Microwave Technology

Summary

A microwave is an electromagnetic wave ranging from one meter to one millimeter. Microwave energy has a frequency ranging from 0.3 gigahertz (GHz) to 300 GHz. The high frequency of microwaves provides the microwave band with a very large information-carrying capacity. The band has a bandwidth thirty times that of the radio spectrum below it. Microwave signals propagate in straight lines, and thus, are limited to line-of-sight transmission. Unlike lower-frequency radio waves, they cannot pass around hills or mountains and are not refracted or reflected by atmospheric layers. In addition to the familiar microwave oven, applications include communications, radar, radio astronomy, navigation, and spectroscopy.

Definition and Basic Principles

In contrast to sound waves, which require a medium such as air or water for propagation, microwaves can travel through a vacuum. In a vacuum such as outer space, they travel at the speed of light (299,800 kilometers per second). Microwaves travel in a straight line, limiting them to line-of-sight applications. In space, microwaves conform to the inverse-square law. The power density of a microwave is proportional to the inverse of the square of the distance from a point source. All microwaves weaken as they travel a distance. At some point, depending on the strength of the signal, the microwave will no longer be discernible. Interference can weaken or destroy a radio signal. Other microwave transmitters in the same frequency range produce interference. Their small wavelength allows small antennae to direct the electromagnetic energy in narrow beams, which can be pointed directly at the receiving antenna. This feature allows nearby microwave equipment to broadcast on the same frequencies without interfering with each other, as lower-frequency radio waves do.

Microwave ovens pass radiation, usually at a frequency of 2.45 GHz, through food. Energy from the microwaves is absorbed by fat, water, and other substances in the food through a process known as dielectric heating. Water and many other molecules have a partial positive charge at one end and a partial negative charge at the other. They rotate in an attempt to align themselves with the alternating electric field of the microwaves. This molecular movement produces heat.

Background and History

Electromagnetic waves were discovered in 1877 by the German physicistHeinrich Hertz, whose name is used to describe radio frequencies in cycles per second. Eight years later, American inventor Thomas Edison obtained a patent for wireless telegraphy by discontinuous (intermittent) waves. A far superior system was developed in 1894 by the Italian inventor Guglielmo Marconi. Marconi initially transmitted telegraph signals over a short distance on land. Subsequently, an improved system was capable of transmitting signals across the Atlantic Ocean.

Much of microwave technology was developed during World War II for radar applications. The technology was developed secretly. It became available for public use only after the war. In 1951, AT&T's new microwave radio-relay skyway carried a telephone call via a series of 107 microwave towers that were spaced about 30 miles apart. This was the first microwave application that could carry telephone conversations across the United States via radio (as opposed to wire or cable). The system could also carry television signals. Three weeks after the first telephone call, at least 30 million people saw and heard President Harry Truman open the Japanese Peace Treaty Conference in San Francisco. Then, in 1946, Percy Spencer, an engineer at the Raytheon Corporation, developed the now-ubiquitous microwave oven.

How It Works

Communication. Because of the short wavelength, microwave radio transmission employs small, highly directional antennae, which are smaller and therefore more practical than ones used for longer wavelengths (lower frequencies). Considerably more bandwidth is available in the microwave spectrum than in lower frequencies. This wider bandwidth is suitable for the transmission of video and audio. Since microwave transmission is a line of sight, distance is limited by the curvature of the Earth. A higher antenna can transmit over a greater distance. Much greater transmission distances between the Earth's surface and an orbiting satellite are possible because the only limitation is the attenuation that occurs from the atmosphere. However, even in the vacuum of outer space, the signal degrades over a distance.

Energy Transmission. The concept of using microwaves for the transmission of power emerged following World War II. High-power microwave emitters, also known as cavity magnetrons, were developed. These emitters can transfer electrical energy from a power source to a target without interconnecting wires. Wireless transmission is reserved for cases in which interconnecting wires are inconvenient, dangerous, or impossible. To be effective and economical, a large part of the energy sent out by the generating plant must arrive at the receiver(s). The short wavelengths of microwave radiation can be made more directional than lower radio frequencies, allowing power to beam over longer distances. A rectenna (rectifying antenna) at the target can convert the microwave energy back into electricity. Conversion efficiencies of more than 95 percent have been achieved with rectennae.

Microwave Ovens. A microwave oven passes non-ionizing microwave radiation, usually at a frequency of 2.45 GHz, through food. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, which means they have a partial positive charge at one end and a partial negative charge at the other. They rotate to align themselves with the alternating electric field of the microwaves. This movement represents heat, which is dispersed as the rotating molecules strike each other.

