Microwaves are a form of electromagnetic radiation that play an integral, if invisible, role in modern life. Any time you use your cell phone, connect to WiFi, warm up leftovers, or check a weather report, you‘re interacting with microwaves. But what exactly are these waves, where do they come from, and how did we discover uses for them? Read on for a comprehensive guide.
A Brief History of the Discovery of Microwaves
Microwaves were first predicted theoretically by James Clerk Maxwell in 1864 when he proposed the existence of electromagnetic waves. However, they weren‘t produced and detected experimentally until 1887 when German physicist Heinrich Hertz demonstrated the first transmission and reception of what would now be called radio waves.
Hertz used an induction coil and improvised capacitor to generate an oscillating electrical spark. This spark produced electromagnetic radiation that could jump over a small gap between two metal electrodes, confirming Maxwell‘s theory. The frequencies produced ranged from tens to hundreds of megahertz – what we now classify as radio frequencies.
In 1895, Guglielmo Marconi expanded on Hertz‘s work to transmit Morse code over distances up to 2.4 kilometres. Marconi recognized that with the development of more sensitive receivers, even longer distance communication would be possible using radio waves.
It wasn‘t until the 1930s-1940s that engineers and scientists learned how to reliably generate and utilize the higher frequency microwave range of 300MHz to 300GHz within the radio portion of the electromagnetic spectrum. Key innovations that enabled the practical use of microwaves included:
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Powerful microwave generators: Devices like the klystron (1937) and magnetron (1940) allowed amplification or radiation of microwaves at levels useful for detection and energy transmission.
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Accurate waveguides: During WWII, microwaves proved useful for military applications like radar. Carefully constructed hollow metal tubes helped direct the waves.
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Improved antennas: New antenna designs like parabolic and horn antennas (1930s) improved transmission, reception, and reflection of beams.
With this foundation of understanding and technology around microwaves laid by mid-century pioneers, the unique properties of these high frequency electromagnetic waves could be applied to transform civilian applications as well.
Defining Microwaves
Microwaves occupy the portion of the electromagnetic spectrum from 300 MHz to 300 GHz. They have frequencies higher than radio waves but lower than infrared waves and wavelengths ranging from about 30 cm down to 1 mm. In comparison, radio waves can be kilometers long and infrared wavelengths are a micrometer (one millionth of a meter) in size.
Some key qualities of microwaves include:
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Penetration ability: Lower frequency microwaves can penetrate haze, clouds, and building materials. Higher frequencies are blocked by metals.
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Reflection: Smooth metal surfaces efficiently reflect microwaves.
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Interaction with water: Microwaves passing through food containing water molecules apply rotating energy for rapid heating.
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Bandwidth utilization: Different microwave frequency ranges each have advantageous applications based on their properties. For organizational purposes, bands designated by letter are used.
Natural Sources of Microwave Radiation
Microwaves arise naturally from astronomical sources, mainly as part of the cosmic microwave background (CMB) leftover from the Big Bang origin of the universe. When astronomers Arno Penzias and Robert Wilson detected this omnipresent low level signal in 1964, it provided compelling evidence for the Big Bang theory.
CMB radiation corresponds almost exactly to that of an absolute black body at 2.7 K (-454°F/-270°C) and peaks at 160.2 GHz frequency, squarely in the microwave range. Variations in the CMB, as measured by space probes like COBE, WMAP, and Planck provide information about the age, composition, and expansion of the universe dating back to its earliest moments 13.8 billion years ago.
In addition to the CMB, microwaves are emitted by a wide array of celestial objects and events, including:
- Molecular clouds within galaxies
- Quasars
- Pulsar stars
- Masers occurring around black holes and star-forming nebulae
- Supernovae explosions
Astronomers analyze this microwave radiation using space telescopes and ground-based radio telescopes equipped with ultra-sensitive antennas, receivers, and processors. Learning about the universe through these invisible to the eye signals unlocks mysteries from its birth to star death and beyond.
Generation of Artificial Microwaves
While galactic events can produce immense amounts of microwave energy, humans require specialized equipment to intentionally generate them closer to home. Vacuum devices that accelerate charge particles are most commonly used:
Klystrons: This microwave amplifier makes use of velocity modulation of electron beams. Internal resonant cavities multiply the frequency. Output power can range from tens of watts up to tens of kilowatts. Klystrons made radar systems effective during WWII for example. Today they have applications from satellite transmitters to particle accelerators.
Magnetrons: In this tube, internal cavities and external magnetic fields interact with an electron beam to cause rapid oscillations. Magnetrons can yield hundreds to thousands of watts and are exceptionally efficient in pulses. The first microwave oven designs capitalized on magnetrons for rapid food heating with most power staying in the oven instead of the tube.
