Also found in: Dictionary, Thesaurus, Medical, Legal, Acronyms, Idioms, Wikipedia.
antenna(ăntĕn`ə), in electronics, system of wires or other conductors used to transmit or receive radio or other electromagnetic waves (see radioradio,
transmission or reception of electromagnetic radiation in the radio frequency range. The term is commonly applied also to the equipment used, especially to the radio receiver.
..... Click the link for more information. ); sometimes called an aerial. The idea of using an antenna was developed by Guglielmo Marconi (c.1897). In a transmitting antenna, the signal from an electronic circuit causes electrons in the antenna to oscillate; these moving electric charges generate electromagnetic radiation, which is transmitted through the air and space. Distribution of the waves depends on the design of the antenna; the transmitting antennas of a radio station might be designed to emit waves in all directions, while an antenna used for radarradar,
system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, the principle of radar is not confined to any particular frequency range.
..... Click the link for more information. or space communications would be designed to focus the waves in a single direction. In a receiving antenna electromagnetic waves cause the electrons to oscillate, inducing a signal that can be detected by an electronic circuit. The antenna has a characteristic frequency related to its physical dimensions; a wire of a given length may be inherently tuned to radio waves whose wavelengths are simple fractions or multiples of the length of the wire. In general, a longer antenna is used to transmit or receive signals of longer wavelength. Although theoretically, the same antenna can be used both for sending and receiving signals, in many instances transmitting antennas are designed differently from receiving antennas, especially if they must handle very high power loads. Any straight vertical conductor may serve as an antenna and will transmit to or receive from all directions. A horizontal antenna radiates or intercepts energy principally at right angles to itself; the use of horizontal antennas enables transmitters to concentrate or beam their signals into desired areas and enables receivers to select one of several signals having the same frequency but arriving with different polarizations. The dish-shaped microwave antenna is highly directional; it uses a parabolic reflector to focus received signals on a small antenna element. Phased array antennas, used for long range radar and radio astronomy, are composed of large groupings of individual antennas; they may be electronically aimed by changing the relative phase of the signal at each element.
antenna(an-ten -ă) (aerial) A device used for the transmission or reception of radio waves. When connected to a transmitter, the oscillating electric currents induced in the antenna launch radio waves into space. When connected to a receiver, incoming radio waves from a distant source generate oscillating electric currents in the antenna, which are detected by the receiver. A practical antenna neither radiates nor receives equally in all directions, which may be demonstrated by its antenna pattern – a plot of relative gain as a function of direction. The plot may be made in terms of the antenna's voltage response or power response, giving a voltage pattern or a power pattern. By the principle of reciprocity the antenna pattern for transmission is identical with that for reception.
A number of distinct lobes may often be identified in the antenna pattern. The lobe corresponding to the direction of best transmission or reception is the main lobe or main beam; all the others are called side lobes and are usually unwanted (see also beam). The ratio of the power received in the main beam to the total power received by all the lobes is called the beam efficiency. The angle between the two directions in the main beam at which the power response has fallen to half its maximum value is called the beamwidth; it is a measure of the antenna's directivity. If the angular separation of two cosmic radio sources is less than the beamwidth, the sources will not be resolved but will be observed as a single source. Some highly directional antennas have pencil beams, which are narrow main beams of circular cross section. Others have fan beams where the cross section of the main beam is greatly extended in one direction. See also array; dish; radiation resistance.
a device for radiating and receiving radio waves. A transmitting antenna transforms the energy of the electromagnetic high-frequency oscillations concentrated at the output oscillatory circuits of a radio transmitter into the energy of radiated radio waves. The transformation is based on the fact that alternating electric current is well known as a source of electromagnetic waves. This property of alternating electric current was first established by H. Hertz in the 1880’s on the principles of J. Maxwell’s research. A receiving antenna performs the inverse function by transforming the energy of propagating radio waves into energy that is concentrated at the input of oscillatory circuits of a receiver. The form, dimensions, and construction of antennas vary and depend on the length of the radiated or received waves and the purpose of the antenna. Antennas may be in the form of a section of wire, combinations of such sections, reflective metal surfaces of different configurations, cavities with metal walls in which slots have been cut, spirals of metal wire, and so on.
