Physics · Ch 15 — Communication System
Propagation of Electromagnetic Waves in the Atmosphere
Propagation of Electromagnetic Waves in the Atmosphere
Three ways to reach the receiver. Once a message signal has been modulated onto a carrier and radiated as an electromagnetic wave from a transmitting antenna, it must still physically travel through the Earth's atmosphere before it can be picked up by a receiving antenna somewhere else. Depending mainly on its frequency, this journey happens in one of three qualitatively different ways, sketched together in this section's figure: as a ground wave, hugging the Earth's surface; as a sky wave, bounced back down from a layer high in the atmosphere; or as a space wave, travelling in essentially a straight line.
The ionosphere. The sky-wave mode depends on a particular feature of the upper atmosphere: the ionosphere, a region roughly 60 km to 400 km above the Earth's surface in which solar radiation strips electrons from atmospheric gas molecules, leaving behind a layer containing large numbers of free electrons and ions. A radio wave of suitable frequency, striking this ionised layer, is reflected (more precisely, progressively bent back) toward the Earth again, in much the same qualitative way that a wave of light is bent by a change of medium -- and it is this reflection that makes sky-wave, long-distance communication possible at all …
What this figure shows. A cross-sectional side view of the curved Earth's surface is drawn, with a transmitting antenna tower standing on the ground at the left. A curved, shaded band well above the Earth's surface represents the ionosphere. Three separate paths leave the base of the antenna: (i) a path that hugs the ground, curving to follow the Earth's curvature for a short distance -- this is the ground wave; (ii) a path that leaves the antenna at a steep upward angle, travels up to the ionosphere band, bends sharply back downward at the point it meets the ionosphere (representing reflection by the ionised layer), and comes back down to a receiving point on the ground far beyond the horizon -- this is the sky wave, with the ground region between where the ground wave's reach ends and where the sky wave lands marked as the 'skip zone', a dead zone reached by neither mode; and (iii) a nearly horizontal straight-line path from the top of the transmitting antenna directly to the top of a receiving antenna, drawn tangential to the Earth's curved surface at both ends and cut off by the visible horizon -- this is the space wave, …
| Mode | Frequency range typically used | Physical mechanism | Typical use |
|---|---|---|---|
| Ground wave | Up to about 2 MHz (low/medium frequency) | Wave glides along the Earth's surface, guided by diffraction and conduction over the curved, conducting ground | Local AM radio broadcast |
| Sky wave | About 2 MHz to 30 MHz (medium/high frequency) | Wave travels upward and is reflected back down by the ionised layers of the ionosphere | Short-wave, long-distance international broadcast |