82D - Electromagnetic Waves

Electromagnetic waves are essentially a self-propagating transverse wave of electric and magnetic fields. Electromagnetic waves are comprised of both electric and magnetic waves. These waves oscillate perpendicularly to and in phase with one another.

The genesis of all electromagnetic waves begins with a charged particle. This charged particle has an electric field. When it moves, the charged particle not only maintains its electric field, but also produces a magnetic field. Once in motion, the electric and magnetic fields a charged particle creates are self-perpetuating: time-based changes in one field (electric or magnetic) affect the other.

The most common wireless technologies use electromagnetic wireless telecommunications, such as radio or infrared signals. With infrared waves, distances are short (such as a few meters for television remote control) while radio waves can reach as far as thousands or even millions of kilometers for deep-space radio communications. It encompasses various types of fixed, mobile, and portable applications, including two-way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking.

82D - Electromagnetic Waves

Here you will learn to:

Explain how electromagnetic waves are produced and how they propogate through a vacuum,

Describe the properties and uses of different waves in the electromagnetic spectrum,

Apply the principles of the Inverse Square Law to the intensity of electromagnetic radiation, and

Explain how electromagnetic waves are used for communication purposes.

What Are Electromagnetic Waves?

Electromagnetic waves are waves that can travel through the vacuum of outer space because they do not need a medium. Mechanical waves, unlike electromagnetic waves, require a medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves. Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. The changing electric field induces and changing magnetic field (and vice-versa) perpendicular to one another and this is what causes the wave to propogate without a medium. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 ms-1 (a speed value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 ms-1.

Electromagnetic waves are formed by the vibrations
of electric and magnetic fields. These fields are
perpendicular to one another in the direction the
wave is traveling. Once formed, this energy travels
at the speed of light until further interaction with matter.

In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations." Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency is named the hertz, in honor of Heinrich Hertz. His experiment with radio waves solved two problems. First, he had demonstrated in the concrete, what Maxwell had only theorized - that the velocity of radio waves was equal to the velocity of light. This proved that radio waves were a form of light. Second, Hertz found out how to generate the electric and magnetic fields needed to produce and detect an electromagnetic wave by making electrons in an antenna oscillate at the same frequency as the wave.

The sources of low frequency electromagnetic waves are accelerating electric charges such as when electrons oscillate. A common example is the generation of radio waves by oscillating electrons in an antenna. When a charge moves in a linear antenna with an oscillation frequency f, the oscillatory motion constitutes an acceleration, and an electromagnetic wave with the same frequency propagates away from the antenna. At frequencies above the microwave region, the classical picture of an accelerating electric charge producing an electromagnetic wave is less and less applicable. In the infrared, visible, and ultraviolet regions, the primary radiators are the charged particles in atoms and molecules. In this regime a quantum mechanical radiation model is far more relevant.

Electromangetic Spectrum

When you tune your radio, watch TV, send a text message, or pop popcorn in a microwave oven, you are using electromagnetic waves. You depend on this energy every hour of every day. Without it, the world you know could not exist. Electromagnetic waves span a broad spectrum from very long radio waves to very short gamma rays. The human eye can only detect only a small portion of this spectrum called visible light. A radio detects a different portion of the spectrum, and an x-ray machine uses yet another portion. NASA's scientific instruments use the full range of the electromagnetic spectrum to study the Earth, the solar system, and the universe beyond.

Examples of radio waves include fm & am radio frequencies. Mircrowaves are used in microwave ovens and their lengths are about the size of a baseball. Infrared waves are much shorter - about the size of a thickness of paper. The heat energy that radiates off humans and other animals is in the infrared region. Television remote controls use infrared light to communicate with the television. Visible light is the only region of the spectrum that our eyes can sense. Ultraviolet light is the higher energy radiation that can cause sunburn. Smaller than the size of a water molecule is the length of an x-ray. An x-ray image of a broken bone is actually the shadow cast by x-rays on a piece of x-ray sensitive film. Gamma rays are the shortest and highest energy wavelengths in the electromagnetic spectrum. These waves can be smaller than atomic nuclei.

