Microwave - Wikiparency

# Microwave **Microwave** is the name given to the shortest of the radio wavelengths, the stretch of spectrum that picks up where [[Ultra_high_frequency|UHF]] leaves off and runs down to about the point where the spectrum starts behaving like infrared rather than radio, roughly one metre to one millimetre. Exactly where that stretch begins and ends depends on who is asked: radio-frequency engineers most often quote 1 to 100 gigahertz (GHz), narrower than the roughly 300 megahertz-to-300 gigahertz span the word covers when used at its loosest, though every version of the definition agrees on the whole super-high-frequency band in between. A frequency inside this range is more often named by its IEEE radar-band letter, S, C, X, Ku, K or Ka among them, than by the frequency itself.[^ieee521] Microwaves share UHF's dependence on a clear [[Line-of-sight_propagation|line of sight]], but more strictly still: at this wavelength there is essentially no bending around terrain and no help at all from the ground or the ionosphere, and near the top of the band the atmosphere's own gases start taking a real bite out of the signal, a loss no lower band has to plan around. That same short wavelength is what makes a beam narrow enough for radar, a satellite dish small enough to be practical, a receiver sensitive enough for radio astronomy, and an oven able to cook food from the inside out. A three.js sketch elsewhere on this site lights the 1-to-300 GHz microwave range within the wider [[Radio_spectrum]], including the line-of-sight bands it shares with UHF. ## Electromagnetic spectrum Where "microwave" begins and ends depends on who is drawing the line. The broadest reading runs from 300 MHz to 300 GHz; the narrower, more common engineering reading runs from 1 to 100 GHz; and the one stretch every version of the definition keeps without dispute, 3 to 30 GHz, corresponds to what radio engineers separately call the super-high-frequency band. All three conventions are drawing arbitrary lines across the same continuous physics: a 2.9 GHz wave and a 3.1 GHz wave differ from each other only in frequency, not in kind, whichever side of a stated boundary each happens to fall on. ### Frequency bands Rather than quote frequency directly, radar and microwave engineers commonly use IEEE letter designations inherited from wartime secrecy conventions, still convenient shorthand today: | Band | Frequency range | |---|---| | L | 1–2 GHz | | S | 2–4 GHz | | C | 4–8 GHz | | X | 8–12 GHz | | Ku | 12–18 GHz | | K | 18–27 GHz | | Ka | 27–40 GHz | Similar but not identical NATO and EU letter schemes exist alongside the IEEE one, a source of persistent confusion since the same letter can mean a different range depending on which standard a given document follows.[^ieee521] The [[Hydrogen_line|hydrogen line]] at 1.42 GHz, one of the most closely watched frequencies in radio astronomy, sits at the bottom of this ladder, in the L band. ## Propagation Microwave propagation follows the same line-of-sight rule as UHF, only more strictly: the shorter the wavelength, the less a wave diffracts around an obstacle, so a microwave link genuinely needs a clear path between its antennas rather than merely a strong signal. [[Free-space_path_loss|Free-space loss]] grows with the square of frequency for a fixed antenna size, which is why microwave links favour high-gain dish antennas over the simple whips lower bands can use. Atmospheric absorption becomes a real design constraint for the first time in this band: water vapour absorbs strongly near 22 GHz and molecular oxygen near 60 GHz, bands avoided for long terrestrial links and deliberately chosen for short-range satellite crosslinks that benefit from not reaching the ground at all. Rain adds further attenuation that grows sharply above about 10 GHz, so a satellite link budget at Ku band or above must plan a fade margin that a UHF link never needs. ### Troposcatter Before satellites made beyond-line-of-sight microwave links routine, tropospheric-scatter ("troposcatter") systems exploited faint scattering from turbulence in the lower atmosphere to bridge distances well beyond the radio horizon, at the cost of very high transmitter power and very large antennas to recover a signal scattered forward at a small fraction of the power that hit the scattering volume. Troposcatter's chief historical use was linking remote sites, such as the Arctic radar chain and island outposts, where no line-of-sight relay chain was practical and no satellite yet existed; it survives today mainly as a fallback where satellite capacity is unavailable or deliberately avoided. ## Antennas Because a microwave wavelength is centimetres or less, an antenna many wavelengths across, and therefore highly directional, is still a practically sized object: a one-metre dish is over a hundred wavelengths wide at 30 GHz. Parabolic reflector and horn antennas dominate the band for exactly this reason, concentrating power into a narrow beam whose gain rises with the antenna's area relative to the wavelength. [[Phased_array|Phased arrays]], which steer that same narrow beam