Navigation. Global Positioning Systems (GPSs) operate in microwave frequencies ranging from about 1.2 GHz to 1.6 GHz. GPS is a space-based global navigation satellite system, which provides accurate location and time information at any place on Earth with an unobstructed line of sight to four or more GPS satellites. GPS can function under any weather conditions and anywhere on the planet. Since it cannot function underwater, a submarine must surface to use a GPS. The technology depends upon triangulation, just as land-based systems do, to locate a discrete point. GPS has three segments—the space segment, the control segment, and the user segment. The U.S. Air Force operates and maintains both the space and control segments. The space segment is made up of satellites, which are in medium-space orbit. The satellites broadcast signals from space, and a GPS receiver (user segment) uses these signals to calculate a three-dimensional location (latitude, longitude, and altitude). The signal transmits the current time, accurately within nanoseconds.

Radar. Radar is an acronym for radio detecting and ranging. The device consists of a transmitter and a receiver. The transmitter emits radio waves, which are deflected from a fixed or moving object. The receiver, which can be a dish or an antenna, receives the wave. Radar circuitry then displays an image of the object in real-time. The screen displays the distance of the object from the radar. If the object is moving, consecutive readings can calculate the speed and direction of the object.

If the object is airborne, and the device is so equipped, the altitude is displayed. Radar is invaluable in foggy weather when visibility can be severely reduced.

Radio Astronomy. Radio astronomy is a subfield of astronomy that examines celestial objects, which emit radio frequencies. Much of radio astronomy is focused on the microwave band.

Spectroscopy. Spectroscopy involves using spectrometers and spectroscopes to analyze the distribution of atomic or subatomic particles in a system, such as a molecular beam. Microwave spectroscopy is a form of spectroscopy in which information is obtained on the structure and chemical bonding of molecules and crystals by measuring the wavelengths of microwaves emitted or absorbed by them. Microwave spectroscopy can be conducted only on gases.

Applications and Products

Communication. Microwaves are commonly used by communication systems on the Earth's surface, in satellite communications, and in deep-space radio communications. Microwaves are commonly used by television news media to transmit audio and video from a specially equipped van to a television station. Mobile telephone networks operate at the lower end of the microwave band, while others operate at frequencies just beneath the microwave band. Networks of microwave relay links have largely been replaced by fiber-optic networks. Wireless transmission employing local area network (LAN) protocols such as Bluetooth operates in the 2.4 GHz microwave band. Other LAN protocols operate at higher microwave frequencies. Wireless Internet services operate in the 3.5–4.0 GHz range.

Energy Transmission. Although wireless power transmission via microwaves is well-proven, many applications are still experimental. In 1964, a miniature helicopter propelled by microwave power was demonstrated. In 2008, 20 watts of power were transmitted 92 miles from a mountain on Maui to the big island of Hawaii. The U.S. military uses microwave energy transmission as a form of sublethal weaponry. The application, known as an Active Denial System, uses microwaves to heat a thin layer of human skin to an unbearable temperature, which forces the recipient to move away from the energy source. The skin can be heated to a temperature of 54 degrees Celsius at a depth of 0.4 mm with a two-second burst of a 95-GHz-focused beam.

Microwave Ovens. A microwave oven is a common household appliance. It rapidly heats frozen foods, pops popcorn, bakes potatoes, and boils water. Its compact size makes it beneficial when space is at a premium. It is used on commercial airlines for meal preparation and is found in many other locations, including offices. In addition, microwave heating is employed in many industrial processes for drying and curing products.

Navigation. GPS applications, which operate in the microwave band, are used for navigation over land and water, and numerous military, civilian, and commercial applications exist. GPS devices are used for navigation on military and commercial vessels, on pleasure boats, and in automobiles. The device gives a visual display of the vehicle's position on a map overlay. Useful information such as distance to the next turn and the destination is given visually and aurally if the GPS is equipped with an audio system. Some vehicle GPS devices also provide information like the nearest gasoline stations, rest areas, restaurants, and hospitals. GPS units for off-road use are also available. The motorist on a budget can purchase an inexpensive handheld GPS. These devices are popular with campers and hikers. Many small aircraft and virtually all large aircraft contain a GPS. GPS is also used for space navigation. Another application of GPS is an ankle monitor, which tracks the location of individuals under house arrest. In addition to their use in GPS technologies, microwaves are used in the collision-detection systems of self-driving cars.

Radar. Radar is an essential component of any vessel that operates offshore. Navy vessels and many pleasure boats have radar. Larger vessels often have several. Aircraft are guided by land-based radar installations, known as airport surveillance radar (ASR), located at civilian and military airfields. An ASR tracks airport positions and weather conditions in the vicinity of the airport.