Our sun is a source of energy across the full spectrum, and its electromagnetic radiation bombards our atmosphere constantly. However, the Earth's atmosphere protects us from exposure to a range of higher energy waves that can be harmful to life. Gamma rays, x-rays, and some ultraviolet waves are "ionising," meaning these waves have such a high energy that they can knock electrons out of atoms. Exposure to these high-energy waves can alter atoms and molecules and cause damage to cells in organic matter. These changes to cells can sometimes be helpful, as when radiation is used to kill cancer cells, and other times not, as when we get sunburned.

The electromagnetic spectrum and associated sources, energies, wavelengths and frequencies.

NASA spacecraft provide scientists with a unique vantage point, helping them "see" at higher-energy wavelengths that are blocked by the Earth's protective atmosphere. Electromagnetic radiation is reflected or absorbed mainly by several gases in the Earth's atmosphere, among the most important being water vapor, carbon dioxide, and ozone. Some radiation, such as visible light, largely passes (is transmitted) through the atmosphere. These regions of the spectrum with wavelengths that can pass through the atmosphere are referred to as "atmospheric windows." Some microwaves can even pass through clouds, which make them the best wavelength for transmitting satellite communication signals. While our atmosphere is essential to protecting life on Earth and keeping the planet habitable, it is not very helpful when it comes to studying sources of high-energy radiation in space. Instruments have to be positioned above Earth's energy-absorbing atmosphere to "see" higher energy and even some lower energy light sources such as quasars.

Inverse Square Law

The relationships in the inverse square law as as
result of radiation being spread out in three
dimensions from a source.

Electromagnetic waves decrease in their energy and intensity as they move away from a srouce such as a light or antenna. This is because the energy spreads out in three dimensions away from the source and therefore becomes less intense.

The mathematical rendering of the inverse
square law.

If you imagine an antenna as the source of an electromagnetic wave, the waves spread out in three dimensions from the source. Imagine a series of consecutive spheres with different radii and the radiation passing through each sphere.

At a distance of one unit from the source, the diagram shows a certain amount of electromagnetic radiation passing through one area unit. When the distance is doubled, the same amount of electromagnetic radiation has to pass through an area nine times larger. Tripling the distance means that same amount of radiation has to pass through an area 16 times larger. Said another way, each time the distance increased proportionally, the intensity of the radiation passing through the same area is decreased to the inverse of the square.

If the distance is doubled, the intensity is reduced to one quarter and if the distance is tripled the intensity is reduced to one ninth. If the distance is halved, the intensity will increase by four times. This is known as the inverse square law.

The attenuation (decreasing intensity) of a transmitted signal can be a problem for communications that use electromagnetic radiation. You notice this as the radio station you are listening to on a long drive begins to lose reception as your distance from home increases. This problem can be remedied by transmitting at a very high power, or by amplifying the signal at the reception point. Amplification is essential if the transmission uses repeater stations or when the signal goes via a satellite. The mathematical relationship between the intensity of the energy falling on a given area and the distance of that area from the source of the energy is known as the inverse square law, summarised by the expressions on the left.

Communication with Electromagnetic Waves

Wireless communication is the transfer of information between two or more points that are not connected by an electrical conductor. The term is commonly used in the telecommunications industry to refer to telecommunications systems (e.g., radio transmitters and receivers, remote controls, etc.) that use some form of energy (e.g., radio waves, acoustic energy, etc.) to transfer information without the use of wires. Information is transferred in this manner over both short and long distances. Wireless operations permit services, such as long-range communications, that are otherwise impossible (or impractical) to implement with the use of wires.

The most common wireless technologies use electromagnetic wireless telecommunications, such as radio or infrared signals. With infrared waves, distances are short (such as a few meters for television remote control) while radio waves can reach as far as thousands or even millions of kilometers for deep-space radio communications. It encompasses various types of fixed, mobile, and portable applications, including two-way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of applications of radio wireless technology include GPS units, garage door openers, wireless computer mice, keyboards and headsets, headphones, radio receivers, satellite television, broadcast television, and cordless telephones. Less common methods of achieving wireless communications include the use of light, sound, magnetic, or electric fields.

The use of the atmosphere is radio and microwave communication.

