• Tags: radio, microwave, communication
• Difficulty: Foundation

Radio waves (f < 10^9 Hz, lambda > 30 cm) are produced by oscillating electrons in antennas driven by LC circuits. AM radio (530-1710 kHz) uses amplitude modulation — the signal varies the amplitude of the carrier wave. FM radio (88-108 MHz) uses frequency modulation — more resistant to noise. TV signals (54-890 MHz) use both. Radio waves can be reflected by the ionosphere (for long-distance communication) or propagate as ground waves. Microwaves (1-300 GHz, lambda = 1 mm to 30 cm) are produced by klystrons and magnetrons. Microwave ovens operate at 2.45 GHz — the resonant frequency of the water molecule's rotational mode. Microwaves are used in radar (reflection timing gives distance), satellite communication (penetrate the ionosphere), and WiFi/Bluetooth. They travel in straight lines (line-of-sight propagation).