Traveling wave tubes: These have a wire conducting a stream of electrons that interacts with a traveling electromagnetic wave moving at close to the electron speed. It‘s like a mix between amplifier klystron and oscillator magnetron principles. Traveling wave tubes generate high levels of microwave power from boxes just several centimeters in size.
Transistor amplifiers: At frequencies up to around 100 GHz currently, transistors amplify power by energizing the electrons across their junction layers. High electron mobility transistors made from gallium arsenide rather than silicon are often utilized. While less efficient than klystron or magnetrons, transistor amps are more tunable, durable, and affordable.
Frequency multipliers: As the name suggests, these components multiply an input frequency by a set integer or decimal number. They contain nonlinear components that selectively output harmonics of the input frequency. Multipliers widen the frequency coverage options from microwave sources.
Major Applications of Microwaves
The unique properties of microwaves at their short yet penetrating wavelengths have led to diverse beneficial applications since their initial development including:
Communications
Microwaves relay signals between cell phones, WiFi networks, and billions of devices globally each second through frequencies like:
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UHF: Ranges from 300 MHz to 3 GHz utilized for things like mobile phones, UHF radio/television, GPS, WiFi, Bluetooth.
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C band: Used widely in WiFi at levels around 5 GHz. Offers a good balance of coverage and capacity.
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Ka band: Used for satellite communications thanks to good tradeoff between attenuation and antenna performance during heavy rain conditions. Employed by broadband providers like ViaSat and HughesNet.
Even higher bands up to 90 GHz are being tested for ultra-short range applications. The rise of 5G cellular taps into previously unused higher microwave frequencies to deliver fast, low latency service. New faster successor networks already in development like 6G aim go hit 1 terahertz frequencies.
Radar
During WWII, radar leveraging pulsed magnetrons operating from 1-10 GHz proved highly effective for detecting enemy ships and aircraft. Civilian applications followed including:
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Weather radar: Networks like the Next-Generation Radar Doppler system transmit 10 cm microwaves to track precipitation and storm dynamics.
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Speed monitors: Traffic control radar guns work on the Doppler effect at around 10-25 GHz frequencies to catch speeding vehicles.
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Aircraft monitoring: Air route surveillance monitors plane positions with 1 GHz signals while airplane landing systems assist with high accuracy utilizing 5 GHz beams.
Spectroscopy
Microwaves offer a preferred frequency range for studying the rotations and vibrations of molecules through absorption spectroscopy and other techniques. Microscopy modalities like near-field scanning microwave microscopy achieve nanoscale resolutions. Overall microwave spectroscopy contributes to fields from chemistry to astronomy.
Heating
At around 2.45 GHz frequency specifically, microwaves easily penetrate most food items without excessive absorption by dishware. The waves energize water and fat molecules for rapid heating. This selectivity gives microwave ovens advantageous speed and efficiency for thawing, cooking, and reheating foods.
Industrial microwave heating is also becoming more widespread for processes like drying lumber, vulcanizing rubber, and recycling plastics. The direct energy transfer minimizes heating of surrounding materials for faster, more consistent results.
Real World Microwave Examples
Microwaves already impact lives worldwide in fallen snow forecasts, satellite weather bureaus, precisely thawed medical treatments, evenly baked potato skins, and streaming binges. Here are some other notable implementations:
Doppler Weather Radar
The National Weather Service employs 160 high resolution weather radars that bounce 10cm microwaves off precipitation and clouds to track their direction. This informs predictions from local thunderstorms to the next potential hurricane landfall.
Global WiFi
The new WiFi 6E standard taps into the 6GHz band for faster yet reliable service. Microwave frequencies here allow signals to be focused into beams that stay cramped indoor areas instead of dissipating through walls and windows into the ether.
Microwave Ovens
Affordable microwave ovens first reached consumers in the late 1960s and now appear in over 90% of US households. Convenient keys, turntables, and multi-stage cooking presets make them staples for quick meals, even if they can‘t achieve the satisfying crispness of other methods.
VLBI Radio Telescope Networks
Groups of radio telescopes leverage very long baseline interferometry (VLBI) at microwave frequencies to achieve the resolution of dishes thousands of kilometers across. Facilities like the Event Horizon Telescope produced the first black hole images with this technique in 2019.
Microwave Position on the EM Spectrum
Microwaves inhabit the radio wave portion on the higher frequency end of the electromagnetic spectrum, which also contains, in order of decreasing wavelength and increasing energy: radiowaves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.
Their position gives them an optimal balance of penetration depth and information/energy carrying capacity. Microwaves also constitute the dominant natural signal from the early universe in the form of the cosmic microwave background.
