Physics · Ch 13 — Electromagnetic Waves and Communication System
Propagation of EM Waves
Propagation of EM Waves
A television antenna picking up a signal from a broadcasting tower, or from a satellite, is a familiar everyday example of EM-wave communication in action: in radio-wave communication generally, an antenna at the transmitter radiates EM waves that travel outward through space and are eventually picked up by a receiving antenna at the other end. As the wave moves away from the transmitter, its strength steadily decreases, and several factors influence exactly what path it follows and how far it can travel -- among the most important of these is the layered structure of the Earth's own atmosphere (shown schematically in Fig. 13.3), which behaves very differently at different altitudes and can either help or hinder a wave's journey depending on its frequency. …
What this figure shows. A cross-sectional diagram of the Earth's surface with the layered structure of its atmosphere drawn rising above it as a series of curved, roughly concentric bands, each labelled with the name of the atmospheric layer it represents (such as the troposphere nearest the ground, followed by higher layers up to the ionosphere region, referenced elsewhere in the chapter as lying roughly between 60 km and 1000 km altitude). The figure establishes the vertical geometry -- Earth's curved surface plus a layered atmosphere above it -- that the ground-wave, space-wave and sky-wave propagation diagram (Fi …
What this figure shows. A schematic diagram showing a transmitting antenna mounted on the Earth's curved surface and a receiving antenna some distance away, with three distinct wave paths drawn between them to illustrate the three propagation modes described in the following subsections: (1) a ground/surface-wave path that hugs the curved surface of the Earth all the way from transmitter to receiver, (2) a space-wave path shown as a straight line-of-sight ray travelling directly from the top of the transmitting antenna to the receiving antenna (with an alternative branch shown reflecting off the ground or the troposphere), and (3) a sky-wave path shown travelling upward from the transmitting antenna, reflecting off the ionosphere layer high above the Earth, and returning back down to a distant receiving antenna. The Earth's curvature and the antenna heights relative to the …