electronically by adjusting the phase fed to many small elements rather than by moving a dish, extend the idea to radar and, increasingly, to communication systems that need to point a beam faster than a mechanical mount can turn. ## Design and analysis Because a microwave wavelength is only centimetres long, ordinary resistors, capacitors and inductors stop behaving as simple lumped components partway through the band, and circuits are more often analysed and built from sections of [[Transmission_line|transmission line]] than from discrete parts. Design work leans on the reflection coefficient, `Γ = (Z_L − Z0)/(Z_L + Z0)`, and its graphical form, the Smith chart, which turns an impedance-matching problem into a locus that traces a clockwise circle as a line is lengthened, with inductive loads plotting in one half of the chart and capacitive loads in the other.[^smith] [[Standing_wave_ratio|Standing wave ratio]] is the same mismatch expressed as a single number rather than a chart position, and both describe how much of a signal a connector, a filter or an antenna feed reflects back toward its source instead of passing through. A receiver's own noise, expressed as a noise figure that cascades stage by stage through an amplifier chain, is set almost entirely by the very first amplifier a microwave antenna feeds, which is why that stage gets the most design attention even though it is only one component among many.[^noisefig] ## Sources Microwaves reach a receiver from both natural and artificial sources, and the two matter for different reasons. Every object above absolute zero radiates some [[Thermal_radiation|thermal radiation]] as a rough [[Black-body_radiation|blackbody]], and at microwave frequencies that includes the [[Emission_spectrum|emission]] of the cosmic microwave background near 2.7 kelvin, the Sun's disc at anywhere from about a thousand to a million kelvin depending on conditions and frequency, and the Moon near 200 kelvin — numbers a sensitive receiver's own noise floor must beat before any of them can be detected at all.[^ell104] Artificial sources divide broadly into vacuum-electronic devices, chiefly the [[Cavity_magnetron|cavity magnetron]] and the klystron, and solid-state devices such as the Gunn diode, which generate microwave power directly from a biased semiconductor without any moving parts or heated cathode. Radar transmitters, communication-link amplifiers and ordinary microwave ovens between them account for nearly all of the artificial microwave power generated on Earth. ## Applications ### Communication Point-to-point microwave relay once carried the bulk of long-distance telephone and television traffic between line-of-sight towers, a role satellite and fibre-optic links have since taken over for most routes, though microwave relay remains common for shorter backhaul links, including from cellular base stations back to the wired network. Satellite communication uses microwave uplinks and downlinks almost exclusively, since the same short wavelength that demands a clear path from a ground station also lets a modestly sized dish achieve the gain a satellite link needs. The 5 and 6 GHz Wi-Fi bands, unlike the 2.4 GHz one, lie inside the microwave range as usually defined, another reminder that the UHF-microwave boundary is drawn through the middle of technologies that do not otherwise change at that line. ### Navigation Satellite navigation downlinks, including GPS's civilian signal near 1.575 GHz, sit at the low edge of the microwave range by the common 1-to-100-GHz definition, in the same neighbourhood UHF claims by its own broader ITU definition — one more place the two bands' textbook boundary is more convention than physics. Microwave landing systems, an aviation precision-approach aid operating in the same neighbourhood, use the band's tight beam control to guide aircraft along paths a wider low-frequency beam could not define precisely enough. ### Radar [[Radar]] is one of microwave's original large-scale applications and still one of its largest: air-traffic and weather radars generally run in S and C band, where rain scatters less strongly and range is favoured over resolution, while marine, automotive and precision-tracking radars move up to X, Ku and Ka band, trading range for the finer angular and Doppler resolution a shorter wavelength and smaller antenna can deliver. ### Radio astronomy Radio astronomy depends on the microwave band because much of the universe's own emission peaks there: the cosmic microwave background, the Sun and the Moon all radiate detectable microwave brightness, described above under Sources, and a radio telescope's antenna temperature is essentially that brightness averaged over whatever the antenna's beam happens to be pointed at.[^ell104] The discovery of the cosmic microwave background itself came out of microwave engineering rather than astronomy: a horn antenna built for satellite-communication research picked up a faint, uniform excess noise that could not be tuned away, traced eventually to the radiation predicted decades earlier as a relic of the early universe.