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Extremely high frequency
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Extremely high frequency
Extremely high frequency
Frequency range
30 to 300 GHz
Wavelength range
10–1 mm
Related bands
  • K / L / M (NATO)
  • Ka / V / W / mm (IEEE)
Millimetre band (IEEE)
Frequency range
110 to 300 GHz
Wavelength range
2.73 to 1 mm
Related bands
EHF (IEEE)
Radio bands
ITU
1 (ELF) 2 (SLF) 3 (ULF) 4 (VLF)
5 (LF) 6 (MF) 7 (HF) 8 (VHF)
9 (UHF) 10 (SHF) 11 (EHF) 12 (THF)
EU / NATO / US ECM
IEEE
Other TV and radio

Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band in the electromagnetic spectrum from 30 to 300 gigahertz (GHz).[1][2] It is in the microwave part of the radio spectrum, between the super high frequency band and the terahertz band. Radio waves in this band have wavelengths from ten to one millimeter, so it is also called the millimeter band and radiation in this band is called millimeter waves, sometimes abbreviated MMW or mmWave.[3] Some define mmWaves as starting at 24 GHz, thus covering the entire FR2 band (24.25 to 71 GHz), among others.[4][5]

Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Absorption increases with frequency until at the top end of the band the waves are attenuated to zero within a few meters. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances. However the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential. Millimeter waves are used for military fire-control radar, airport security scanners, short range wireless networks, and scientific research.

In a major new application of millimeter waves, certain frequency ranges near the bottom of the band are being used in the newest generation of cell phone networks, 5G networks.[6] The design of millimeter-wave circuit and subsystems (such as antennas, power amplifiers, mixers and oscillators) also presents severe challenges to engineers due to semiconductor and process limitations, model limitations and poor Q factors of passive devices.[7]

Propagation

[edit]
Atmospheric attenuation in dB/km as a function of frequency over the extremely high frequency band. Peaks in absorption at specific frequencies are a problem, due to atmosphere constituents such as water vapour (H2O) and molecular oxygen (O2). The vertical scale is double logarithmic, as dB are themselves logarithmic.

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 as lower frequency radio waves do.[8] At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage.[8][9][10] Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, this absorption is maximum at a few specific absorption lines, mainly those of oxygen at 60 GHz and water vapor at 24 GHz and 184 GHz.[9] At frequencies in the "windows" between these absorption peaks, millimeter waves have much less atmospheric 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.[9][10] The high free space loss and atmospheric absorption limit useful propagation to a few kilometers.[8] Thus, they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse.[8]

Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and dielectric lenses around 5 to 30 cm (2 inches to 1 foot) 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.[8] Multipath propagation, particularly reflection from indoor walls and surfaces, causes serious fading.[10][11] Doppler shift of frequency can be significant even at pedestrian speeds.[8] 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.

Applications

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Scientific research

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Part of the Atacama Large Millimeter Array (ALMA), Chile, America, a millimeter wave radio telescope

This band is commonly used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array (ALMA) due to atmospheric absorption issues.

Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The International Telecommunication Union non-exclusive passive frequency allocation at 57–59.3 GHz is used for atmospheric monitoring in meteorological and climate sensing applications and is important for these purposes due to the properties of oxygen absorption and emission in Earth's atmosphere. Currently operational U.S. satellite sensors such as the Advanced Microwave Sounding Unit (AMSU) on one NASA satellite (Aqua) and four NOAA (15–18) satellites and the special sensor microwave/imager (SSMI/S) on Department of Defense satellite F-16 make use of this frequency range.[12]

Telecommunications

[edit]

In the United States, the band 36.0–40.0 GHz is used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range (1.7 km) data links with data throughputs up to 2.5 Gbit/s. It is used commonly in flat terrain.

The 71–76, 81–86 and 92–95 GHz bands are also used for point-to-point high-bandwidth communication links. These higher frequencies do not suffer from oxygen absorption, but require a transmitting license in the US from the Federal Communications Commission (FCC). There are plans for 10 Gbit/s links using these frequencies as well. In the case of the 92–95 GHz band, a small 100 MHz range has been reserved for space-borne radios, limiting this reserved range to a transmission rate of under a few gigabits per second.[13]

A CableFree MMW link installed in the UAE installed for Safe City applications, providing 1 Gbit/s capacity between sites. The links are fast to deploy and have a lower cost than fibre optics.

The band is essentially undeveloped and available for use in a broad range of new products and services, including high-speed, point-to-point wireless local area networks and broadband Internet access. WirelessHD is another recent technology that operates near the 60 GHz range. Highly directional, "pencil-beam" signal characteristics permit different systems to operate close to one another without causing interference. Potential applications include radar systems with very high resolution.

The Wi-Fi standards IEEE 802.11ad and IEEE 802.11ay operate in the 60 GHz (V band) spectrum to achieve data transfer rates as high as 7 Gbit/s and at least 20 Gbit/s, respectively.

Uses of the millimeter wave bands include point-to-point communications, intersatellite links, and point-to-multipoint communications. In 2013 it was speculated that there were plans to use millimeter waves in future 5G mobile phones.[14] In addition, use of millimeter wave bands for vehicular communication is also emerging as an attractive solution to support (semi-)autonomous vehicular communications.[15]

Shorter wavelengths in this band permit the use of smaller antennas to achieve the same high directivity and high gain as larger ones in lower bands. The immediate consequence of this high directivity, coupled with the high free space loss at these frequencies, is the possibility of a more efficient use of frequencies for point-to-multipoint applications. Since a greater number of highly directive antennas can be placed in a given area, the net result is greater frequency reuse, and higher density of users. The high usable channel capacity in this band 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.[16]

Weapons systems

[edit]
Millimeter wave fire control radar for CIWS gun on Soviet aircraft carrier Minsk, Russia