Fundamental characteristics and parameters. For the majority of transmitting antennas, the intensity of the radiation is a function of direction; that is, the antenna has directivity of radiation. This antenna property is depicted graphically by the radiation pattern, which shows the directional dependence of the electrical field strength for the radiated wave (measured at a large, fixed distance from the antenna). The directivity of an antenna produces an increase in the field strength of the wave for the direction of maximum radiation and thus creates an effect equivalent to that produced by an increase of radiated power. For a quantitative evaluation of the equivalent gain in radiated power, the concept of antenna directivity has been adopted; this indicates how many times the radiated power must be increased to replace a specific real antenna with a hypothetical nondirectional antenna (isotropic radiator) so that the electromagnetic field strength remains unchanged. Not all the power supplied to an antenna is radiated; part is lost in the antenna’s conductors and insulators, as well as in the environment (the earth, the antenna’s supporting structures, and so on). The ratio of the radiated power to the total power supplied to an antenna is called the radiation efficiency. The product of the antenna directivity and the efficiency is called the gain of the antenna.
Similarly, a receiving antenna is characterized by the shape of its radiation pattern, its directivity factor, its efficiency, and its gain. Its radiation pattern depicts the electromotive force produced by the antenna at the receiver’s input as a function of the direction from which a wave arrives. It is assumed here that the field strength at the receiving point is independent of the wave’s direction of arrival. The antenna directivity indicates by what factor the power introduced by the antenna into the input circuit of the receiver for a wave arriving from the direction of maximum reception is greater than the average power (taken over all directions), assuming that the field strength is independent of the wave’s direction of arrival. The antenna directivity of a receiving antenna characterizes its spatial selectivity, which determines the possibility of distinguishing a received signal from a background of interference created by radio signals arriving from different directions and generated by a variety of sources. The term “efficiency” for a receiving antenna is understood to be the same as for a transmitting antenna. The gain is defined as the product of the antenna directivity and the efficiency. The shape of the radiation pattern, the antenna directivity, and the gain of any antenna are identical for the transmitting and receiving modes. This reciprocity characteristic of the transmitting and receiving processes makes it possible to limit the description of antenna characteristics to the transmitting mode alone.
Antenna theory and design methods are based on the radiation theory of an elementary electrical dipole (Figure 1, a), which was published by H. Hertz in 1889. An elementary electrical dipole is understood to mean a conductor which has a length many times less than the wavelength λ of the radiated wave and in which a radio frequency current is flowing with the same amplitude and phase over its entire length. Its radiation pattern in a plane containing the axis has the form of a figure eight (Figure 1, b). In a plane perpendicular to the axis, the radiation is nondirectional and the pattern is circular (Figure 1, c). The antenna directivity of an elementary dipole is equal to 1.5. A practical example of an elementary dipole is the Hertz dipole. Any antenna can be considered to be an ensemble of many elementary dipoles.
First practical antenna. An antenna in the form of an asymmetric dipole was proposed by the inventor of radio, A. S. Popov, in 1895. A dipole (asymmetric with respect to the energy supply point) is a long vertical conductor between whose lower end and ground a transmitter or receiver is connected (Figure 2, a). Grounding is usually accomplished by a radial system of conductors buried in the earth at a shallow depth. These conductors are attached by a common lead to one of the transmitter or receiver terminals. The radiation pattern of a vertical asymmetric dipole having a length that is small compared with λ has a shape in the vertical plane (when the conductivity of the ground is high) of a half figure eight (Figure 2, b); in the horizontal plane it has the shape of a circle. The directivity of such an antenna is equal to 3. As may be seen from Figure 2, b, a vertical asymmetric dipole provides intense radiation along the earth’s surface and therefore is widely used in radio communication and radio broadcasting for long and medium wavelengths. At these wavelengths the properties of the soil are close to those of a high-conducting medium, and intense radiation over the earth’s surface must usually be provided.