[^penzias] ### Heating and power application Microwave ovens heat food by driving water molecules to rotate at [[Cavity_magnetron|magnetron]]-generated frequencies near 2.45 GHz, a frequency chosen for how well it penetrates typical foods without being absorbed entirely at the surface; the same dielectric-heating principle scales up to industrial drying, curing and processing lines. At a very different scale, high-power microwave and millimetre-wave sources heat the confined plasma inside a fusion reactor by driving electrons at their [[Fusion_power|cyclotron resonance frequency]], one of several heating methods a fusion experiment combines to reach the temperatures a reaction needs. ### Spectroscopy Many small molecules have rotational energy levels spaced closely enough that transitions between them fall in the microwave band, so microwave spectroscopy reads a gas's rotational fingerprint directly; the same lines, observed instead as the [[Emission_spectrum|emission]] or absorption features of the cold gas that fills much of the [[Interstellar_medium|interstellar medium]], are how radio astronomers identify specific molecules in space without ever sampling them directly. ## Frequency measurement A microwave signal's frequency is generally too high to count directly with ordinary digital logic, so measurement leans on comparison rather than brute counting: a signal can be mixed down against a known reference to a much lower difference frequency that ordinary counters handle easily, or checked against the sharp resonance of a tuned cavity that only accepts power at, or very near, its own resonant frequency. Modern frequency counters and spectrum analysers automate this heterodyne process, but the underlying idea, comparing an unknown against a known standard rather than counting cycles outright, is the same one a cavity wavemeter used decades earlier. ## Effects on health The one biological effect of microwave exposure that is well established is thermal: microwave energy absorbed by tissue is converted to heat by the same dielectric mechanism a microwave oven uses on food, which is why exposure guidelines are framed around a specific absorption rate, a power-per-kilogram limit rather than a field-strength limit alone. Claims of non-thermal biological effects at exposure levels below those thermal limits remain scientifically contested and are not established with the same confidence; standards bodies have generally declined to regulate against them absent reproducible evidence, while continuing to revisit the question as research continues. ## History ### Hertzian optics Heinrich Hertz's own experiments in the late 1880s, the same ones that first generated and detected radio waves in the laboratory, used apparatus short enough in wavelength to demonstrate reflection, refraction and polarization directly, showing that the newly discovered waves behaved optically in miniature rather than only in Maxwell's equations.[^hertz1888] The name "Hertzian optics" survives for this style of tabletop demonstration, an early hint that the microwave-adjacent part of the spectrum would eventually be treated as much like light, with lenses, mirrors and beams, as like the long-wave radio of the same era. ### First microwave communication experiments Jagadish Chandra Bose's work in Calcutta in the 1890s pushed generated wavelengths down into the millimetre range, far shorter than any contemporary commercial radio system attempted, using it to demonstrate reception through obstacles and across a lecture hall rather than for any communication service.[^bose] Practical microwave communication links waited for vacuum-tube sources capable of real power at these frequencies, which did not arrive in useful form until decades later. ### Radar development Centimetric radar, as opposed to the longer-wavelength early-warning sets already in wartime service, became practical only once the [[Cavity_magnetron|cavity magnetron]] could generate enough pulsed power at microwave frequencies to be worth building into an aircraft or a warship; the smaller antenna a short wavelength allows was as important militarily as the finer resolution it gave, since it let a usable radar fit into an aircraft nose rather than needing a tower.[^magnetron1940] ### Post World War II exploitation Peacetime uses grew quickly out of the wartime investment in microwave hardware: transcontinental microwave relay chains carried the bulk of long-distance telephone and television traffic through the 1950s and 1960s, wartime radar hardware and expertise seeded civilian weather radar networks, and radio astronomy grew directly out of radar engineers' recognition that the same receivers built to detect faint echoes could detect faint celestial emission instead.[^relay] ### Solid state microwave devices Vacuum-tube sources such as the klystron and the travelling-wave tube dominated microwave generation and amplification for decades, valued for the power they could deliver even as they demanded a heated cathode and, often, a bulky magnet. The Gunn diode, which generates microwave oscillation directly from a steady voltage across a small semiconductor chip through a purely electronic instability rather than any moving electron beam, brought solid-state microwave sources into practical use from the 1960s onward, and the wider spread of microwave [[Transistor|transistors]] and [[Semiconductor_device|semiconductor devices]] gradually displaced tubes from all but the highest-power roles.