Millimeter wave radar is used in short-range fire-control radar in tanks and aircraft, and automated guns (CIWS) on naval ships to shoot down incoming missiles. The small wavelength of millimeter waves allows them to track the stream of outgoing bullets as well as the target, allowing the computer fire control system to change the aim to bring them together. [citation needed]

With Raytheon the U.S. Air Force has developed a nonlethal antipersonnel weapon system called Active Denial System (ADS) which emits a beam of millimeter radio waves with a wavelength of 3 mm (frequency of 95 GHz).[17] The weapon causes a person in the beam to feel an intense burning pain, as if their skin is going to catch fire. The military version had an output power of 100 kilowatts (kW),[18] and a smaller law enforcement version, called Silent Guardian that was developed by Raytheon later, had an output power of 30 kW.[19]

Security screening

[edit]
Main article: Millimeter wave scanner
Millimeter wave security scanner at Bonn airport

Clothing and other organic materials are transparent to millimeter waves of certain frequencies, so a recent application has been scanners to detect weapons and other dangerous objects carried under clothing, for applications such as airport security.[20] Privacy advocates are concerned about the use of this technology because, in some cases, it allows screeners to see airport passengers as if without clothing.

The TSA has deployed millimeter wave scanners to many major airports.

Prior to a software upgrade the technology did not mask any part of the bodies of the people who were being scanned. However, passengers' faces were deliberately masked by the system. The photos were screened by technicians in a closed room, then deleted immediately upon search completion. Privacy advocates are concerned. "We're getting closer and closer to a required strip-search to board an airplane," said Barry Steinhardt of the American Civil Liberties Union.[21] To address this issue, upgrades have eliminated the need for an officer in a separate viewing area. The new software generates a generic image of a human. There is no anatomical differentiation between male and female on the image, and if an object is detected, the software only presents a yellow box in the area. If the device does not detect anything of interest, no image is presented.[22] Passengers can decline scanning and be screened via a metal detector and patted down.[23]

According to Farran Technologies, a manufacturer of one model of the millimeter wave scanner, the technology exists to extend the search area to as far as 50 meters beyond the scanning area which would allow security workers to scan a large number of people without their awareness that they are being scanned.[24]

Thickness gauging

[edit]

Recent studies at the University of Leuven have proven that millimeter waves can also be used as a non-nuclear thickness gauge in various industries. Millimeter waves provide a clean and contact-free way of detecting variations in thickness. Practical applications for the technology focus on plastics extrusion, paper manufacturing, glass production and mineral wool production.

Medicine

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Low intensity (usually 10 mW/cm2 or less) electromagnetic radiation of extremely high frequency may be used in human medicine for the treatment of diseases. For example, "A brief, low-intensity MMW exposure can change cell growth and proliferation rates, activity of enzymes, state of cell genetic apparatus, function of excitable membranes and peripheral receptors."[25] This treatment is particularly associated with the range of 40–70 GHz.[26] This type of treatment may be called millimeter wave therapy or extremely high frequency therapy.[27] This treatment is associated with eastern European nations (e.g., former USSR nations).[25] The Russian Journal Millimeter waves in biology and medicine studies the scientific basis and clinical applications of millimeter wave therapy.[28]

Police speed radar

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Traffic police use speed-detecting radar guns in the Ka-band (33.4–36.0 GHz).[29]

History

[edit]

Millimeter-length electromagnetic waves were first investigated by Jagadish Chandra Bose, who generated waves of frequency up to 60 GHz during experiments in 1894–1896.[3]

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Extremely high frequency

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from Grokipedia

Extremely high frequency (EHF) is the designation by the (ITU) for the portion of the spanning 30 to 300 gigahertz (GHz). This band corresponds to wavelengths between 10 millimeters and 1 millimeter, classifying it within the millimeter-wave regime. EHF signals exhibit short wavelengths that enable compact antennas and high but are prone to significant from atmospheric gases, , and oxygen absorption, limiting distances to line-of-sight paths typically under a few kilometers. Key applications of EHF leverage its capacity for high data rates and precise targeting, including fire-control radars for weapon systems, downlinks in upper bands like Q/V, and experimental high-capacity point-to-point communications. Despite challenges with signal loss, advancements in and low-noise amplifiers have expanded its use in secure, short-range wireless networks and , where empirical data confirms superior resolution over lower frequencies. Defining characteristics include the need for highly directional antennas to mitigate and the band's role in bridging and terahertz regimes for future systems.

Definition and Technical Characteristics

Frequency Range and Designation

The extremely high frequency (EHF) band spans from 30 GHz to 300 GHz, corresponding to wavelengths of 10 mm to 1 mm. This range positions EHF as the highest subdivision within the traditional spectrum before transitioning into terahertz frequencies above 300 GHz. The EHF designation originates from IEEE Standard Letter Designations for radar-frequency bands, where it denotes the 30–300 GHz interval, and is similarly adopted by the (ITU) for radio and radar applications. These standards provide a consistent across , , and contexts, distinguishing EHF from adjacent bands like (SHF, 3–30 GHz) and avoiding overlap with informal or regional allocations. Within this band, sub-divisions such as Ka (26.5–40 GHz), V (40–75 GHz), and (75–110 GHz) are sometimes used for specific or applications, though they do not alter the overarching EHF classification.

Wavelength and Wave Properties

The wavelengths of extremely high frequency (EHF) signals span from 10 mm at the lower frequency limit of 30 GHz to 1 mm at the upper limit of 300 GHz, determined by the fundamental relation λ=c/f\lambda = c / f, where cc is the in (3×1083 \times 10^8 m/s) and ff is the . This short range classifies EHF as millimeter waves, distinguishing it from longer-wave lower-frequency bands. EHF waves propagate as transverse electromagnetic waves with electric and magnetic fields oscillating perpendicular to the direction of travel, exhibiting properties such as polarization (linear, circular, or elliptical) that can be manipulated for applications requiring specific field orientations. In free space, their phase and group velocities equal the , but short wavelengths result in inherently narrow when generated by antennas sized proportionally to λ/2\lambda/2, enabling high directivity compared to longer-wavelength radio bands. Unlike , EHF waves carry insufficient (on the order of micro-electronvolts) to break chemical bonds, behaving as with thermal interaction dominant at high intensities.

