Chapter Summary

This chapter introduced wireless communication — the sending of information through free space on radio waves. A basic electronic communication system is a chain of blocks: source, transmitter, channel/medium, receiver and destination, with unavoidable noise entering at the channel. Noise can be reduced but never removed, and its effect is measured through the signal-to-noise ratio $\left(SNR = \frac{\text{Signal Power}}{\text{Noise Power}}\right)$, the noise ratio $\left(NR = \frac{\text{SNR at input}}{\text{SNR at output}}\right)$ and the noise figure $\left(NF = 10\log_{10}(NR)\ \text{dB}\right)$.

Information is carried by electromagnetic (radio-frequency) waves, in which perpendicular electric and magnetic fields travel through free space at $3 \times 10^{8}$ m/s. The radio spectrum, spanning about $10^{4}$–$10^{14}$ Hz, is split into bands (VLF, LF, MF, HF, VHF, UHF, SHF, EHF), each with its own uses.

Radio waves reach a receiver by three paths. Ground waves run along the earth's surface (LF and MF bands, useful for marine communication). Space waves, or line-of-sight waves (VHF), combine a direct and a ground-reflected wave and reach only up to the radio horizon, which is about $\frac{4}{3}$ of the optical horizon. Sky waves are bent back to earth by the ionosphere, the sun-ionized region from about 50 km to 400 km made of the D, E and F layers (the E layer is also called the Kennelly–Heaviside layer, the F layer the Appleton–Barnett layer; by day the F layer splits into $F_1$ and $F_2$). …