Radio Astronomy. Most radio astronomy applications operate in the microwave spectrum. Usually, naturally occurring microwave radiation is observed. However, radio astronomy has been used to measure distances precisely within the solar system. Radio astronomy has also been employed to map the surface of Venus, which is not visible via optical telescopes because of its dense cloud cover. The technology has expanded astronomical knowledge and has led to the discovery of new objects, including radio galaxies, pulsars, and quasars. Radio astronomy allows objects that are not detectable with an optical telescope to be seen. These objects are some of the most extreme and energetic physical processes that exist in the universe. Since microwaves penetrate dust, radio astronomy techniques can study the dust-shrouded environments where stars and planets are born. Radio astronomy is also used to trace the location, density, and motion of hydrogen gas, which constitutes about 75 percent of the ordinary matter in the universe. Another focus of radio astronomy is mapping the cosmic microwave background radiation (CMBR), a faint background radiation left over from the Big Bang that fills the universe. By studying the CMBR, radio astronomers can learn about the universe's formation and the conditions shortly afterward.

Spectroscopy. Microwave radiation is used for electron paramagnetic resonance, also called electron spin resonance, analysis, which has been crucial in developing the most fundamental theories in physics, including quantum mechanics, the special and general theories of relativity, and quantum electrodynamics. Microwave spectroscopy is essential for developing a scientific understanding of electromagnetic and nuclear forces.

Careers and Course Work

Many technical and nontechnical careers are available in microwave technology. The technical fields require a minimum of a bachelor's degree in engineering or another scientific field, but many also require a master's degree or doctorate. Coursework should include mathematics, engineering, computer science, and robotics. Positions are available in the government and private sectors. The ability to be a team player is often of value for these positions because ongoing research is often a collaborative effort.

Technicians are needed in a variety of fields for equipment repair. Many of these positions require some training beyond high school at a community college or trade school. If a company employs several technicians, supervisory positions may be available. Numerous opportunities are available for sales.

Social Context and Future Prospects

Microwave technology is an integral component of everyday life in the modern world. Given the continuous advances in radio technology, including microwaves, further advances are likely. Additionally, new applications for existing microwave technology, such as the use of microwaves in steering autonomous vehicles, are still being discovered. In 2021, the Xaver 1000 handheld scanner was developed by Israel's Camero Tech. The scanner uses microwave technology to track movement through walls, such as people trapped in a fire. The technology could also be used to locate space objects traveling toward Earth.

The microwave oven demonstrates that electromagnetic waves are a form of energy capable of heating, thus damaging tissue. Microwave ovens are shielded to prevent exposure, but other microwave devices are not. For example, if one stands near or touches a microwave transmitter, severe burns can result. The heating effect of an electromagnetic wave varies depending on its power and frequency. The Institute of Electrical and Electronics Engineers (IEEE) and many national governments have established safety limits for exposure to various frequencies of electromagnetic energy. Controversy still exists as to whether microwave energy can be harmful to humans and animals.

Low levels of microwave radiation have no proven harmful effect. Examples of low-level microwave radiation are the small amount of leakage from a microwave oven and microwave transmission from a cell phone. Some experts express concern that prolonged low-level exposure (for example, holding a cell phone to one's ear for extended periods) might be harmful. Countering this concern is research that has involved exposing multiple generations of animals to microwave radiation at the cell phone intensity or higher, and no health issues have been found. High levels are harmful. An example is the U.S. military's Active Denial System, which can heat human skin to an intolerable level.

Bibliography

Balbi, Amedeo. The Music of the Big Bang: Cosmic Microwave Background and the New Cosmology. Springer-Verlag, 2010.

Benford, James, et al. High Power Microwaves. 3rd ed., CRC Press, 2016.

Eldridge, Alison, and Stephen Eldridge. Understanding and Using Microwaves. Enslow Publishing, 2022.

Hu, Q., et al. Microwave Technology: A Novel Approach to the Transformation of Natural Metabolites. Chin Med, vol. 16, no. 87, 2021. doi.org/10.1186/s13020-021-00500-8.

Kamenetskii, Eugene O., et al. Fano Resonances in Optics and Microwaves: Physics and Applications. Springer Nature Switzerland AG, 2018.

"Microwaves." NASA, 2023, science.nasa.gov/ems/06‗microwaves. Accessed 20 May 2024.

Rao, Rahul. "A New Microwave Scanner Can Track Moving Objects through Walls, Superman-Style." Popular Science, 9 July 2021, www.popsci.com/science/microwave-imaging-technology-see-through-walls. Accessed 9 June 2022.

Rudel, Anthony. Hello, Everybody! The Dawn of American Radio. Houghton Mifflin Harcourt, 2008.

"Silicon Breakthrough Could Make Key Microwave Technology Much Cheaper and Better." Phys.org, 24 May 2018, phys.org/news/2018-05-silicon-breakthrough-key-microwave-technology.html. Accessed 30 Aug. 2018.

Strobel, Otto. Optical and Microwave Technologies for Telecommunication Networks. Wiley, 2016.