Comparison to Adjacent Bands

The extremely high frequency (EHF) band, designated from 30 to 300 GHz with wavelengths of 10 mm to 1 mm, immediately follows the super high frequency (SHF) band of 3 to 30 GHz (wavelengths 100 mm to 10 mm) in the radio spectrum classification. This adjacency results in EHF inheriting some microwave propagation traits from SHF, such as predominant line-of-sight transmission, but with markedly increased challenges due to higher frequencies. Free-space path loss in EHF exceeds that of SHF by factors scaling with the square of frequency, yielding roughly 20 dB additional loss when comparing the upper SHF limit (30 GHz) to the lower EHF onset under equivalent distances and conditions, as derived from the Friis transmission equation. Atmospheric effects further differentiate EHF from SHF: while SHF signals penetrate and with moderate (typically 0.01 to 0.1 dB/km in heavy ), EHF incurs 5 to 20 dB/km from rain scatter and gaseous absorption by oxygen and , especially above 50 GHz, confining practical ranges to under 1 km without relays even in clear . Antenna designs reflect this progression; SHF often employs parabolic dishes of 0.5 to 2 m for gain, whereas EHF demands smaller, higher-directivity horns or phased arrays (effective apertures under 10 cm) to achieve comparable beam efficiency, enabling finer resolution in but complicating alignment. Applications diverge accordingly: SHF supports longer-range satellite downlinks and with bandwidths up to hundreds of MHz, while EHF prioritizes short-haul, high-capacity links (gigabits per second) in millimeter-wave backhaul and precision military sensing. Above EHF lies the tremendously high frequency (THF) or terahertz band, conventionally starting at 300 GHz and extending to 3 THz (wavelengths 1 mm to 0.1 mm), marking a shift from fully electronic to hybrid optoelectronic generation and detection methods due to the "terahertz gap" in solid-state device performance. Propagation in THF amplifies EHF limitations, with becoming negligible and behavior approximating —signals attenuate exponentially over millimeters in air from enhanced molecular absorption, restricting non-line-of-sight paths almost entirely and necessitating or confinement for any distance. Unlike EHF's viable electronic amplifiers and oscillators (e.g., via GaAs or InP transistors up to 300 GHz), THF relies on quantum cascade lasers or photoconductive sources, incurring higher power inefficiency and cost, though offering sub-wavelength resolution for unattainable in EHF. This boundary underscores EHF as the upper practical limit for conventional radio systems before optical paradigms dominate.

Propagation Characteristics

Atmospheric Attenuation and Absorption

Electromagnetic waves in the extremely high frequency (EHF) band, spanning 30 to 300 GHz, undergo substantial due to absorption by atmospheric gases, particularly oxygen and . This gaseous absorption arises from molecular resonances, resulting in frequency-selective peaks that limit range. For instance, oxygen exhibits strong absorption near 60 GHz, with peak specific values approaching 15 dB/km under standard atmospheric conditions. Water vapor contributes additional absorption lines within the EHF range, notably around 183 GHz, where attenuation can exceed several dB/km depending on humidity levels. ITU-R Recommendation P.676 provides models for computing this specific gaseous attenuation, accounting for variables such as temperature, pressure, and water vapor density; clear-sky attenuation remains below 1 dB/km in atmospheric windows away from resonance peaks, such as between 70 and 100 GHz. Beyond gaseous absorption, hydrometeorological effects like , , and clouds induce further through and additional absorption, scaling approximately with the square of frequency. in the EHF band can reach tens of dB/km during heavy precipitation, severely restricting link budgets for terrestrial and satellite communications. and clouds contribute lesser but cumulative losses, with driving opacity at these wavelengths. These combined effects confine EHF primarily to line-of-sight paths under favorable , necessitating high-gain antennas and adaptive modulation in practical systems.

Line-of-Sight Propagation and Limitations

Extremely high frequency (EHF) electromagnetic waves, with wavelengths of 1 to 10 mm, propagate predominantly via line-of-sight (LOS) paths due to their short wavelengths, which severely limit diffraction and bending around obstacles. Unlike lower-frequency bands such as medium frequency (MF) and high frequency (HF), which enable skywave propagation via ionospheric reflection, millimeter waves (30–300 GHz) do not reflect off the ionosphere and experience negligible refraction or reflection by it. EHF frequencies greatly exceed the ionosphere's critical frequency (typically up to 10 MHz), allowing signals to pass through unimpeded. This confines propagation solely to LOS paths. This behavior resembles optical propagation, requiring a direct, unobstructed visual path between transmitter and receiver for effective signal transmission. Unlike lower-frequency radio waves, EHF signals exhibit minimal diffraction, as the wavelength is much smaller than typical environmental features like building edges or terrain variations, preventing significant energy spillover into shadowed regions. Key limitations stem from geometric constraints and material interactions. Obstacles such as buildings, , , and even vehicles cause complete or partial signal blockage, with poor penetration through common materials; for instance, foliage induces losses of approximately 19 dB at 40 GHz over 10 meters depth. In non-line-of-sight (NLOS) scenarios, propagation relies on reflections or , but high frequencies render most surfaces relatively rough, favoring diffuse scattering over , which results in substantial power reduction and unreliable links. further restricts range, governed by the LFSL(dB)=92.4+20log⁡f+20log⁡RL_{FSL} (dB) = 92.4 + 20 \log f + 20 \log R where ff is in GHz and RR is distance in km, yielding practical LOS link distances typically under 5-20 km for fixed services, depending on , antenna gains, and power levels. To mitigate these limitations, systems employ high-gain directional antennas, elevated towers, or to maintain clear LOS, though urban or forested environments often necessitate dense deployment. Empirical measurements confirm that signal strength drops sharply beyond LOS, with NLOS paths exhibiting 20-40 dB additional loss compared to direct paths in urban settings. These characteristics make EHF suitable for short-range, high-capacity applications like point-to-point backhaul but challenging for broad-area coverage without auxiliary technologies.

