What is the first thing that comes to your mind when you hear the term “Microwave?” Well, it must be the microwave oven that you might have used recently for reheating or cooking food. A microwave oven is indeed the most popular application of microwave radiation; however, if you think that cooking is the only thing microwaves are good for, you are broadly undervaluing their importance in our daily life. Before we discuss other uses of microwaves, let’s first understand what do we mean by the term “Microwave?” The visible light that allows us to see the things present around us is a part of the Electromagnetic spectrum that contains several other types of radiations. Essentially, all the EM radiations are transverse electric and magnetic waves traveling at the speed of light (only in free space) with different frequencies and wavelengths. Microwave is one such EM radiation whose wavelength lies between the range of {10}^{-3} to {10}^{-1} meters, hence the name “microwave.” The corresponding frequencies lie between the range of {3}{×}{10}^{9} - {3}{×}{10}^{11} Hz, which means that microwaves are more energetic, and hence these are more suitable for carrying signals with less attenuation to a far range. Such dimensions have made microwaves employable in several applications over time. Let’s take a look at few uses of microwaves:
Uses of Microwaves
1. Wireless Communication
Wireless communication is one of the most outstanding marvels of microwave technology. Let’s try to understand how microwaves help transmit data around the globe. Whenever you use your mobile phone (or other wireless devices such as laptops, tablets, etc.), either for the internet or making a voice call, it sends or receives information in the form of invisible microwave radiations. These microwaves are picked up by the cell antenna, transmitted towards the destination antenna, and then finally to the end-user. More than half the world’s mobile transmission takes place over secure microwave network links. Cost-effectiveness is one of the most prominent peculiarities that makes microwaves a preferable option for wireless communication among the EM spectrum. Microwaves are cheaper to generate, far quicker to install, and almost as secure as the cable transmission. Due to their low attenuation, microwaves can travel efficiently through air, smoke, rain, or frost; however, their range is limited by the curvature of the earth as the microwave is a line-of-sight technology. This problem is encountered by incorporating optical fibers in the transmission process. Optical fiber helps transmit the data to the regions where microwaves are inadequate because of the earth’s curvature, or the areas where the mountains may cause the hindrance. Over short-range distances (a few kilometers), microwave links can provide gigabits of capacity, enough for millions of people to upload a Facebook post at the same time. Here is the list of few microwave-based communication technologies that you may come across in your daily life.
- Bluetooth
- GSM, 2G through 4G.
- Wireless Broadband Systems (Wi-Fi)
- Wireless Local Area Networks (WLAN)
- Outdoor Broadcasting Transmission (e.g. News Vans)
- Linking remote and regional telephone exchanges to main exchanges without the need for copper/optical fiber lines
- Aircraft Communications Addressing and Reporting System (ACARS)
- Satellite-Dish Antenna
- Spacecraft Communication Systems
2. Navigation
For centuries, mankind has been developing several methods to bring precision to their perception of geological position and navigation. Thanks to the satellite navigation systems (satnavs), humans no longer rely on stars to guide them through a non-familiar terrain. Many of us are familiar with Global Positioning System (GPS) that locates our position on earth. It is a three-part system including satellites, ground stations, and receivers. GPS uses microwaves with each signal having a unique frequency, wavelength, amplitude, phase, or some combination of these parameters. The microwave signals emitted by these satellites are used to calculate how far they are from each other (at least three satellites), and also from the device whose location they are measuring. This process is known as Trilateration. Nowadays, there are several regional satnav systems like the USA’s GPS, India’s NAVIC, Russia’s GLONASS, China’s BieDuo Navigation System, European Union’s Galileo, etc.
3. Radar
Microwave technology has been an integral part of several military applications since the beginning of the Second World War. In fact, microwave technology is widely considered as something that changed the course of World War II. In particular, the device that incorporated microwave technology was the radar (Radio Detection And Ranging). It is a radiolocation technique in which a radio wave beam is emitted and recollected after it bounces back from any obstacle in the path. Before the Second World War, shortwave radio waves, with frequencies ranging from 3-30 MHz, were used for the detection of aircraft, ships, and other artillery vessels. With the advancement in airforce technology, these frequencies were not that efficient for defense. Although long-range microwaves had been discovered long before the Second World War, the tools required for their generation were not available till 1920, when Albert Hull, an American physicist, first discovered the cavity magnetron. The Hull magnetron was tested as an amplifier in radio receivers and also as a low-frequency oscillator. It was found to generate a power of 15 kW at a frequency of 20 kHz. During World War II, John Randall and Harry Boot built the modern cavity magnetron based on Hull’s concept, the first device that could produce high-power microwave frequencies, resulting in centimeter-band radar. Nowadays, his technology is being used in several sectors of various industries including aircraft location, marine traffic navigation, by meteorologists for weather forecast operations, and also by law enforcers to keep a check on overspeeding vehicles by measuring the doppler effect.
