Extremely high frequency

Lore & Background

Millimeter waves propagate solely by line-of-sight paths. They are not refracted by the ionosphere nor do they travel along the Earth as ground waves. At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency, with maximum absorption at specific lines, mainly oxygen at 60 GHz and water vapor at 24 GHz and 184 GHz. Frequencies in the 'windows' between these absorption peaks have much less attenuation and greater range, so many applications use these frequencies. Millimeter wavelengths are the same order of size as raindrops, so precipitation causes additional attenuation due to scattering (rain fade) as well as absorption. The high free space loss and atmospheric absorption limit useful propagation to a few kilometers, making them useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse.

Millimeter waves show 'optical' propagation characteristics and can be reflected and focused by small metal surfaces and dielectric lenses around 5 to 30 cm in diameter. Because their wavelengths are often much smaller than the equipment that manipulates them, the techniques of geometric optics can be used. Diffraction is less than at lower frequencies, although millimeter waves can be diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so diffuse reflection increases. Multipath propagation, particularly reflection from indoor walls and surfaces, causes serious fading. Doppler shift of frequency can be significant even at pedestrian speeds. In portable devices, shadowing due to the human body is a problem. Since the waves penetrate clothing and their small wavelength allows them to reflect from small metal objects, they are used in millimeter wave scanners for airport security scanning.

The design of millimeter-wave circuit and subsystems (such as antennas, power amplifiers, mixers and oscillators) presents severe challenges to engineers due to semiconductor and process limitations, model limitations and poor Q factors of passive devices.

Reader's Guide

The significance of the extremely high frequency band lies in its unique combination of short wavelength and high atmospheric attenuation, which together enable extremely dense frequency reuse. This makes it ideal for high-capacity, short-range communications networks, including the newest generation of cell phone networks, 5G networks, which use certain frequency ranges near the bottom of the band. The band is also critical for scientific research, including radio astronomy at high-altitude sites such as the Atacama Large Millimeter Array (ALMA), and satellite-based remote sensing near 60 GHz for atmospheric temperature monitoring. In telecommunications, licensed and unlicensed bands support high-speed point-to-point data links, with the 60 GHz band used for unlicensed short-range links achieving data throughputs up to 2.5 Gbit/s, and the 71–76, 81–86, and 92–95 GHz bands used for licensed point-to-point high-bandwidth communication links. The Wi-Fi standards IEEE 802.11ad and IEEE 802.11ay operate in the 60 GHz spectrum to achieve data transfer rates as high as 7 Gbit/s and at least 20 Gbit/s, respectively. Military applications include short-range fire-control radar in tanks and aircraft, and automated guns on naval ships. The band also supports a nonlethal antipersonnel weapon system called Active Denial System (ADS) that emits a beam of 95 GHz millimeter waves. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential, and the high usable channel capacity might allow it to serve some applications that would otherwise use fiber-optic communication or very short links such as for the interconnect of circuit boards.

Did You Know?

• Millimeter waves are absorbed by atmospheric gases, with peaks at oxygen (60 GHz) and water vapor (24 GHz and 184 GHz).
• The Wi-Fi standard IEEE 802.11ad operates in the 60 GHz band with data rates up to 7 Gbit/s.
• The Active Denial System (ADS) uses a 95 GHz millimeter wave beam as a nonlethal antipersonnel weapon.

Spectral Identity and Naming Conventions

The International Telecommunication Union designates the electromagnetic band from 30 to 300 gigahertz as Extremely High Frequency, placing it squarely within the microwave portion of the radio spectrum. It occupies the stretch between the super high frequency band on its lower side and the terahertz band above. Because wavelengths in this range fall between ten and one millimeter, the band is widely known as the millimeter band, and the radiation itself is routinely called millimeter waves, abbreviated MMW or mmWave. Not every practitioner draws the boundary identically: some define mmWaves as beginning at 24 GHz, a choice that would sweep in the entire FR2 band spanning 24.25 to 71 GHz. The 60 GHz region is sometimes referred to as the V band. This nomenclature matters because it shapes how engineers, regulators, and researchers discuss allocation, hardware design, and application scope across a spectrum that bridges conventional microwave engineering and the emerging terahertz frontier.

Propagation Physics and Atmospheric Limitations

Millimeter waves travel exclusively along line-of-sight paths; they are neither refracted by the ionosphere nor carried along the Earth's surface as ground waves. At typical power densities, building walls block them entirely, and foliage causes heavy attenuation. Atmospheric absorption is a persistent challenge throughout the band and grows with frequency, peaking sharply at specific molecular lines—oxygen at 60 GHz and water vapor at 24 and 184 GHz. Between these absorption peaks, so-called windows offer considerably lower loss and extended range, which is why many practical systems target those frequencies. Because millimeter wavelengths are comparable in size to raindrops, precipitation introduces both scattering and absorption, producing what engineers call rain fade. The combined effect of high free-space loss and atmospheric absorption confines useful propagation to just a few kilometers. On the positive side, this short range enables much smaller frequency-reuse distances, and the short wavelength lets modest-sized antennas produce narrow beam widths, further multiplying spectrum efficiency in densely packed networks.

Telecommunications and the 5G Push

In the United States, the 36.0–40.0 GHz range is allocated for licensed high-speed microwave data links, while the 60 GHz band supports unlicensed short-range links of up to 1.7 km delivering throughputs as high as 2.5 Gbit/s, particularly in flat terrain. Higher bands at 71–76, 81–86, and 92–95 GHz serve point-to-point high-bandwidth links; these avoid oxygen absorption but require an FCC transmitting license. Plans exist for 10 Gbit/s links in these ranges, though a 100 MHz slice of the 92–95 GHz band is reserved for space-borne radios, capping its rate below a few gigabits per second. The Wi-Fi standards IEEE 802.11ad and 802.11ay operate in the 60 GHz V band, achieving data rates up to 7 Gbit/s and at least 20 Gbit/s respectively. WirelessHD also operates near 60 GHz. A major new application is the use of certain lower-band frequencies in 5G cell phone networks. The highly directional, pencil-beam nature of these signals allows adjacent systems to coexist without mutual interference, making the band attractive for dense urban deployments.

Scientific Research and Security Applications

Radio astronomy and remote sensing are two of the band's most established scientific uses. Ground-based radio astronomy is restricted to high-altitude observatories such as Kitt Peak and the Atacama Large Millimeter Array, because atmospheric absorption at sea level would swallow the faint signals from space. Satellite-based remote sensing near 60 GHz exploits oxygen emission to infer upper-atmosphere temperature as a function of pressure. The ITU's non-exclusive passive allocation at 57–59.3 GHz underpins meteorological and climate monitoring; operational U.S. sensors include the Advanced Microwave Sounding Unit aboard NASA's Aqua satellite and four NOAA satellites numbered 15 through 18, as well as the SSMI/S sensor on the Department of Defense F-16 satellite. In the security and defense realm, millimeter waves penetrate clothing and reflect off small metal objects, making them ideal for airport security scanners. The same short-range, high-resolution properties serve military fire-control radar systems.

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