One important characteristic of this type of antenna is the radiation resistance, Rrad. When the length of a dipole l≤¼λ, the radiation resistance is usually assumed to be the ratio of the radiated power to the square of the effective current strength measured at the lower end of the dipole. As Rrad increases, the radiated power (for a given current in the dipole) and the efficiency increase, the frequency pass band becomes broader, and the maximum strength of the electrical field developed on the surface of the antenna conductor decreases for a given amount of power supplied. Since the maximum field strength should not exceed a specified value to avoid ionization of the surrounding air and breakdown of the insulators supporting the antenna, an increase in Rrad means an increase in the maximum power that can be supplied to the antenna. Rrad increases as the ratio l/λ rises, and also as uniformity of the current distribution over the length of the dipole increases. A broadening of the frequency pass-band and a reduction of the maximum field intensity are also achieved by increasing the diameter of the antenna’s conductor or by using several conductors connected in parallel (a reduction of the antenna’s wave impedance).
Long-wave antenna. In the long-wave region, antenna improvements went along the lines of an increase in their geometric height up to 300 m, equalization of the current distribution by adding horizontal and inclined conductors (T-shaped, Γ-shaped, and umbrella-shaped antennas, Figure 3), and construction of the vertical and horizontal portions from several parallel conductors in order to reduce the wave impedance. The directivity of long-wave antennas is approximately equal to 3. As the wavelength is shortened, construction becomes easier, since antenna height is commensurate with λ. In this case there is no need for added horizontal or inclined conductors. Consequently, during the 1930’s the radio broadcasting stations operating in the wavelength range from 200 to 2,000 m began to use a vertical asymmetric dipole in the form of self-supporting metal antenna towers insulated from the ground, or antenna masts supported by guy wires which were divided into short sections by insulators to reduce the currents induced in them by the electromagnetic field from the dipole. The directivity factor of antenna masts or towers is a function of the ratio of their height to λ. When this ratio is equal to 0.63, the directivity has a maximum value of 6. If directional radiation in the horizontal plane is desired because of operating conditions in this wavelength range, a composite antenna is employed (Figure 4, a). Such an antenna usually consists of two vertical asymmetric dipoles, one of which (the active dipole) is supplied directly from the transmitter, and the other (the passive reflector, which is identical in construction to the first) is excited as a result of the spatial electromagnetic coupling with the active dipole. When the passive reflector is suitably adjusted, the resulting interference of the waves radiated from the active dipole and the passive reflector produces a radiation pattern having the characteristic shape in the horizontal plane shown in Figure 4, b. As may be seen, the use of a reflector causes the radiated intensity to be substantially attenuated in a semispace. The directivity of such an antenna is approximately twice that of the dipole alone.
Medium-wave antenna. In the radio broadcasting range from 200 to 550 m, extensive use is made of the so-called antifading antenna, which makes it possible to reduce the fading effect in the electromagnetic field that takes place at short distances from an antenna (starting at 40 to 60 km) during the evening and at night. The fading effect is due to interference between the spatial wave (reflected from, the ionosphere) and the ground wave propagated along the earth’s surface. The current distribution in the dipole of an antifading antenna is chosen so that reception of the spatial wave is substantially attenuated. In addition to asymmetric dipoles, for reception at long and medium wavelengths the loop antenna (Figure 5) and the so-called magnetic antenna (or “loopstick”) are utilized, as are complex antennas composed of a loop antenna and a vertical symmetrical dipole. These receiving antennas have directional properties in the horizontal plane and thus can attenuate radio receiving interference if the source of the interference is located in the directions of the radiation pattern minima. A further improvement in interference protection can be achieved for reception at the long and medium wavelengths by using a Beverage antenna, which is a long horizontal conductor suspended at a height, of several meters above the earth and oriented toward the station to be received.
Shortwave antenna. The performance of short wavelength antennas depends essentially on the length of the communications links. For short links (up to several dozen km), communication is carried on by means of waves propagating along the earth’s surface. On such links vertical asymmetric dipoles—similar to medium- and long-wave dipoles—as well as vertical symmetrical dipoles (Figure 6, a), are used. On long links (from 50 to 100 km or more), communication is accomplished by means of radio waves reflected one or more times from the ionosphere. On such links, the antennas generally used are made of horizontal symmetric dipoles (Figure 6, b) which produce maximum radiation at a certain angle to the horizontal plane. Around-the-clock and year-round communication on short wavelengths requires frequent shifting of λ. During the day, in summer, and in years of high solar activity, shorter wavelengths are needed than during the night, in winter, and in years of low solar activity. Therefore it is preferable to use wideband antennas that operate over a broad frequency range without any tuning. One of the simplest wideband antennas is a symmetrical horizontal dipole known as the Nadenenko dipole (Figure 7). This antenna has a low wave impedance, so that its input impedance
anee changes little with wavelength over a broad range; this ensures a good match with the input in a range of more than twice the bandwidth without tuning. The directivity factor of the Nadenenko dipole (taking into account the effect of the earth, which eliminates radiation in the lower semispace) varies between 6 and 12.