Environmental and Material Interactions

EHF signals experience pronounced attenuation from environmental factors beyond gaseous absorption, particularly precipitation and obscurants like fog and clouds, which scatter and absorb millimeter waves due to their wavelength comparable to water droplet sizes. Rain represents the dominant impairment, with empirical models deriving from measurements showing attenuation proportional to rainfall rate and frequency; for instance, at 30 GHz, moderate rain (10 mm/h) induces approximately 5-10 dB/km loss, escalating to over 20 dB/km in heavy downpours (50 mm/h), and worsening at higher EHF frequencies due to increased Mie scattering. Fog and clouds contribute additional losses of 0.5-2 dB/km depending on liquid water content, as verified in propagation experiments over paths up to several kilometers. Vegetation and terrain further degrade EHF propagation through , absorption, and . Foliage penetration yields high specific , typically 15-30 dB/m through dense tree canopies at 60 GHz, attributed to leaf properties and random orientation causing beam broadening and . Urban or forested environments amplify these effects via diffuse multipath from rough surfaces—where millimeter wavelengths render even moderately textured obstacles (e.g., brick or bark) highly —resulting in signal depths of 10-20 dB beyond line-of-sight. In terms of material interactions, EHF waves reflect strongly from conductors like metals, with reflection coefficients approaching -1 for smooth surfaces, but exhibit diffuse from rough or materials due to sub-wavelength surface irregularities. Water-rich substances, including biological tissues, absorb heavily; the , modeled as a lossy medium with high , reflects most incident power while limiting penetration to 0.1-1 mm at 30-100 GHz, as confirmed by studies measuring surface-specific absorption rates. Transmission through non-metallic barriers like or is feasible but attenuated by 5-15 dB depending on thickness and composition, with empirical data from shielding tests showing frequency-selective absorption in thin films designed for mmWave bands. Building materials such as impose losses exceeding 20 dB for typical wall thicknesses, restricting indoor penetration and favoring line-of-sight applications.

Applications

Telecommunications and Data Transmission

Extremely high frequency (EHF) bands, spanning 30 to 300 GHz, enable high-capacity wireless through point-to-point and point-to-multipoint links, particularly in the E-band (70-80 GHz). These systems support transmission rates up to 10 Gbps full-duplex, facilitating mobile backhaul, extensions, and fiber backups in urban environments where rapid deployment is essential. The U.S. (FCC) allocation of unlicensed spectrum in 71-76 GHz, 81-86 GHz, and 92-95 GHz bands in 2003 spurred commercial adoption by providing 13 GHz of available bandwidth for such applications. In networks, EHF frequencies within the millimeter-wave range (mmWave) deliver ultra-high throughput and low latency for dense, short-range access networks, supporting data-intensive uses like real-time video streaming, (V2X) communications, and infrastructure. E-band links, operating at 71-86 GHz, can handle 15 to 20 times more traffic than traditional mid-microwave bands (14-25 GHz), making them vital for aggregating traffic from in high-density areas. Multiband integration with lower frequencies further enhances capacity for fronthaul and backhaul. Satellite communications leverage EHF bands such as V-band (40-75 GHz) and Q-band (33-50 GHz) for (HTS) systems, enabling terabit-per-second aggregate capacities through wider bandwidth availability compared to lower bands like Ka-band. These frequencies support internet delivery, with potential for multi-Gbps user links in future very high-throughput satellite (VHTS) architectures, though they require advanced mitigation for propagation losses. EHF's large contiguous spectrum blocks allow efficient modulation schemes to achieve these rates, positioning it as a for next-generation orbital data relay.

Radar Systems and Sensing

Extremely high frequency (EHF) systems leverage wavelengths of 1 to 10 mm to achieve high , enabling precise detection and imaging of targets. This resolution stems from the inverse relationship between and angular accuracy, allowing EHF radars to resolve small features that lower-frequency systems cannot distinguish. Compact antenna sizes are possible due to the short wavelengths, facilitating integration into small platforms like and missiles. In automotive sensing, 77 GHz and 79 GHz EHF radars dominate advanced driver-assistance systems (ADAS), supporting functions such as , blind-spot monitoring, and automatic emergency braking. These radars use frequency-modulated continuous-wave (FMCW) techniques to measure range, , and with sub-meter accuracy, even in adverse weather, outperforming ultrasonic or camera-based sensors in range and reliability. By 2022, regulations mandated phasing out lower 24 GHz bands in favor of these higher EHF frequencies to mitigate interference and enhance performance. Military applications exploit EHF for fire-control radars and active , where narrow beamwidths—often milliradians—enable tracking of agile targets like or projectiles at short to medium ranges. For instance, millimeter-wave in precision-guided munitions provide resistance to electronic countermeasures through high-frequency operation and rapid beam agility. However, EHF limitations, including high atmospheric absorption by and oxygen, restrict effective ranges to line-of-sight distances typically under a few kilometers, necessitating high transmitter power and sensitive receivers. Sensing beyond radar includes remote detection in industrial settings, such as non-contact thickness of materials, where EHF waves' sensitivity to surface variations allows micron-level precision. Challenges like signal attenuation in or demand advanced mitigation, including arrays and error-correction algorithms, to maintain reliability. Despite these, the bandwidth availability supports high-resolution (SAR) modes for detailed terrain mapping and target classification in defense reconnaissance.