4. Spectroscopy
Spectroscopy is an analytical technique primarily based on the interaction of matter with EM radiations. It is one of the most important tools to understand the structure and behavior of molecules. Atoms and molecules change their state when they interact with EM radiations. The change can be observed as the emission of photons, causing a change in some of the specific properties of the atom or molecule under study. Using microwaves for spectroscopy is mainly concerned with the transition of rotational energy levels in the molecules; however, only molecules with a permanent dipole that changes upon rotation can be investigated using microwave spectroscopy. This is because there must be a charge difference across the molecule for the oscillating field of the photon to impart a torque upon the molecule around an axis that is perpendicular to this dipole and that passes through the molecule’s center of mass. Microwave spectroscopy utilizes the photons in the microwave region to cause transitions among the quantum rotational energy levels of the molecules. One of the most widely used spectroscopy techniques that incorporates microwave frequency is:
ESR or EPR: Electron Spin Resonance, also known as Electron Paramagnetic Resonance, is a spectroscopy technique used to study the molecules with unpaired electrons. When a magnetic field is applied to such an electron, it exerts a torque on the dipole moment of the electron (the dipole moment of the electron arises from the intrinsic angular momentum, or “spin” of the electron). This torque causes the splitting of the otherwise sharp spectral lines associated with the principal quantum number n into multiple closely spaced lines associated with the spin quantum number, specifying the orientation of the electron in the space (Zeeman Effect). When microwaves are applied to such a system, a microwave photon gets absorbed by the electron, causing the transition among two spin quantum states, satisfying the resonance condition. It helps in the determination of Lande’s g-factor by measuring the field and the frequency at which resonance occurs, which in turn gives information about the nature of the atomic or molecular orbital containing the unpaired electron.
5. Radio Astronomy
Since the dawn of human civilization, our ancestors were fascinated with the tiny sparkling objects present in the night sky, which we now classify as celestial objects (e.g., stars, planets, moons, asteroids, etc.). Thanks to microwave technology that we can widen the scope of our understanding, not only to the present but also to the past of our universe. Most of us are familiar with the static, or noise (black and white pixels dancing randomly), that we see on an analog tv screen when there is no specific signal coming through the dish antenna. On accounting for all the interferences that may occur in the atmosphere, a sizeable amount of the signal for which this static account is the electromagnetic waves that fall under the microwave region of the spectrum. What is the source of these microwaves? Well, it may surprise you, but some of that static is a picture of our newly born universe.
The best understanding that we have about the origin of our universe is through the Big Bang Theory. When the Big Bang occurred 13.8 billion years ago, the whole universe came into existence as a blob of an enormous amount of energy. Around 400,000 years later, it was a hot and dense sphere of supercharged plasma with several thousand degrees of temperature. Just like every hot thing emits light, this superhot ionic plasma was also emitting EM radiations and since the temperature was too high for neutral atoms to form, these EM radiations couldn’t travel very far before they run into an electron and bounce back. As this temperature cooled down below the ∼ 3000K mark, the neutral atoms began to form, allowing the previously trapped EM radiation to expand as far as the universe goes. The wavelength of EM radiation changes as they travel through the expanding universe (cosmological redshift). Given 13 billion years of expansion, that trapped light is now present everywhere in the universe as the Cosmic Microwave Background Radiation (the oldest light in the universe). In 2003, Wilkinson Microwave Anisotropy Probe (WMAP) mapped the pattern of tiny fluctuations in the Cosmic Microwave Background (CMB) radiation and produced the first fine-resolution (0.2 degrees) full-sky map of the microwave sky. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy. In addition to receiving naturally-occurring microwave radiation, radio telescopes have been used in active radar experiments to bounce microwaves off planets in the solar system, to determine the distance to the Moon, or map the invisible surface of Venus through cloud cover.
The full-sky image of the temperature fluctuations (shown as color differences) in the cosmic microwave background, made from nine years of WMAP observations. These are the seeds of galaxies, from a time when the universe was under 400,000 years old.
Credits: NASA
6. Microwave Ablation
From the standpoint of microwaves being non-ionizing in nature, they can be safely employed for medicinal applications. They have energy significant enough to penetrate the tissue without causing any harm to it. One of the most prominent applications of microwaves in the field of medicine is microwave ablation (Ablation is a term used in medicine to describe the removal of tissue either by surgery or less invasive techniques). It is a form of interventional radiography that helps in the treatment of benign tumors and cancer. In this process, microwave energy is used to create localized dielectric heating to desiccate the unwanted tissues. Common medical areas of application include oncology, cardiology, gynecology, rhizotomy, otolaryngology (ENT), ophthalmology, cosmetic treatments, and dental treatments. Cancer patients who are poor surgical candidates can also benefit from microwave ablation as it is minimally invasive. Another important factor is the availability of a frequency that can be decided depending on the size of the tumor; however, while using microwaves to treat any condition, it is vital to take into account the changing dielectric properties of tissue during the treatment. Any imprecision in these measurements has the potential to result in either insufficient power resulting in poor treatment or excessive power inadvertently causing serious patient injury.
7. Microwave Oven
The microwave oven is a well-known by-product of the above-mentioned radar technology. The magnetron tubes, which were initially used in the development of long-range military radar, gained their commercial application after World War II. Although the scientific community was familiar with the heating characteristics of radio waves since the 1920s, it was not until 1945 that Percy Spencer, a self-taught American engineer, accidentally discovered the thermal effect of a high-powered microwave beam. On 8th October 1945, Spencer patented the microwave cooking process and an oven under the Raytheon company. For the detailed description of the microwave oven’s working, kindly refer to Microwave Oven Working Principle.