Radio waves
Animation of a half-wave dipole antenna radiatingradio waves, showing the electric field lines. The antenna in the center is two vertical metal rods connected to a radio transmitter (not shown). The transmitter applies an alternating electric current to the rods, which charges them alternately positive (+) and negative (−). Loops of electric field leave the antenna and travel away at the speed of light; these are the radio waves. In this animation the action is shown slowed down enormously.
Radio waves are a type of electromagnetic radiation with wavelengths in theelectromagnetic spectrum longer thaninfrared light. Radio waves have frequenciesas high as 300 GHz to as low as 3 kHz, though some definitions describe waves above 1 or 3 GHz as microwaves, or include waves of any lower frequency. At 300 GHz, the corresponding wavelength is 1 mm (0.039 in), and at 3 kHz is 100 km (62 mi). Like all other electromagnetic waves, they travel at thespeed of light. Naturally occurring radio waves are generated by lightning, or byastronomical objects.
Artificially generated radio waves are used for fixed and mobile radio communication,broadcasting, radar and other navigation systems, communications satellites, computer networks and innumerable other applications. Radio waves are generated byradio transmitters and received by radio receivers. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves can diffract around obstacles like mountains and follow the contour of the earth (ground waves), shorter waves can reflect off the ionosphere and return to earth beyond the horizon (skywaves), while much shorter wavelengths bend or diffract very little and travel on a line of sight, so their propagation distances are limited to the visual horizon.
To prevent interference between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the International Telecommunications Union(ITU), which defines radio waves as "electromagnetic waves of frequenciesarbitrarily lower than 3 000 GHz, propagated in space without artificial guide".[1] The radio spectrum is divided into a number of radio bands on the basis of frequency, allocated to different uses.
Diagram of the electric fields (E) and magnetic fields (H) of radio waves emitted by a monopoleradio transmitting antenna (small dark vertical line in the center). The E and H fields are perpendicular as implied by the phase diagram in the lower right.
Discovery and exploitation
Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.
Radio waves were first predicted by mathematical work done in 1867 by Scottish mathematical physicist James Clerk Maxwell.[2] Maxwell noticed wavelike properties of light and similarities in electrical and magnetic observations. His mathematical theory, now called Maxwell's equations, described light waves and radio waves as waves of electromagnetism that travel in space, radiated by a charged particle as it undergoes acceleration. In 1887, Heinrich Hertz demonstrated the reality of Maxwell's electromagnetic waves by experimentally generating radio waves in his laboratory,[3]showing that they exhibited the same wave properties as light: standing waves, refraction,diffraction, and polarization. Radio waves, originally called "Hertzian waves",[4] were first used for communication in the mid 1890s byGuglielmo Marconi, who developed the first practical radio transmitters and receivers.
Propagation
Radio waves passing through different environments experience reflection,refraction, polarization, diffraction, andabsorption. The study of radio propagation; how radio waves move in free space and over the surface of the Earth, is vitally important in the design of practical radio systems. Different frequencies experience different combinations of these phenomena in the Earth's atmosphere, making certain radio bands more useful for specific purposes than others. Practical radio systems use three different techniques of radio propagation to communicate:[5]
- Line of sight: This refers to radio waves that travel in a straight line from the transmitting antenna to the receiving antenna. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the only method of propagation possible at microwavefrequencies and above. On the surface of the Earth, line of sight propagation is limited by the visual horizon to about 64 km (40 mi). This is the method used by cell phones, FM and television broadcastingand radar. By using dish antennas to transmit beams of microwaves, point-to-point radio relay links transmit telephone and television signals over long distances up to the visual horizon, and ground stations can communicate with satellitesand spacecraft billions of miles from Earth.
- Indirect propagation: Radio waves can reach points beyond the line-of-sight bydiffraction and multipathpropagation.[5] Diffraction allows a radio wave to bend around obstructions such as a building edge, a vehicle, or a turn in a hall. In multipath, radio waves reflect from surfaces such as walls, floors, ceilings, vehicles and the ground. These propagation methods occur in short range radio communication systems such as cell phones, cordless phones, walkie-talkies, and wireless networks.