Scientific and Industrial Uses

EHF radiation finds application in for detecting emissions from celestial bodies, including the and molecular spectral lines in interstellar gas clouds, due to the band's sensitivity to cold, distant sources. Facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) utilize EHF frequencies up to 950 GHz, though core operations overlap with the 30-300 GHz range for high-resolution imaging of star-forming regions and protoplanetary disks. In , EHF enables of molecules, aiding studies of atmospheric composition and chemical reactions under controlled conditions. Industrial applications leverage EHF's ability to deliver precise, volumetric heating for material processing, achieving rapid of ceramics, oxides, ferrites, and metal-ceramic composites with an 83 GHz system, reducing processing times by orders of magnitude compared to conventional methods while minimizing energy use. This technique exploits the waves' penetration into materials for uniform heating without surface overheating, applied in advanced for high-temperature treatments. Additionally, millimeter-wave systems support non-destructive testing and inline in industries like automotive, food , and , detecting defects, moisture content, and structural anomalies through high-resolution imaging without physical contact. Such methods enhance efficiency in fabrication and curing, where EHF's short wavelengths allow sub-millimeter precision.

Medical and Terahertz Imaging

Terahertz (THz) imaging, employing frequencies from approximately 0.1 to 3 THz that overlap with the upper EHF band (above 100 GHz), enables non-ionizing, non-invasive visualization of biological tissues by detecting contrasts in and absorption arising from differences in , cellular , and biomolecular signatures. This approach contrasts with or MRI methods by avoiding risks while providing chemical specificity through spectroscopic analysis, though it is confined to superficial applications due to rapid in hydrated tissues. Empirical studies, primarily and , demonstrate feasibility for early detection, with ongoing efforts toward portable, real-time systems integrating metamaterials and AI for enhanced . In dermatological and oncological contexts, THz differentiates malignant from healthy by exploiting elevated and alterations in tumors; for example, it identifies and with clear contrasts at 0.1–2 THz. depth assessment benefits from hydration mapping, revealing second-degree partial-thickness injuries via reduced signals in necrotic zones. For , handheld THz pulsed on excised samples yields 87–88% accuracy, 86–87% sensitivity, and 96% specificity in reflection mode (0.1–1.8 THz), outperforming some optical methods for ductal carcinoma. Brain detection leverages higher absorption in tumor regions (0.4–2.53 THz), achieving sub-wavelength resolution in attenuated total reflection setups. These applications extend to oral, gastric, and colorectal cancers, where THz sensitivity to fingerprints enables label-free biosensing with reported sensitivities up to 80–82% in select studies. Dental and orthopedic uses include caries identification through enamel-dentin contrasts and evaluation, as THz waves penetrate dry or low-water matrices better than soft tissues. Continuous-wave and pulsed THz systems facilitate real-time monitoring, such as permeation in or corneal hydration for . Limitations persist, including penetration depths of 0.2–0.3 mm from water absorption, hindering deep-tissue imaging, alongside equipment bulkiness, low detector sensitivity, and challenges distinguishing from tumors. While preclinical evidence supports diagnostic utility, clinical translation requires addressing variability and standardization, with millimeter-wave subsets (30–300 GHz) explored adjunctively for rather than high-resolution imaging.

Security, Weapons, and Defense Systems

Extremely high frequency (EHF) signals, spanning 30–300 GHz, enable high-resolution systems critical for defense, offering superior angular precision and weather penetration compared to lower-frequency bands. These s support fire control, tracking, and threat detection in degraded visual environments, such as or , where optical systems fail. For instance, millimeter-wave s developed since the late have been adapted for applications, including precise targeting in airborne and ground-based systems. The U.S. funds programs like Millimeter-Wave Digital Arrays () to advance digital array technologies for next-generation Department of Defense (DoD) capabilities at these frequencies. In and anti-radiation systems, EHF s enhance seeker performance for engaging mobile threats. The U.S. Navy's AGM-88E Advanced Anti-Radiation Guided (AARGM) incorporates millimeter-wave to improve accuracy against hostile air defenses, enabling operation in electronic warfare environments. Similarly, munitions like the Hellfire use millimeter-wave seekers to generate target images matched against onboard models, distinguishing threats from decoys via high-frequency resolution. EHF's narrow beamwidth supports low-probability-of-intercept (LPI) operations, reducing detectability by adversaries, as outlined in analyses of over 60 EHF applications. Secure communications represent another defense pillar, with EHF providing jam-resistant, high-data-rate links via narrow, directional beams. The U.S. (AEHF) , operational since 2011, uses EHF for uplinks and crosslinks to ensure survivable command-and-control in nuclear or contested scenarios, outperforming super-high-frequency (SHF) downlinks in electronic warfare resilience. These systems maintain global connectivity for strategic forces, with EHF's atmospheric paradoxically aiding by limiting unintended . For non-lethal weapons, EHF enables directed-energy systems like the (ADS), which emits a 95 GHz millimeter-wave beam to induce skin-surface heating, repelling personnel without penetration beyond 0.4 mm. Deployed by the U.S. DoD since demonstrations in 2007, ADS travels at light speed for rapid engagement, with effects tunable for or perimeter defense, as validated in safety studies showing reversible thermal sensations. Solid-state variants, developed by the U.S. Army Research, Development and Engineering Center since 2013, enhance portability and efficiency for non-lethal denial of area. In security contexts, EHF imaging detects concealed weapons or explosives by differentiating materials through clothing or covers, supporting checkpoint and perimeter surveillance with minimal false alarms.