One of the best-known examples of wireless technology is the mobile (or cellular) phone, with more than 4.6 billion mobile cellular subscriptions worldwide as of the end of 2010. These wireless devices use radio waves to enable their users to make phone calls from many locations worldwide. They can be used within range of the mobile telephone sites that house the necessary equipment to transmit and receive the radio signals these devices emit. Wireless data communications are also an essential component of mobile computing. The various available technologies differ in local availability, coverage range, and performance. In some circumstances, users must be able to employ multiple connection types and switch between them. To simplify the experience for the user, connection manager software is available, or a mobile VPN can be utilized to handle the multiple connections as a secure, single virtual network.

Mobile Satellite Communications may be used where other wireless connections are unavailable, such as in largely rural areas or remote locations. Satellite communications are especially important for transportation, aviation, maritime, and military use. Satellites generally use microwaves as they are able to pass through the atmosphere which is is essential for GPS satellites which are beyond the atmospheric limits. Radio waves are reflected by the atmosphere and this can act to transfer radio waves from one part of the earth to another where there is no direct line of site.

Modulation of Radio Waves

Amplitude modulation for AM radio. (a) A carrier wave at
the station's basic frequency. (b) An audio signal at much lower
audible frequencies. (c) The amplitude of the carrier is
modulated by the audio signal without changing its basic

Information can be added to a carrier wave by either superimposing signals of varying frequency (or wavelength, since these are dependent on each other), or signals of varying amplitude. Adding information in this way is known as modulation. If the information is added by superimposing a wave with varying frequency, we have frequency modulation or FM. If it is added by superimposing a wave with varying amplitude we have amplitude modulation or AM.

The information transmitted by the many radio and TV stations is very similar. They all need to broadcast information with the same frequencies (20 Hz to 20 000 Hz - the range of human hearing) and amplitudes. If they did so, then we would not hear any of them clearly. All the different signals from the different stations would interfere with each other and we would receive a jumbled mess. To avoid this problem, each station is assigned a particular broadcast frequency (the carrier wave) onto which they superimpose the data they wish to transmit using the frequencies in a narrow band either side of the carrier frequency. This range of frequencies is known as the band width of that radio station, while the carrier wave frequency is the tuning frequency - the one we turn our dial to receive that station. Circuitry in the receivers decodes the information and processes it into the appropriate sound wave frequencies.

Frequency modulation for FM radio.
(a) A carrier wave at the station's
basic frequency. (b) An audio signal
at much lower audible frequencies.
(c) The frequency of the carrier is
modulated by the audio signal without
changing its amplitude.

Receivers can be tuned to pick up the carrier wave, and because no two radio stations have the same carrier wave, they should not interfere with each other. In fact, they still do a little some times because there are so many stations using a limited range of the electromagnetic spectrum, that the carrier waves of different stations are not very different to the carrier waves of other stations. Circuitry in the receiver subtracts the carrier wave from the combined signal, interprets the frequency or amplitude variations in the signal wave and produces the sounds we hear from our radios or TV. This process is known as demodulation.

Microwaves are also modulated to carry information. Because the available band width for microwas is greater, and because there are not as many users, microwaves are used to transmit mobile phone and Internet cable data. This also means that many more signals can be added to the same carrier wave. Up to 20 000 independent signals can be transmitted simultaneously on a single carrier wave. In addition, because their wavelength is smaller, the carrier waves do not spread out as much as radio waves, so more data and more reliable data reach receivers.

Visible light is also used to transmit data. High energy laser light with a fixed but small frequency range is amplitude modulated to carry data. Because the frequency range within a laser beam is very narrow, FM is not efficient so amplitude modulation is used. In addition, the narrow frequency range of a laser means it suffers more from interference than the alternate, broader band communications by radio or microwave. Their use in open air communications is therefore very limited - reliable only to about 200 m distance. Laser transfer of data requires fibre optic cable if the data is to be transmitted more than 200 m to eliminate external interference.

Advantages and Disadvantages of AM and FM Radio Transmission

AM uses a much narrower range of frequencies than FM, so many more AM stations fit into the limited radio bandwidth of the electromagnetic spectrum. However, it is much easier for circuitry in receivers to filter out variations in amplitude in an incoming FM signal than it is to filter out frequency variations in an incoming AM signal, so FM reception is usually much clearer than AM reception. For this reason, it is the preferred way to broadcast music - hence TV music concerts with "simulcast FM radio sound".

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