Electromagnetic Waves

When an electric charge oscillates, it will produce both electric and magnetic fields that vary sinusoidally with time. The two fields couple with each, such that the two fields are perpendicular to each other, and travel as an electromagnetic wave. This is shown below.

All electromagnetic waves are transverse waves that travel at the speed of light (3.0 x 10⁸ ms^-1) in a vacuum.

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Electromagnetic Spectrum

The electromagnetic (EM) spectrum is the range of all possible electromagnetic radiation. The electromagnetic spectrum, shown in the chart, extends from just below the frequencies used for modern radio (at the long-wavelength end) to gamma radiation (at the short-wavelength end), covering wavelengths from thousands of kilometres down to fractions of the size of an atom. It is commonly said that EM waves beyond these limits are uncommon, although this is not actually true. The short wavelength limit is likely to be the Planck length, and the long wavelength limit is the size of the universe itself, though in principle the spectrum is infinite.

Electromagnetic energy at a particular wavelength λ (in vacuum) has an associated frequency f and photon energy E. Thus, the electromagnetic spectrum may be expressed equally well in terms of any of these three quantities. They are related according to the equation:

wave speed (c) = frequency x wavelength

where v (or sometimes indicated as c) is 3.0 x 10⁸ ms^-1 in a vacuum or air.

The table below summarises the different regions of the electromagnetic region.

Region	Frequency Range (f) / Hz	Wavelength Range (l) / m	Sources	Uses
Radio waves	<>9	> 10-1	Sparks or alternating current cause a radio antennae to oscillate the atoms within it to the correct frequency	Radio, television, mobile phones, magnetic resonance imaging
Microwaves	1011 – 109	10-3 – 10-1	Atoms or molecules are oscillated within klystron and magnetron tubes	Cooking, long distance communication, radar, terrain mapping
Infrared	1014 – 1011	10-7 – 10-3	Oscillation of atoms or molecules due to the absorption of heat energy	Heating and drying, night vision cameras, remote controls, satellite remote sensing
Visible	7.5 x 1014 – 4.3 x 1014	4 x 10-7 – 7 x 10-7	Oscillation due to heat energy or electron transitions within an atom	What the typical eye and film can see
Ultraviolet	1016 – 1014	10-8 – 7 x 10-7	Electron transitions within an atom	Photochemicals, photoelectric effects, hardening casts in medicine
X-rays	1019 – 1016	10-11 – 10-8	Electron transitions or braking	Medicine, crystallography, astrophysics, remote sensing
Gamma Rays	> 1019	<>-11	Nuclear transitions	Nuclear research, geophysics, mineral exploration.

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Inverse Square Law

Another problem with long distance communications is the attenuation of the transmitted signal - it decreases in strength as distance from the transmitter increases. 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 expression (right).

Where I = intensity of energy per unit area, and d = distance from source (m).

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Communicating With Electromagnetic Waves

Modulation

Waves carry energy from one place to another. If the amount of energy they carry varies constantly, they can also carry information. The energy variations act as a type of "code". Light, radio and microwaves can be encoded in this way.

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.

Amplitude modulation (AM)

Frequency modulation (FM)

Modulation of radio waves

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 decode the information and process it into the appropriate sound wave frequencies.

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.

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".

Microwave modulation

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 modulation

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.