Health, Safety, and Biological Effects

Thermal Heating Mechanisms

Extremely high frequency (EHF) , spanning 30 to 300 GHz, induces thermal heating in biological tissues primarily through losses, where the oscillating causes polar molecules—predominantly —to rotate and generate frictional via molecular collisions. This process is governed by the tissue's complex , with the imaginary component representing as ; , which constitutes 50-70% of most soft tissues, dominates absorption at these frequencies due to its high dipole moment. Empirical measurements confirm peak absorption in around 60-140 GHz, aligning with EHF bands, resulting in specific absorption rates (SAR) that scale with incident . Due to the short wavelengths (1-10 mm), EHF penetration depth in skin is limited to approximately 0.3-1 mm, concentrating deposition in the and rather than deeper organs. For instance, at 94 GHz, the 1/e in is about 0.56 mm, leading to surface temperature rises that can exceed 1°C per W/cm² of in controlled exposures, though blood and sweat provide rapid cooling . Finite element models of mmWave exposure at 60 GHz demonstrate localized heating in cutaneous structures like nerves and capillaries, with temperature gradients dissipating heat conductively beyond the absorption zone. This superficial heating contrasts with lower-frequency microwaves, which penetrate deeper (e.g., centimeters at 2.45 GHz), and forms the basis for thermal safety assessments using metrics like SAR (typically limited to 1.6 W/kg averaged over 1 g of tissue). Thermal equilibrium is achieved when absorbed power balances metabolic and thermoregulatory dissipation, but high-intensity exposures (e.g., >10 /cm²) can overwhelm , causing burns or cataracts via corneal heating, as observed in with focused EHF beams. trials with 28-60 GHz mmWaves at security scanner levels (0.1-1 /cm² for seconds) show negligible bulk changes (<0.1°C), with effects confined to sensory warmth due to nerve stimulation from localized gradients rather than uniform heating. These mechanisms are empirically validated through calorimetry and infrared thermography, underscoring that EHF thermal risks are intensity- and duration-dependent, with no evidence of cumulative effects below thresholds where cooling dominates.

Non-Thermal Effects and Empirical Evidence

Studies on non-thermal effects of extremely high frequency (EHF) radiation, spanning 30–300 GHz, have primarily focused on in vitro and in vivo models, reporting potential alterations in cellular processes such as ion channel activity, membrane permeability, and gene expression without significant temperature increases. For instance, exposure of yeast cells to non-ionizing millimeter waves (MMWs) at frequencies around 42 GHz and power densities below 1 mW/cm² has demonstrated inhibited cell division, attributed to mechanisms involving disrupted microtubule dynamics rather than heating. Similarly, human cell lines exposed to 35–94 GHz MMWs at sub-thermal intensities (specific absorption rates <1 W/kg) exhibited changes in calcium ion influx via voltage-gated channels, potentially leading to downstream effects like reactive oxygen species production. Animal studies provide mixed evidence; rats exposed to 94 GHz MMWs at low power densities (0.1–10 mW/cm²) for durations up to 30 minutes showed behavioral changes, such as altered pain thresholds, independent of skin temperature rise, suggesting possible neural modulation. However, replication has been inconsistent, with some experiments failing to distinguish non-thermal from subtle thermal artifacts due to localized heating in superficial tissues. In vitro work on human skin cells and bacteria has indicated non-thermal impacts on proliferation and antibiotic sensitivity at 70–80 GHz, potentially via resonance effects on water clusters or protein conformations, though these findings are preliminary and lack large-scale validation. Comprehensive reviews highlight the controversy: while select studies propose mechanisms like electron tunneling or ionic rearrangements in DNA at sub-thermal EHF exposures, major health agencies such as ICNIRP conclude no established adverse non-thermal effects, emphasizing that observed responses often occur near thermal thresholds or stem from methodological flaws like inadequate dosimetry. A 2024 analysis of MMW therapy literature notes potential non-thermal benefits for wound healing via enhanced cellular signaling but cautions against extrapolating to risks without further epidemiological data, given EHF's limited penetration depth (typically <1 mm in skin). Empirical evidence remains sparse for human health outcomes, with no meta-analyses confirming causal links to pathology; claims of effects like oxidative stress are debated, as they may reflect indirect thermal influences or bias in non-Western studies favoring lower exposure limits.

Regulatory Standards and Exposure Limits

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) established updated guidelines in 2020 for limiting exposure to radiofrequency electromagnetic fields from 100 kHz to 300 GHz, emphasizing protection against thermal effects such as tissue heating, which predominate at extremely high frequencies (EHF) due to shallow penetration depths (e.g., ~0.1-1 mm in skin). For frequencies above 6 GHz, including EHF (30-300 GHz), basic restrictions focus on absorbed power density (S_ab) rather than specific absorption rate (SAR), averaged over 4 cm² for localized exposure, to account for superficial absorption primarily in the skin and eyes. Occupational basic restrictions limit S_ab to 100 W/m² (averaged over 6 minutes), with a peak of 400 W/m² over 1 cm² above 30 GHz; general public limits are 20 W/m² (6 minutes) and 40 W/m² peak. Whole-body average SAR remains 0.4 W/kg occupational and 0.08 W/kg general public (30 minutes), though less relevant for EHF due to negligible deep-body penetration. Corresponding reference levels, derived conservatively from basic restrictions to simplify compliance measurement, specify incident power density (S_inc) of 50 W/m² occupational and 10 W/m² general public for 2-300 GHz (averaged over 6 minutes for occupational, 30 minutes for public), with frequency-dependent adjustments above 6 GHz (e.g., decreasing slightly to 20 W/m² public at 300 GHz). For brief exposures under 6 minutes, energy density thresholds apply (e.g., 72 kJ/m² general public over 2 cm²). These limits incorporate safety factors of 10-50 below thresholds for adverse effects like pain or burns, based on empirical data from animal and human studies showing no confirmed non-thermal hazards at compliant levels. In the United States, the Federal Communications Commission (FCC) enforces maximum permissible exposure (MPE) limits under 47 CFR §1.1310, extended to 100 GHz for EHF applications like mmWave 5G, aligning with power density metrics for frequencies above 1.5 GHz. Uncontrolled (general public) environments limit incident power density to 1 mW/cm² (10 W/m²) averaged over 30 minutes, while controlled (occupational) settings allow 5 mW/cm² (50 W/m²); these match ICNIRP reference levels and apply to EHF via beamformed evaluations for devices, without adopting separate >6 GHz metrics despite 2020 proposals for localized peaks up to 20 mW/cm² occupational. Compliance for mmWave transmitters often uses peak spatial-average power density over 1 cm², reflecting shallow EHF absorption, with no updates as of 2025 altering these thresholds. The IEEE Std C95.1-2019 standard, covering 0 Hz to 300 GHz, sets similar safety levels to prevent thermal injury, with reference levels for incident above 3 GHz at 10 / for unrestricted environments and 50 / for restricted (occupational), evaluated over 6 minutes and small areas (e.g., 4 cm²) to address localized heating in EHF. These derive from dosimetric models calibrated to empirical rise data (<1°C in ), incorporating averaging over body regions and time to ensure margins against nociceptor activation or cataract thresholds observed in high-exposure animal tests. Many national regulators, including those in the EU and Canada, adopt or harmonize with ICNIRP or equivalent limits, prioritizing verifiable thermal endpoints over unconfirmed non-thermal claims lacking causal replication in controlled studies.
GuidelineFrequency RangeGeneral Public Limit (Incident Power Density)Occupational Limit (Incident Power Density)Averaging Area/Time
ICNIRP 202030-300 GHz10 /50 /4 cm² / 6-30 min
FCC MPE>1.5-100 GHz10 /50 /Whole body / 30 min (uncontrolled)
IEEE C95.1-2019>3-300 GHz10 /50 /4 cm² / 6 min

