Industry commercial radio communication, broadcasting and television antennas
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In radio transmission a radiating antenna is used to convert a time-varying electric current into an electromagnetic wave or field , which freely propagates through a nonconducting medium such as air or space. In a broadcast radio channel, an omnidirectional antenna radiates a transmitted signal over a wide service area. In a point-to-point radio channel, a directional transmitting antenna is used to focus the wave into a narrow beam, which is directed toward a single receiver site.
In either case the transmitted electromagnetic wave is picked up by a remote receiving antenna and reconverted to an electric current. Radio wave propagation is not constrained by any physical conductor or waveguide. This makes radio ideal for mobile communications, satellite and deep-space communications, broadcast communications, and other applications in which the laying of physical connections may be impossible or very costly.
On the other hand, unlike guided channels such as wire or optical fibre, the medium through which radio waves propagate is highly variable, being subject to diurnal, annual, and solar changes in the ionosphere , variations in the density of water droplets in the troposphere , varying moisture gradients, and diverse sources of reflection and diffraction.
The range of a radio communications link is defined as the farthest distance that the receiver can be from the transmitter and still maintain a sufficiently high signal-to-noise ratio SNR for reliable signal reception.
The received SNR is degraded by a combination of two factors: beam divergence loss and atmospheric attenuation. Beam divergence loss is caused by the geometric spreading of the electromagnetic field as it travels through space. As the original signal power is spread over a constantly growing area, only a fraction of the transmitted energy reaches a receiving antenna. The other cause of SNR degradation , atmospheric attenuation, depends on the propagation mechanism, or the means by which unguided electromagnetic waves travel from transmitter to receiver.
Radio waves are propagated by a combination of three mechanisms: atmospheric wave propagation, surface wave propagation, and reflected wave propagation. They are described below. In atmospheric propagation the electromagnetic wave travels through the air along a single path from transmitter to receiver.
The propagation path can follow a straight line, or it can curve around edges of objects, such as hills and buildings, by ray diffraction. Diffraction permits mobile phones to work even when there is no line-of-sight transmission path between the phone and the base station. Atmospheric attenuation is not significant for radio frequencies below 10 gigahertz.
Above 10 gigahertz under clear air conditions, attenuation is caused mainly by atmospheric absorption losses; these become large when the transmitted frequency is of the same order as the resonant frequencies of gaseous constituents of the atmosphere, such as oxygen O 2 , water vapour H 2 O , and carbon dioxide CO 2. Additional losses due to scattering occur when airborne particles, such as water droplets or dust, present cross-sectional diameters that are of the same order as the signal wavelengths.
Scattering loss due to heavy rainfall is the dominant form of attenuation for radio frequencies ranging from 10 gigahertz to gigahertz microwave to submillimetre wavelengths , while scattering loss due to fog dominates for frequencies ranging from 10 3 gigahertz to 10 6 gigahertz infrared through visible light range. For low radio frequencies, terrestrial antennas radiate electromagnetic waves that travel along the surface of the Earth as if in a waveguide.
The attenuation of surface waves increases with distance, ground resistance, and transmitted frequency. Attenuation is lower over seawater, which has high conductivity, than over dry land, which has low conductivity. At frequencies below 3 megahertz, surface waves can propagate over very large distances. Ranges of km about 60 miles at 3 megahertz to 10, km 6, miles at 1 kilohertz are not uncommon.
Sometimes part of the transmitted wave travels to the receiver by reflection off a smooth boundary whose edge irregularities are only a fraction of the transmitted wavelength. When the reflecting boundary is a perfect conductor, total reflection without loss can occur.
However, when the reflecting boundary is a dielectric, or nonconducting material, part of the wave may be reflected while part may be transmitted refracted through the medium—leading to a phenomenon known as refractive loss. When the conductivity of the dielectric is less than that of the atmosphere, total reflection can occur if the angle of incidence that is, the angle relative to the normal, or a line perpendicular to the surface of the reflecting boundary is less than a certain critical angle.
Some terrestrial radio links can operate by a combination of atmospheric wave propagation, surface wave propagation, ground reflection, and ionospheric reflection. In some cases this combining of propagation paths can produce severe fading at the receiver. Fading occurs when there are significant variations in received signal amplitude and phase over time or space.
Fading can be frequency-selective—that is, different frequency components of a single transmitted signal can undergo different amounts of fading. A particularly severe form of frequency-selective fading is caused by multipath interference, which occurs when parts of the radio wave travel along many different reflected propagation paths to the receiver.
When the geometry of the reflected propagation path varies rapidly, as for a mobile radio traveling in an urban area with many highly reflective buildings, a phenomenon called fast fading results. Fast fading is especially troublesome at frequencies above one gigahertz, where even a few centimetres of difference in the lengths of the propagation paths can significantly change the relative phases of the multipath signals.
