Radio Wave Propagation: Ground Waves, Sky Waves and the Ionosphere

Three ways a radio wave gets from A to B

A transmitted radio wave can reach a receiver by three genuinely different routes. The ground wave hugs the Earth's curved surface by diffraction, useful mainly at low frequencies where the wavelength is large. The sky wave travels upward and bounces back down off the ionosphere, allowing signals to leap far beyond the horizon. The line-of-sight wave simply travels in a nearly straight line and needs a direct, mostly unobstructed path between transmitter and receiver — the only option left once the frequency is too high for the other two.

The ionosphere as an imperfect mirror

Solar ultraviolet radiation strips electrons from atoms in the upper atmosphere, creating layers of free electrons (named D, E, F1 and F2 by altitude) that behave like a weakly conducting plasma. A radio wave passing through a plasma of electron density N sees an effective refractive index that depends on the wave's own frequency f relative to the layer's plasma frequency f_p:

f_p ∝ sqrt(N)                    (plasma frequency rises with electron density)
n(f) = sqrt(1 - (f_p / f)²)          (refractive index of the layer)

Why HF bounces and VHF punches straight through

When the transmitted frequency is below the layer's critical frequency (for a wave sent straight up) the refractive index effectively bends the ray back toward the ground rather than letting it pass through — the layer acts as a mirror. Sending the wave in at an angle instead of straight up raises the frequency that can still be reflected, following an approximate secant law:

MUF ≈ f_c / cos(θ)      (MUF = maximum usable frequency, θ = angle from vertical)

That is why the HF (shortwave) band supports intercontinental radio by repeatedly reflecting between the ionosphere and the ground, while VHF and above (FM radio, television, mobile phones) sit above the maximum usable frequency of the ionosphere at any reasonable angle, pass straight through it, and are limited to essentially line-of-sight range.

Ground wave: following the curve of the Earth

At low and medium frequencies the wavelength becomes comparable to, or larger than, the obstacles and the Earth's own curvature, so the wave diffracts around the horizon rather than travelling in a strict straight line — this is how AM (mediumwave) broadcast stations reach out to the horizon and somewhat beyond during the day, entirely by ground wave, with no ionosphere involved at all.

Day, night, and why AM travels further after dark

The lowest ionospheric layer, the D layer, absorbs a significant fraction of medium-wave and HF energy passing through it during the day, but it forms only in sunlight and recombines within minutes after sunset because it has no source of ionising radiation at night. Once the D layer disappears, medium-wave signals that would have been absorbed on the way up now reach the higher F layer, reflect efficiently, and travel hundreds of kilometres further as sky wave — which is exactly why distant AM stations that are inaudible during the day can often be picked up after dark.

Frequently asked questions

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