Physics · Ch 10 — Communication Systems
PROPAGATION OF ELECTROMAGNETIC WAVES
PROPAGATION OF ELECTROMAGNETIC WAVES
Once a baseband signal has been modulated onto a carrier wave and radiated by the transmitting antenna, it must actually travel across the intervening space to reach the receiving antenna at the other end -- a journey during which the signal's strength keeps decreasing the further it travels. Wireless communication as a whole spans a huge range of frequencies, from about 2 kHz up to 400 GHz, and depending on exactly where in this range a given signal's frequency falls, it reaches the receiver by one of three physically distinct propagation modes. Ground wave (or surface wave) propagation, used for roughly 2 kHz to 2 MHz signals, is where the wave glides along the Earth's surface from transmitter to receiver. Sky wave (or ionospheric) propagation, used for roughly 3 MHz to 30 MHz signals, is where the wave is radiated upward and bounced back down to Earth by the ionosphere. Space wave propagation, used for roughly 30 MHz to 400 GHz signals, is where the wave travels in an essentially straight line directly from transmitter to receiver (line-of-sight …
What this figure shows. A four-panel figure summarising all three propagation modes together. Panel (a) shows ground-wave propagation: a radio tower near the Earth's surface radiating a wave that visibly glides along the curved ground to reach a home receiver nearby. Panel (b) shows sky-wave propagation: a wave radiated upward from the Earth at large angles reflecting off the ionosphere layer and returning to Earth at a distant point, with the sky-wave coverage zone, the ground-wave coverage zone, and a skip zone lying between them (marked by points labelled 1, 2 and 3) where neither wave is received. Panel (c) shows space-wave propagation between a transmitter and receiver: a direct line-of-sight wave travelling straight between the two antennas, alongside a second wave that reaches the receiver only after reflecting off the Earth's surface. Panel (d) is a combined summary diagram showing the ionosphere layer, a communication satellite in orbit above it, and ground-water level, with ground wave, sky wave and space wave (marked LOS, line-of-sight) all dra …