What Is Radio?

Radio is a technology for transmitting information over distance using electromagnetic waves in the radio frequency (RF) portion of the spectrum, which extends from approximately 3 Hz to 300 GHz. A transmitter encodes information onto a carrier wave through modulation, the receiver recovers the information by reversing that modulation, and the signal propagates through space or guided structures between the two. Radio underlies broadcasting, mobile telephony, satellite communication, radar, and wireless networking, making it one of the most widely applied technologies in electrical engineering.

Radio technology traces its origins to James Clerk Maxwell's 1865 theoretical prediction of electromagnetic wave propagation and Heinrich Hertz's 1887 laboratory demonstrations confirming it. Guglielmo Marconi's practical development of wireless telegraphy in the 1890s established radio as a commercial and military technology. The IEEE commemorates key milestones in radio's development in the IEEE Milestones program, which recognizes Fessenden's 1906 first voice broadcast and other foundational achievements.

Electromagnetic Spectrum and Frequency Bands

The radio spectrum is divided into named frequency bands, each with distinct propagation characteristics and regulatory allocations. Extremely low frequency (ELF) waves below 3 kHz penetrate seawater and are used for submarine communication. Medium wave (300 kHz to 3 MHz) and shortwave (3 to 30 MHz) bands support long-range amplitude modulated (AM) broadcasting, with shortwave propagating globally through ionospheric reflection. Very high frequency (VHF, 30 to 300 MHz) and ultra high frequency (UHF, 300 MHz to 3 GHz) are used for FM broadcasting, television, mobile networks, and public safety radio. Microwave bands above 3 GHz support satellite uplinks, point-to-point backhaul links, and Wi-Fi and 5G wireless networks. The ITU coordinates international spectrum allocation, and individual nations implement these allocations through their regulatory agencies, such as the FCC in the United States.

Modulation and Signal Transmission

Modulation is the process of encoding information onto a carrier wave by varying one of its parameters. Amplitude modulation (AM) varies the carrier's envelope in proportion to the signal; frequency modulation (FM) shifts the carrier frequency; and phase modulation (PM) changes the carrier's phase angle. Digital radio systems use more spectrally efficient schemes: quadrature amplitude modulation (QAM) encodes bits by varying both amplitude and phase simultaneously, with higher-order QAM variants such as 256-QAM packing more bits per symbol at the cost of requiring higher signal-to-noise ratios. Spread-spectrum techniques, including frequency hopping and direct-sequence spreading, distribute a signal across a wide frequency band to improve interference resilience and security. A technical introduction to these methods is available from Analog Devices' overview of spread-spectrum communications.

Receivers and Transceivers

A radio receiver extracts the transmitted information from a received signal that has been attenuated by path loss and mixed with interference and thermal noise. The superheterodyne receiver architecture, developed in the early twentieth century, downconverts the received signal to a fixed intermediate frequency (IF) before demodulation, enabling high selectivity and sensitivity through narrow IF filtering. Modern software-defined radio (SDR) replaces analog signal processing stages with high-speed analog-to-digital converters and digital signal processors, allowing the same hardware platform to support multiple modulation schemes, frequency bands, and protocols through software reconfiguration. Aerospace instrumentation relies on precision radio receivers for telemetry downlinks from aircraft and spacecraft, navigation using distance-measuring equipment (DME) and instrument landing systems (ILS), and air traffic control radar systems. The IEEE history of radio science and early transmitters traces how receiver architectures evolved alongside transmitter developments.

Applications

Radio has applications in a range of fields, including:

• Aerospace instrumentation including aircraft navigation, telemetry, and air traffic control radar
• Mobile telephony and broadband wireless access through cellular network infrastructure
• Satellite communications for broadcasting, GPS positioning, and deep-space telemetry
• Emergency services and public safety communications using land mobile radio systems
• Scientific radio astronomy and remote sensing of Earth's atmosphere and surface