Effective compensation for fast fading requires the use of sophisticated diversity combining techniques, such as modulation of the signal onto multiple carrier waves, repeated transmissions over successive time slots, and multiple receiving antennas.
Before the radio spectrum above 30 megahertz was virtually empty of man-made signals. Today, civilian radio signals populate the radio spectrum in eight frequency bands , ranging from very low frequency VLF , starting at 3 kilohertz, and extending to extremely high frequency EHF , ending at gigahertz. It is frequently convenient to express radio frequencies in terms of wavelength , which is the ratio between the speed of light through a vacuum approximately million metres per second and the radio frequency.
The wavelength of a VLF radio wave at 3 kilohertz is thus km about 60 miles , while the wavelength of an EHF radio wave at gigahertz is only 1 mm about 0.
An important measure of the efficiency with which a transmitting antenna delivers its power to a remote receiving antenna is the effective isotropic radiated power EIRP , measured in watts per metre squared. To achieve high EIRP the antenna dimensions should be several times larger than the largest transmitted wavelength. For frequencies below the medium frequency MF band, where wavelengths range upward from metres about feet , this is usually not practical; in these cases transmitters must compensate for low EIRP by transmitting at higher power.
This makes frequency bands up through high frequency HF unsuitable for such applications as handheld personal radios, radio pagers, and satellite transponders, in which small antenna size and power efficiency are essential. Two radio links can share the same frequency band or the same geographic area of coverage, but they cannot share both without interference.
Therefore, international use of the radio spectrum is tightly regulated by the International Telecommunication Union ITU , while domestic radio links are regulated by national agencies such as the U. Each radio link is assigned a specific frequency band of operation, a specific transmitter radiation pattern, and a maximum transmitter power. For example, a broadcast radio or television station may be authorized to broadcast only in certain directions and only at certain times of the day.
Frequency bandwidths also are allocated , ranging from hertz for radiotelegraphs to 10 kilohertz for voice-grade radiotelephones to more than megahertz for multichannel digital radio relays in the telephone network to about megahertz for cellular telephones. The very low frequency to medium frequency VLF-MF bands extend from 3 kilohertz to 3 megahertz, or wavelengths of km to metres.
These bands are used for low-bandwidth analog services such as long-distance radio navigation, maritime telegraph and distress channels, and standard AM radio broadcasting. Owing to insufficient available bandwidth, they are unsuitable for broadband telecommunication services such as television and FM radio. Because of the high conductivity of salt water , maritime radio transmissions at VLF can propagate via surface waves for thousands of kilometres.
High-frequency HF radio is in the to metre wavelength band, extending from 3 megahertz to 30 megahertz. Much of the HF band is allocated to mobile and fixed voice communication services requiring transmission bandwidths of less than 12 kilohertz. International shortwave radio broadcasting also is conducted in the HF band; it is allocated to seven narrow bands between 5. The primary mode of propagation for HF radio transmissions is reflection off the ionosphere , a series of ionized layers of the atmosphere ranging in altitude from about 50 to km about 30 to miles above the Earth.
Ionization is caused primarily by radiation from the Sun, so that the layers vary in height and in reflectivity with time. During the day the ionosphere consists of four layers located at average altitudes of 70 km D layer , km E layer , km F 1 layer , and km F 2 layer.
At night the D and E layers often disappear, and the F 1 and F 2 layers combine to form a single layer at an average altitude of km. Reflective conditions thus change with time. At night, however, it can reflect only off the high-altitude F layer, creating very long transmission ranges. The D layer is nonreflecting at HF frequencies and merely attenuates the propagating radio wave. In the lower HF band, transmission ranges of many thousands of kilometres can be achieved by multiple reflections, called skips, between the Earth and layers of the ionosphere.
Strong ionospheric reflections occur only below a maximum usable frequency MUF , which is determined by the zenith angle of the incident ray and by the ionization density of the reflecting layer. In general, the MUF is higher at larger zenith angles and higher ionization densities.
During the peaks of the year sunspot cycle , solar ultraviolet radiation produces the highest ionization densities. These sunspot peaks can last several days or months, depending on the persistence of sunspot visibility, producing a sporadic E layer that often can be used for multiple-skip communications by amateur radio operators at frequencies up to megahertz—well into the VHF band.
The very high frequency to ultrahigh frequency VHF - UHF bands are in the wavelength range of 10 metres to 10 cm 33 feet to 4 inches , extending from 30 megahertz to 3 gigahertz. Some of these bands are used for broadcast services such as FM radio in the United States, 88— megahertz , VHF television 54—88 megahertz for channels 2—6, — megahertz for channels 7—13 , and UHF television frequency slots scattered within — megahertz.
The UHF band also is used for studio and remote-pickup television relays, microwave line-of-sight links 1. Parts of the band are used for radio navigation applications, such as instrument landing systems — megahertz , military aircraft communications — megahertz , air-traffic control radio beacons 1.