[^gunn] ### Microwave integrated circuits Just as digital electronics moved from discrete transistors to [[Integrated_circuit|integrated circuits]], microwave design moved toward putting an entire amplifier, mixer or filter stage on a single chip rather than assembling it from individual waveguide or transmission-line sections, trading some of the last decibel of performance a hand-tuned discrete circuit could reach for the repeatability, size and cost that only integration provides at production volume. ## Microsims This article carries no p5.js sketch of its own. A three.js sketch elsewhere on this site lights the 1-to-300 GHz microwave range within the wider [[Radio_spectrum]], picking out the line-of-sight bands microwave shares with UHF against the sky-wave and ground-wave bands below them. The interactive sketches that go with the specific ideas raised above belong to neighbouring articles, and this section points to three of them. *Try:* in the [[Radar]] sketch, shorten the pulse and watch the resolution inset separate a close pair of targets that a wider pulse merges into one blip — the same trade-off that pushes automotive and precision-tracking radar up to Ku and Ka band for the resolution a shorter microwave wavelength buys. *Try:* in the [[Doppler_effect]] sketch, raise the source speed and watch the frequency spread between an approaching and a receding observer; a weather radar reads exactly this shift off raindrops to map wind rather than only rainfall, and the shift itself is proportionally larger at microwave frequencies than at UHF for the same target speed. *Try:* in the [[Sonar]] sketch, compare how quickly a two-way delay resolves a range with how a microwave radar would perform the same trick: light covers in a microsecond what sound needs most of a second to cover underwater, which is why microwave radar electronics must be so much faster than sonar's for a comparable range. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Microwave) : [Wikitube](https://en.wikitube.io/wiki/Microwave) Skeleton mirrored at revision 1374516171. Prose, emphasis and the microsims are Wikitube's own. ## See also - [[Radio_spectrum]] - [[Ultra_high_frequency]] - [[Line-of-sight_propagation]] - [[Free-space_path_loss]] - [[Radar]] - [[Transmission_line]] - [[Cavity_magnetron]] ## References Standard electromagnetic theory used here — line-of-sight propagation, antenna gain and aperture, the reflection coefficient and blackbody radiation — is textbook material used throughout radio engineering and physics and is not separately footnoted, per the Wikitube style guide's §6.1. Page numbers below are PDF pages of the open editions linked from External links and this portal's book shelf. [^ieee521]: Citation needed: the current edition of IEEE Std 521, which defines the L, S, C, X, Ku, K and Ka radar-band boundaries, would confirm the exact figures quoted here and their relation to the similar NATO and EU letter schemes. [^smith]: Steer, M. *Microwave and RF Design: Networks*. 2019, pp. 67-85 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/microwave-and-rf-design-networks . CC BY-NC. [^noisefig]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 96-103 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^ell104]: Ellingson, S. *Radio Systems Engineering - Revised First Edition*. 2023, pp. 104-107 (PDF pages). Open Textbook Library: https://open.umn.edu/opentextbooks/textbooks/radio-systems-engineering . CC BY-NC. [^hertz1888]: Citation needed: the specific paper and date of Heinrich Hertz's late-1880s demonstrations of the optical behaviour of electromagnetic waves, to confirm the wavelengths used and the effects shown. [^bose]: Citation needed: a primary account of Jagadish Chandra Bose's 1890s millimetre-wave experiments in Calcutta, to confirm the wavelengths generated and the dates of the demonstrations. [^magnetron1940]: Citation needed: the University of Birmingham laboratory record or the original patent for John Randall and Harry Boot's 1940 resonant-cavity magnetron, to confirm the date and the priority claim. [^relay]: Citation needed: a telecommunications-history record of a specific post-war transcontinental microwave relay network (such as AT&T's), to confirm the dates and route quoted here. [^penzias]: Citation needed: Arno Penzias and Robert Wilson's original 1965 paper reporting the excess antenna temperature later identified as the cosmic microwave background, to confirm the date, the frequency and the journal record. [^gunn]: Citation needed: J. B. Gunn's original report of the Gunn effect in the early 1960s, to confirm the date and the semiconductor used. ## External links - Authoritative microwave frequency-band and radar-band standards (for example the current IEEE Std 521), to be pinned once specific editions are confirmed.