Controversies and Debated Risks

Concerns over potential health risks from extremely high frequency (EHF) , particularly in millimeter-wave (mmWave) applications like , have sparked debates centered on non-thermal biological effects beyond established tissue heating. Proponents of caution, including some researchers, argue that mmWave exposure at low intensities may induce subtle cellular changes, such as altered or immune modulation, based on and showing effects like increased skin temperature or protein synthesis linked to stress responses. However, these findings often involve exposures exceeding typical environmental levels, and mechanisms remain unestablished, with critics noting methodological limitations like small sample sizes and lack of replication in . A key controversy involves claims of deeper-than-expected penetration and systemic effects, such as neurological alterations evidenced by EEG changes in exposed subjects, challenging the physical principle of shallow absorption (typically <1 mm at 30-300 GHz). Such assertions, advanced by figures like Martin Pall, contrast with biophysical models and dosimetry data confirming minimal internal propagation, attributing observed outcomes to indirect pathways like sweat duct interactions rather than direct tissue impact. Peer-reviewed syntheses, including a 2021 state-of-the-science review, find no consistent evidence for adverse non-thermal effects at exposure densities below international limits (e.g., 10 W/m² for public mmWave), emphasizing that public fears amplified during 5G rollouts in 2019-2020 often stem from conflating correlation with causation absent longitudinal data. Regulatory debates highlight tensions between precautionary approaches and evidence-based standards, with bodies like ICNIRP and FCC maintaining guidelines focused on averting thermal damage (e.g., <1°C skin temperature rise), updated in 2020 to address >6 GHz frequencies. Critics, including petitions signed by over 400 scientists in 2017 and subsequent appeals, contend these overlook potential non-thermal sensitivities in vulnerable groups like children, citing rodent studies on or risks from eye exposure, though human trials show no or DNA damage at compliant levels. A engineering analysis reinforces that mmWave deployments yield peak exposures orders of magnitude below thresholds, with no epidemiological uptick in cancers or symptoms post-5G activation in urban areas. Despite this, ongoing gaps—such as long-term low-dose cumulative effects—fuel calls for independent monitoring, as evidenced by EU-funded projects like GOLIAT launched in 2021. The preponderance of empirical data from controlled exposures supports safety within limits, attributing much to selective interpretation amid institutional biases favoring alarmism in non-peer-reviewed advocacy.

Historical Development

Early Theoretical and Experimental Work

The theoretical foundation for extremely high frequency (EHF) electromagnetic waves, spanning 30 to 300 GHz, rests on formulated in 1865, which unified electricity and magnetism and predicted the existence of electromagnetic waves propagating at the across all frequencies without theoretical upper limits. experimentally verified these predictions in 1886–1888 using spark-gap transmitters and resonators to generate and detect waves with wavelengths around 33 cm to several meters, demonstrating reflection, , and polarization akin to . However, extending experiments to millimeter wavelengths required overcoming practical challenges such as precise quasi-optical alignment and sensitive detection, as longer-wave apparatus proved inadequate for shorter scales. Jagadish Chandra Bose pioneered the first generation and detection of millimeter waves in 1894–1896 at Presidency College in Calcutta, , motivated by investigating the shortest feasible electromagnetic wavelengths to test Maxwell's theory at optical boundaries. Bose employed a with a mercury interruption for pulsed generation, achieving wavelengths from 25 mm down to 5 mm (corresponding to frequencies up to 60 GHz), using cylindrical gratings for dispersion and dielectric lenses for focusing. Detection relied on his invention of a point-contact semiconductor-like receiver, initially a galena crystal coherer improved for selectivity, which responded to the high-frequency signals without rectification at the time. In a landmark 1895 demonstration, Bose transmitted 5 mm waves over 23 meters through two intervening walls, successfully activating a receiver to ring an and ignite , marking the earliest documented millimeter-wave communication system. These quasi-optical experiments confirmed wave properties including polarization, double in crystals, and selective absorption by substances, with Bose reporting findings to the Royal Institution in 1897. His work, conducted without contemporary vacuum tubes or , highlighted the feasibility of EHF despite atmospheric , laying groundwork for later high-frequency research while predating systematic exploration of the full EHF band by decades.

