What are Microwaves?
Microwaves are electromagnetic (EM) waves whose wavelengths range from one millimetre up to one metre, with frequencies that range from 300MHz to 300GHz.
These waves are located between radio waves and infrared waves on the EM spectrum. Microwaves are classified as non-ionising radiation: this means that they excite molecules and atoms of substances (such as food, water, and fat) but don’t possess enough energy to break them up into ions. Thus, they are relatively safe to handle and non-carcinogenic.
A typical, modern example of microwave technology: a microwave tower with four antennas, peeking over the roof of a building. Microwave towers are designed to be sufficiently high to provide a clear ‘line of sight’ for data transmission.
History of Microwaves
It was not until over two decades later, however, in 1888, when physicist Heinrich Hertz successfully detected microwaves in the electromagnetic spectrum using several experiments that confirmed Maxwell’s theories. Hertz used a spark-gap radio transmitter, namely a device that generated EM waves from an electric spark, to produce microwaves in the ultra-high frequency (or UHF) regions.
In 1931, a U.S.-French consortium, led by pioneer Andre Clavier, successfully created a microwave relay link over the English Channel using parabolic antennas. This set-up allowed communications via telephone, telegraph, and facsimile (i.e. fax) between the United Kingdom and France.
American Telephone and Telegraph (AT&T) went on to adopt microwaves for its long-distance telephone and television lines in the 1950s. Microwave is still being used today, as we’ll discuss shortly.
A picture of a magnetron part, which is used to produce microwaves.
How Microwaves Are Produced
Microwaves can be produced by special vacuum tube devices that contain Magnetron (see image above) or Klystron diodes. These high-powered vacuum tubes use the kinetic energy of a focused electron beam to amplify high-frequency signals.
Properties of Microwaves
Hertz’s experiments showed that microwaves had similar properties to light: they can be reflected, refracted, diffracted, polarised, and subject to interference—all of which make them valuable for signal transmission.
Some physical properties of microwaves are as follows:
They can pass through the atmosphere
They are absorbed by water molecules
They can pass through transparent objects, such as plastics or glass
They induce heating in certain materials by causing the molecules to vibrate
Microwave technology is not just a vital part of communications. Pictured: a typical microwave oven on a kitchen counter.
Modern Applications of Microwaves
Microwave technology enables a host of modern domestic, industrial, and commercial applications. These include:
Point-to-point Communication Systems
Microwaves enable point-to-point communications on earth and in space. On Earth, microwave beams can be used to transmit and receive voice, video, and data information (in analogue or digital form) across varying distances. This system is known as a microwave link.
In space, microwaves are used in deep space communications, such as NASA’s Deep Space Network. Satellite communication systems utilise electromagnetic waves in the microwave region for fixed satellite service or direct broadcast satellite.
Radar and Navigation Systems
The short wavelength and high frequency of microwaves allow for the accurate location of objects, such as air traffic control stations for aircraft and navigation systems for ships and submarines. Parabolic antennas are used to direct the microwaves which cause wide reflections as they bounce off the surface of aircraft or marine vessels. Navies use radar systems for collision avoidance or monitoring the position of enemy ships under the sea.
Domestic and Industrial Heating
A microwave oven is a type of electric cooker that increases the temperature of food by exposing it to microwave radiation. The principle of operation is to heat the food in a confined metal space by introducing polar molecules, which produce heat energy by exciting the particles—a process known as dielectric heating. Microwave ovens are used to re-heat already-cooked food. At industrial facilities, dielectric heating can be used to dehydrate or cure products.
An example (based in Bavaria, Germany of a parabolic satellite for microwave transmission. Image Credit: Wikimedia Commons.
Comparing Microwave and RF Waves
Although some engineers tend to use the terms ‘microwave’ and ‘radio frequency waves’ interchangeably, there are some distinctions between them: microwaves have higher frequencies (300MHz to 300GHz) than RF waves (20kHz to less than 300GHz).
Also, RF waves are mostly used in mobile, AM/FM radios, and TV communications, while microwaves are used more often in satellite, radar, and space communications.
Limitations of Microwaves
The most significant drawback of microwave technology is the requirement of a clear line of sight between end nodes to prevent interference in the transmitted signals. Clear lines of sight are pathways free from obstructions, such as tall buildings and trees).
To minimise interference, engineers usually erect transmitters and receivers on tall structures like masts and towers, which increases the costs of building microwave links. Atmospheric conditions can also impact the integrity of microwave communications: in humid weather, for instance, microwaves are attenuated by molecular oxygen in the atmosphere.
Taking Stock of Modern Microwave Technology
Microwaves continue to be a useful form of EM radiation for several technological purposes. Although line-of-sight technology is challenging to design and manage in communication systems, high-gain parabolic antennas have been designed to send microwave beams over great distances.
Owing to their higher frequencies, microwave bands have some of the largest information-carrying capacities in the electromagnetic spectrum—certainly a major reason why the technology is still relevant today.