Radio propagation

Radio propagation is a term used to explain how radio waves behave when they are transmitted, or are propagated from one point on the Earth to another.

In free space, all electromagnetic waves (radio, light, X-rays, etc) obey the inverse-square law which states that the power density of an electromagnetic wave is proportional to the inverse of the square of the distance [radius] from the source.

Electromagnetic wave propagation is also affected by several other factors determined by its path from point to point. This path can be a direct line of sight path or an over-the-horizon path aided by refraction in the ionosphere.

The ionosphere is the uppermost part of the atmosphere, distinguished because it is ionized by solar radiation.

The D layer is the innermost layer, 50 km to 90 km above the surface of the Earth.

The E layer is the middle layer, 90 km to 120 km above the surface of the Earth.

The F layer or region, also known as the Appleton layer, is 120 km to 400 km above the surface of the Earth.

Radio frequencies and their primary mode of propagation Band

Frequency Wavelength Propagation via
VLF Very Low Frequency 3–30 kHz 100–10 km Guided between the earth and
the ionosphere.
LF Low Frequency 30–300 kHz 10–1 km Guided between the earth and
the D layer of the
ionosphere.

MF Medium Frequency 300–3000 kHz 1000–100 m E, F layer ionospheric
refraction at night, when D
layer absorption weakens.

HF High Frequency 3–30 MHz 100–10 m E layer ionospheric
refraction.
F1, F2 layer ionospheric
refraction.

VHF Very High Frequency 30–300 MHz 10–1 m Infrequent E ionospheric
refraction. Extremely rare
F1,F2 layer ionospheric
refraction during high
sunspot activity up to 80
MHz. Generally direct wave.
Sometimes tropospheric
ducting.

UHF Ultra High Frequency 300–3000 MHz 100–10 cm Direct wave. Sometimes
tropospheric ducting.

SHF Super High Frequency 3–30 GHz 10–1 cm Direct wave.

EHF Extremely High Freq 30–300 GHz 10–1 mm Direct wave limited by
absorption.

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