Mid-20th Century Advancements

Following , research into extremely high frequency (EHF) signals advanced through improvements in devices capable of operating at millimeter wavelengths. The and , refined during wartime efforts, were adapted for higher frequencies, enabling initial EHF experiments in the late 1940s. In 1946, Robert Dicke invented the Dicke , a switching that improved sensitivity for detecting weak millimeter-wave emissions, foundational for passive sensing applications. By the early , backward-wave oscillators (BWOs) and traveling-wave tubes (TWTs) emerged as key sources, allowing tunable generation up to 100 GHz with sufficient power for and studies, though limited by atmospheric . A pivotal breakthrough occurred in 1953 when Charles Townes and colleagues at demonstrated the (microwave amplification by of radiation), the first device to produce coherent amplification at EHF-relevant frequencies using gas, paving the way for lasers and precise spectroscopic measurements. This enabled detailed investigations into EHF propagation, revealing strong absorption bands—such as oxygen-induced loss at 60 GHz and effects around 22 GHz and 183 GHz—that constrained long-range uses but highlighted potential for short-range, high-resolution systems. Military applications focused on compact radars for aircraft ground mapping and harbor navigation, where EHF's narrow beams offered superior over lower microwave bands. In 1958, the UK's Royal Radar Establishment conducted the first airborne nadir-viewing radiometric imaging at 35 GHz, successfully mapping docklands from an and demonstrating EHF's viability for penetration and all-weather sensing despite high atmospheric variability. Communication prospects were explored in 1959 by H.M. Barlow, who proposed low-loss TE01 mode in circular filled with non-polar gases to mitigate for high-capacity trunk lines, though practical deployment lagged due to fabrication challenges and signal instability. These efforts, primarily driven by defense needs in the , , and , established EHF's dual-edged profile: immense bandwidth potential exceeding 10 GHz per channel, offset by rapid signal decay in humid conditions, spurring and over-the-horizon mitigation research into the .

Late 20th to Early 21st Century Commercialization

In the 1990s, millimeter-wave systems in the extremely high frequency (EHF) range achieved commercial success primarily in telecommunications for short-haul, high-capacity point-to-point links serving as alternatives to leased lines or fiber for backhaul in cellular networks and early internet infrastructure. The decade's explosive growth in data traffic, driven by expanding mobile services and internet adoption, created demand for systems capable of gigabit rates over short distances, often in bands above 30 GHz such as 38 GHz. These deployments marked a shift from military-dominated applications to civilian uses, with equipment enabling reliable connectivity in urban environments where trenching for fiber was impractical. Regulatory actions facilitated this expansion; by the mid-1990s, the U.S. (FCC) had begun allocating millimeter-wave spectrum for public use, including licensed bands for fixed services around 28-39 GHz auctioned for local multipoint distribution systems (LMDS). This enabled companies to develop and deploy point-to-multipoint architectures for broadband delivery, though high equipment costs and propagation challenges limited initial scale. Advances in monolithic integrated circuits (MMICs) during the period reduced complexity and improved affordability, supporting prototypes and early field trials. Entering the early , E-band (60-90 GHz) technologies gained traction for even higher capacities, with systems delivering 1-10 Gbps over 1-3.5 km under favorable conditions, comprising about 75% of U.S. . In , the FCC opened additional millimeter-wave bands, including 71-76 GHz and 81-86 GHz, on a licensed basis, accelerating deployments for metro backhaul amid surging mobile needs. This period saw market dominance by legacy architectures evolving into more efficient designs, though atmospheric absorption at these frequencies necessitated line-of-sight paths and adaptive modulation. Commercial adoption remained niche compared to lower frequencies until cost reductions and availability matured further.

Recent Developments in mmWave and Beyond

As of July 2025, 203 operators across 56 countries and territories have invested in mmWave (24-100 GHz), with 24 networks launched commercially and over 150 compatible devices available, though global spending on such deployments has declined since 2020 due to economic pressures and slower-than-expected returns. By mid-2025, worldwide connections exceeded 1.9 billion, driven partly by mmWave's high data rates and low latency in dense urban areas, though adoption remains concentrated in regions like and parts of where auctions have progressed. The mmWave market is projected to reach USD 44.26 billion in 2025, growing at a 25.52% CAGR to USD 137.93 billion by 2030, fueled by telecom providers' investments in auctions and to support backhaul and access. Technological innovations have addressed mmWave's propagation challenges, such as and blockage. In July 2025, researchers demonstrated analog repeaters that extend coverage in non-line-of-sight scenarios by relaying signals without digital , improving feasibility for urban mobile networks. Advancements in antennas include hybrid beamforming with phased arrays and dynamic beam-steering, enabling efficient in beyond-5G systems; these designs mitigate interference and support higher throughput at 28-60 GHz bands. In October 2024, released mmWave integrated circuits optimized for factory and , enhancing precision sensing in industrial environments. mmWave at 60 GHz has seen application growth in automotive advanced driver-assistance systems (ADAS) and smart infrastructure, with the market valued at USD 1.2 billion in 2024 and expected to reach USD 3.5 billion by 2033. Looking beyond current , mmWave forms a foundation for research targeting commercialization around 2030, with emphasis on sub-THz (100-300 GHz) and THz bands for data rates exceeding 100 Gbps. Beam-steering antennas at mmWave frequencies are advancing for , incorporating reconfigurable intelligent surfaces and AI-driven optimization to handle Doppler shifts and channel dynamics in high-mobility scenarios. frameworks are being explored to manage mmWave allocation in , addressing bandwidth and power constraints while enabling privacy-preserving adaptations. Global data traffic forecasts predict a 2.5-fold increase from 2024 to 2030, necessitating these higher-frequency innovations alongside novel waveforms resilient to mmWave and THz impairments. mmWave sensing has also progressed for non-contact health monitoring, leveraging high-resolution imaging for detection, though empirical validation remains limited to controlled studies.

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