Microwave Bands for Communications, Radar, and More
To better organize utilization for radar and telecommunications, bands designated by letters subdivide portions of the microwave range. Some common ones include:
| Band Letter | Frequency Range | Example Uses |
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| L band | 1 – 2 GHz | GPS, mobile phones, air traffic control |
| S band | 2 – 4 GHz | Weather radar, microwave ovens, WiFi |
| C band | 4 – 8 GHz | Satellite networks, WiFi |
| X band | 8 – 12 GHz | Military & weather radar, satellite TV |
| Ku band | 12 – 18 GHz | Satellite communications, cable TV |
| K band | 18 – 26.5 GHz | Police radar, amateur radio |
| Ka band | 26.5 – 40 GHz | High-resolution weather & military radar |
| Q band | 33 – 50 GHz | Military radar, airport security scanners |
| V band | 50 – 75 GHz | Millimeter wave imaging, short range communications |
| E band | 60 – 90 GHz | High-speed wireless networks, autonomous vehicles |
| W band | 75 – 110 GHz | Millimeter wave imaging, high-resolution radar |
Microwaves vs. Radio Waves
Radio waves and microwaves are commonly confused or conflated given their adjacent positions on the EM spectrum and overlapping frequencies in the 0.3-3 GHz range. However, there are some notable differences:
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Propagation: Lower frequency radio waves can diffract around large obstacles and travel long distances by bouncing between the ground and ionosphere. Microwaves diffract less and aren’t reflected by the ionosphere, limiting their terrestrial range.
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Information transmission: Thanks to their longer wavelengths, radio waves can impress intelligible audio communications. Microwaves have shorter wavelengths more suited to encoding data.
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Directivity: Due to their shorter wavelengths, microwave beams spread out less and can be focused into tight directional beams using appropriately sized antennas.
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Equipment: While simple passive components suffice to generate radio waves, specialized vacuum tube sources like klystrons or magnetrons are required to produce microwaves.
So in summary – think radio as sound-based long haul communication and microwaves as directed, localized, high frequency data carriers.
Health Concerns Around Microwaves
Microwaves utilized by devices like mobile phones, WiFi routers, smart meters, and ovens operate at levels well below those thought to cause adverse health effects from sustained exposure based on current research.
However, microwaves are a form of non-ionizing radiation, meaning they don‘t carry enough energy per quantum to ionize atoms or molecules. Their primary effect is heating tissue.
Governmental organizations like OSHA provide strict exposure limit guidelines employers must follow to protect workers using microwave devices continuously. The FDA also enforces specific absorption rate (SAR) maximums for modern consumer electronics so heating remains negligible.
More research is still needed to conclusively determine if regular low-level exposure like from cell phones increases cancer risks as some studies suggest. Until then, precautions like not keeping phones in pockets for prolonged periods or directly on the body may mitigate unconfirmed dangers.
Microwaves in Space Exploration
Beyond their discovery in cosmic background radiation decades ago, microwaves continue serving pivotal roles in studying spacetime and celestial phenomena.
Space Telecommunications
Given their high frequency modulated data capacity and penetration through the atmosphere, microwaves enable the vast majority of spacecraft communications and data transfer whether to relay astronauts voices or rover images from across the solar system. Networks of sensitive ground dishes receive and transmit signals.
Spacecraft Navigation
Interplanetary vehicles transmit microwaves that are picked up by giant antenna arrays on Earth to precisely track their speed and trajectory across millions of miles. Without this radar ranging, minute course corrections would not be feasible.
Astronomy
Tree kilometer-scale radio telescope networks with ultra sensitive receivers detect the faintest microwave signals from stars being distorted by blackhole gravity or from molecule clouds coalescing into new star nurseries. Even relic radiation from the universe’s origins falls within microwave frequencies.
The Future of Microwaves
From powering smartphones globally to enabling autonomous vehicles to revealing the hidden universe, microwaves will continue opening new capabilities:
6G Research
With 5G cellular networks just being initially deployed, laboratories have already begun R&D into successors that could operate over 90 GHz into the millimeter wave range. This promises simultaneous connections for millions of devices with extreme low latency – perhaps enabling remote robotic surgery across cities. Realistic implementations are at least a decade down the road however.
High Resolution Radar
Higher frequency microwave radars using shorter millimeter wavelengths and sophisticated processing offer centimeters-scale resolution from satellites, a 10x improvement revealing city details like manholes, mailboxes, and lane markings.
WiFi Advancements
The WiFi 7 standard now in development will tap previously unused 6 GHz and 7 GHz frequencies to deliver smoother streaming and VR experiences in congested urban environments where slower bands are overloaded.
Wireless Data Centers
Technology leaders like Facebook aim to replace cables in data centers with free space lasers and directed microwave beams that never wear out and can dynamically redirect capacity. This could greatly speed information flows and reduce power consumption.
So in summary, from communications, to biological analysis, and perhaps even powering colonies on Mars one day, invisible microwave photons will continue broadening horizons for decades to come through their unique EM abilities.
