

Meteor scatter: meteors entering the atmosphere leave trails of ionized gas that briefly (for 0.1–10 seconds) reflect VHF signals. High-speed digital modes exploit these brief windows for reliable communication up to 2,000 km. Meteor showers (Perseids in August, Leonids in November) dramatically increase opportunities.

Aurora: geomagnetic disturbances ionize polar regions, reflecting radio signals via the visible auroral curtains. Communication via aurora produces a characteristic “hockey rink noise” — rough, rasping CW. Used by Scandinavian and North American operators for 1,000–3,000 km contacts during magnetic storms.

Knowing these modes exists allows operators to exploit them when they occur, and to diagnose anomalous propagation correctly rather than attributing unexpected contacts to equipment malfunction or interference.