In the North American over-the-air digital broadcast system, a television equipped with a QAM quadrature amplitude modulation tuner can decode digital signals, which are broadcast within each 6-megahertz-wide band already assigned to that station—i.
Powerful UHF transmitters can achieve beyond-the-horizon transmission ranges by scattering transmitted energy off layers of the troposphere the lowest layer of the atmosphere, where most clouds and weather systems are contained. Unlike signals in the longer-wavelength HF band, for which layers in the atmosphere appear as relatively smooth reflective surfaces, signals in the shorter-wavelength UHF band reflect off small irregularities in the atmospheric layers as if these irregularities were randomly oriented granular reflectors.
The reflectors disperse the propagating UHF signal in many directions, so that only a fraction of the transmitted signal power may reach the receiver. In addition, owing to unpredictable disturbances in atmospheric conditions, significant fading can occur over a given path, at a given time, and at a given radio frequency.
For this reason a tropospheric scatter relay typically uses combinations of space, time, and frequency diversity techniques. A typical relay links two large terminals across spans of to km to miles and carries up to voice channels. The superhigh frequency to extremely high frequency SHF-EHF bands are in the centimetre to millimetre wavelength range, which extends from 3 gigahertz to gigahertz. Typical allocated bandwidths in the SHF band range from 30 megahertz to megahertz—bandwidths that permit high-speed digital communications up to 1 gigabit per second.
In addition to degradation from fading and from atmospheric attenuation, radio waves in the SHF-EHF band undergo high penetration losses as they propagate through the exterior walls of buildings. Because of the severe atmospheric attenuation, and in particular rainfall scattering losses, the EHF band is currently the least populated radio band for terrestrial communication. However, it has been used for intersatellite communication and satellite radio navigation—applications in which atmospheric attenuation is not a factor.
A line-of-sight microwave link uses highly directional transmitter and receiver antennas to communicate via a narrowly focused radio beam.
The transmission path of a line-of-sight microwave link can be established between two land-based antennas, between a land-based antenna and a satellite-based antenna, or between two satellite antennas.
Broadband line-of-sight links operate at frequencies between 1 and 25 gigahertz the centimetre wavelength band and can have transmission bandwidths approaching megahertz. In the United States, line-of-sight microwave links are used for military communications, studio feeds for broadcast and cable television , and common carrier trunks for inter-urban telephone traffic.
A typical long-distance, high-capacity digital microwave radio relay system links two points 2, km apart by using a combination of nine terrestrial and satellite repeaters. Each repeater operates at 4 gigahertz, transmitting seven megahertz-bandwidth channels at megabits per second per channel.
The maximum range of land-based line-of-sight systems is limited by the curvature of the Earth. For this reason, a microwave radio repeater with transmitter and receiver dishes mounted on metre foot towers has a maximum range of approximately 50 km 30 miles , whereas the maximum range will increase to approximately 80 km 50 miles if the towers are raised to 90 metres feet.
Line-of-sight microwave links are subject to severe fading, owing to refraction of the transmitted beam along the propagation path. Under normal conditions the refractive index of the atmosphere decreases with increasing altitude. This means that upper portions of the beam propagate faster, so that the beam is slightly bent toward the Earth, producing transmission ranges that go beyond the geometric horizon.
However, temporary atmospheric disturbances can change the refractive index profile, causing the beam to bend differently and, in severe cases, to miss the receiver antenna entirely. A telecommunications satellite is a sophisticated space-based cluster of radio repeaters , called transponders , that link terrestrial radio transmitters to terrestrial radio receivers through an uplink a link from terrestrial transmitter to satellite receiver and a downlink a link from satellite transmitter to terrestrial receiver.
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In radio transmission a radiating antenna is used to convert a time-varying electric current into an electromagnetic wave or field , which freely propagates through a nonconducting medium such as air or space. In a broadcast radio channel, an omnidirectional antenna radiates a transmitted signal over a wide service area. In a point-to-point radio channel, a directional transmitting antenna is used to focus the wave into a narrow beam, which is directed toward a single receiver site. In either case the transmitted electromagnetic wave is picked up by a remote receiving antenna and reconverted to an electric current. Radio wave propagation is not constrained by any physical conductor or waveguide. This makes radio ideal for mobile communications, satellite and deep-space communications, broadcast communications, and other applications in which the laying of physical connections may be impossible or very costly.
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Radio is the technology of signaling and communicating using radio waves. They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by a radio receiver connected to another antenna. Radio is very widely used in modern technology, in radio communication, radar , radio navigation , remote control , remote sensing and other applications.
Terrestrial television is a term which refers to modes of television broadcasting which do not involve satellite transmission or via underground cables. Terrestrial television broadcasting dates back to the very beginnings of television as a medium itself and there was virtually no other method of television delivery until the s with the beginnings of cable television, or community antenna television CATV. The first non-terrestrial method of delivering television signals that in no way depended on a signal originating from a traditional terrestrial source began with the use of communications satellites during the s and s of the twentieth century.
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