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| 4 being actively studied for sensing and short-range, extremely high- data-rate communications. 1 Nomenclature for RF technologies and phenomena is often specific to the application, industry, or field of science that uses the concepts. For instance, one engineer may refer to the microwave spectrum as a subset of the RF spectrum, while another may use it to describe the shorter-wavelength and higher-frequency electromagnetic radiation beyond legacy RF communications frequencies. The millimeter-wave (mmWave) spectrum—typically assigned the range of 30GHz to 300GHz but may be extended to 3THz in some contexts—has a higher frequency range than the microwave spectrum. In this case, "millimeter" is used to describe the range of wavelengths that are on the order of a millimeter to hundreds of millimeters. Terahertz, like mmWave, is a physically accurate descriptor that describes the submillimeter-wave frequencies beyond mmWave frequencies and below optical frequencies (which are in the tens of terahertz). Many applications and industries have specific terms for regions of the RF spectrum and frequency bands, such as the radar frequency bands designated using letters, the International Telecommunication Union (ITU) spectrum designations, NATO radar bands, and cellular communications frequency ranges. This chapter uses the term RF to describe the portions of the electromagnetic spectrum (Figure 1) used for wireless communication, data signals, power transfer, and RF-based sensing up to the lower regions of the infrared spectrum. spectrum radiation (RF and beyond), terrestrial sources such as lightning and atmospheric interactions with space weather, and the myriad human-made RF sources and systems. Because RF technology originated in wireless communications, some schools of thought limit RF to relatively low-frequency wireless communications. In some cases, the term RF is used to describe all radiation that could be encompassed within the RF spectrum, whether made by humans or of natural origin. RF is generally considered to encompass a frequency range from roughly 3kHz to 300GHz, with some now considering it to extend into the terahertz (THz) range. The submillimeter-wave spectrum (above 300GHz to 30THz) is Figure 1: Electromagnetic frequency ranges, where radio and microwaves are designated as separate portions of the spectrum. (Source: VectorMine/stock.adobe.com)

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