Radio Wave Propagation

Part II (ii) Propagation · day/night frequencies · skip distance · fading · ground shadow

A vertically-polarized wave at low frequency induces currents in the conducting surface of the Earth; these slow the bottom of the wavefront, tilting it forward so it diffracts around the curve of the planet. The lower the frequency, the better it clings — and the farther it reaches.

Range grows with transmitter power, with lower frequency, and with the conductivity of the surface — so it is much greater over sea water (highly conductive) than over dry land or mountains. This is the path the NDB relies on, giving useful range beyond line-of-sight at low level.

High above the Earth, solar radiation strips electrons from thin air, creating electrically charged layers. These act like a mirror for HF radio, bending the wave back to Earth and giving HF its enormous range.

D layer (lowest, ≈60–90 km) — forms only by day and absorbs lower HF frequencies. E layer (≈90–150 km). F1 & F2 layers (highest, ≈150–400 km) — the main HF-refracting layers; at night F1 and F2 merge into a single, weaker F layer.

Ionisation depends on the Sun, so it varies diurnally (strong by day, weak by night), seasonally, and with the 11-year solar cycle (sunspots). The critical frequency is the highest frequency a layer will reflect at vertical incidence; above it the wave punches straight through.

MUF — Maximum Usable Frequency: the highest frequency the ionosphere will still refract back to a given point. Above it, the wave escapes into space and is lost.
LUF — Lowest Usable Frequency: the lowest frequency that still arrives usable; below it, D-layer absorption kills it.
OWF — Optimum Working Frequency: typically about 85% of the MUF — set safely below the MUF so normal ionospheric variation doesn't push the working frequency above it. The usable window lies between LUF and MUF.

A wave striking the ionosphere at a shallow (grazing) angle is refracted back over a long distance; a steep angle may punch through. By bouncing between the ionosphere and the ground, a sky wave can make several hops, reaching right around the world.

By day the D layer is present and absorbs lower frequencies, so you must use a higher HF frequency to get through. By night the D layer disappears and the ionosphere weakens, so a lower HF frequency works best.

Higher frequency by DAY, lower frequency by NIGHT for HF sky-wave communication. This is why long-haul crews change their HF frequency as they cross the day/night terminator.

Skip distance: the distance from the transmitter to the point where the first sky wave returns to Earth.
Dead space (skip zone): the silent ring between the end of ground-wave coverage and the point where the first sky wave lands — a region where neither wave reaches and reception fails.

VHF and above travel in essentially straight lines (the space wave), so range is set by the radio horizon — and the higher you (or the station) are, the farther that horizon. This is why a VHF call that fails on the ground works once you climb.

At 10,000 ft: d ≈ 1.23 × √10,000 = 1.23 × 100 = 123 NM to the radio horizon.
If the ground station antenna is at 400 ft: its horizon ≈ 1.23 × √400 = 1.23 × 20 = ≈ 25 NM. The total path is the sum: 123 + 25 ≈ 148 NM.
Takeaway: more height, more range — the exact figure isn't usually required, but the principle and the formula are.

Attenuation — the natural weakening of a signal with distance and absorption.
Fading — fluctuation in received strength, typically when two paths (ground + sky, or two sky paths) arrive together and interfere; common on HF at dusk.
Ground shadow — loss of a line-of-sight (space) wave behind a hill, building or terrain that blocks it.

Low on approach behind high ground, your VHF can drop out — that's ground shadow; climb a little and the line-of-sight clears. On HF over the ocean at night you'll hear the signal swell and fade — that's sky-wave fading, eased by selecting a better frequency.