- Ground waves: At lower frequencies, in themedium wave and longwave bands, due todiffraction vertically polarized radio waves can bend over hills and mountains, and propagate beyond the horizon, traveling assurface waves which follow the contour of the Earth. This allows mediumwave and longwave broadcasting stations to have coverage areas beyond the horizon, out to hundreds of miles. As the frequency drops, the losses decrease and the achievable range increases. Military very low frequency(VLF) and extremely low frequency (ELF) communication systems can communicate over most of the Earth, and with submarines hundreds of feet underwater.
- Skywaves: At medium wave and shortwavewavelengths, radio waves reflect off a conductive ionized layer in the atmosphere called the ionosphere. So radio waves directed at an angle into the sky can return to Earth beyond the horizon; this is called "skip" or "skywave" propagation. By using multiple skips communication at intercontinental distances can be achieved. Skywave propagation is variable and dependent on atmospheric conditions; it is most reliable at night and in the winter. Widely used during the first half of the 20th century, due to its unreliability skywave communication has mostly been abandoned. Remaining uses are by militaryover-the-horizon (OTH) radar systems, by some automated systems, by radio amateurs, and by shortwave broadcasting stations to broadcast to other countries.
Speed, wavelength, and frequency
Radio waves in vacuum travel at the speed of light.[6][7] When passing through a material medium, they are slowed according to that object's permeability and permittivity. Air is thin enough that in the Earth's atmosphere radio waves travel very close to the speed of light.
The wavelength is the distance from one peak of the wave's electric field (wave's peak/crest) to the next, and is inversely proportional to thefrequency of the wave. The distance a radio wave travels in one second, in a vacuum, is 299,792,458 meters (983,571,056 ft) which is the wavelength of a 1 hertz radio signal. A 1 megahertz radio signal has a wavelength of 299.8 meters (984 ft).
Radio communication
In order to receive radio signals, for instance from AM/FM radio stations, a radio antennamust be used. However, since the antenna will pick up thousands of radio signals at a time, aradio tuner is necessary to tune in a particular signal.[8] This is typically done via a resonator(in its simplest form, a circuit with a capacitor,inductor, or crystal oscillator, but many modern radios use Phase Locked Loopsystems). The resonator is configured to resonate at a particular frequency, allowing the tuner to amplify sine waves at that radio frequency and ignore other sine waves. Usually, either the inductor or the capacitor of the resonator is adjustable, allowing the user to change the frequency at which it resonates.[9]
Biological and environmental effects
Radio waves are nonionizing radiation, which means they do not have enough energy to separate electrons from atoms or molecules,ionizing them, or break chemical bonds, causing chemical reactions or DNA damage. The main effect of absorption of radio waves by materials is to heat them, similarly to theinfrared waves radiated by sources of heat such as a space heater or wood fire. The oscillating electric field of the wave causespolar molecules to vibrate back and forth, increasing the temperature; this is how amicrowave oven cooks food. However, unlike infrared waves, which are mainly absorbed at the surface of objects and cause surface heating, radio waves are able to penetrate the surface and deposit their energy inside materials and biological tissues. The depth to which radio waves penetrate decreases with their frequency, and also depends on the material's resistivity and permittivity; it is given by a parameter called the skin depth of the material, which is the depth within which 63% of the energy is deposited. For example the 2.45 GHz radio waves (microwaves) in a microwave oven penetrate most foods approximately 2.5 to 3.8 cm (1 to 1.5 inches). Radio waves have been applied to the body for 100 years in the medical therapy ofdiathermy for deep heating of body tissue, to promote increased blood flow and healing. More recently they have been used to create higher temperatures in hyperthermiatreatment, to kill cancer cells. Looking into a source of radio waves at close range, such as the waveguide of a working radio transmitter, can cause damage to the lens of the eye by heating. A strong enough beam of radio waves can penetrate the eye and heat the lens enough to cause cataracts.
Since the heating effect is in principle no different from other sources of heat, most research into possible health hazards of exposure to radio waves has focused on "nonthermal" effects; whether radio waves have any effect on tissues besides that caused by heating. Electromagnetic radiation has been classified by the International Agency for Research on Cancer (IARC) as "Possibly carcinogenic to humans".[10]
Radio waves can be shielded against by a conductive metal sheet or screen, an enclosure of sheet or screen is called aFaraday cage. A metal screen shields against radio waves as well as a solid sheet as long as the holes in the screen are smaller than thewavelength of the waves.
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