For long-distance shortwave communication links, antennas must have a larger directivity factor than symmetrical dipoles. For such antennas a broadside type (Figure 8, a), consisting of a plane array of symmetrical dipoles excited by currents of the same phase, is often used. In the direction perpendicular to the center of the array, at a great distance from the cophasal antenna, the fields produced by the radiation from all the dipoles are in phase because the paths of the waves from the dipoles to the receiving point are practically identical. The field strength is maximal in this direction. In other directions the paths—and consequently the phases of the waves—are different, so that the interference among the waves radiated by individual dipoles causes attenuation of the overall field strength. The greater the number of dipoles in one horizontal row, the narrower the radiation pattern in the horizontal plane. The radiation pattern in the vertical plane is made narrower as the number of horizontal rows (levels) of dipoles is increased. In order to obtain unidirectional radiation and to double the directivity, the arrays are supplemented by a passive reflector in the form of an identical array in which, because of spatial electromagnetic coupling, currents are induced of such magnitude and phase that the radiation in the direction L1 (Figure 8, a) is markedly reduced, but is increased in the direction L2. In order that a broadside antenna may be able to operate over a broad wavelength range (up to double or more) without special arrangements for matching its input impedance to the wave impedance of the supply feeder, the dipoles are often constructed in the form of Nadenenko dipoles. To avoid the necessity of retuning the reflector with a change of λ, it is sometimes made in the form of a dense network of horizontal conductors (aperiodic reflector) which are nearly impervious to the waves radiated by the antenna. The radiation pattern of a shortwave, broadside antenna in the horizontal (Figure 8, b) and vertical planes (Figure 8, c) consists of one large (main) lobe and a number of small (side) lobes. The lower the level of the side lobes, the better the characteristics of the antenna. When transmitting, the side lobes cause a wasteful dissipation of part of the power, and when receiving they increase the probability that interfering signals from various directions will reach the receiver channel. The directivity factor D of a broadside antenna is given approximately by the formula
D = k · 4 π S/λ2
where S is the area of the antenna array in sq m, λ is the wavelength of the wave in m, and k is a factor that takes account of the earth, the separation between dipoles, the length of the dipole arms, and other factors. For broadside, shortwave antennas k is in the range from 2 to 3. The directivity of these antennas runs to several hundred and even thousands, and the efficiency is close to 1.
In addition to the broadside array, the rhombic antenna is also used for short waves. A feature of this antenna is that it can be operated over a broad range of wavelengths (up to fourfold). The directivity factor of a rhombic antenna, depending on λ, lies between the limits of 20 and 200, and the efficiency is between 0.5 and 0.8. A disadvantage of the rhombic antenna is the relatively high level of the side lobes. In addition to the symmetrical dipole and rhombic antennas for reception at shortwave radio communication centers, the traveling-wave antenna (Figure 9) is used; it has a wide operating wavelength range (up to sixfold) and a low side-lobe level in the horizontal plane, thus providing excellent interference protection for reception. The directivity of a traveling-wave antenna lies between 40 and 250, and the efficiency is between 0.05 and 0.5. Due to its low efficiency, it is not utilized for transmitting. For nonprofessional shortwave reception, listeners use asymmetric dipoles, loop and magnetic antennas, and Beverage antennas.
The work of the following Soviet scientists has been of great importance in developing the circuits and the theory of long, medium, and short wavelength antennas: G. Z. Aizen-berg, B. V. Braude, I. G. Kliatskin, V. D. Kuznetsov.G. A. Lavrov, A. L. Mints, A. M. Model’, S. I. Nadenenko, M. S. Neiman, L. K. Olifin, A. A. Pistol’kors, V. V. Tatarinov, M. V. Shuleikin, and others; among the foreign scientists are G. Howe of England, L. Brillouin of France, P. Carter and G. Brown of the United States, and E. Hallén of Sweden.
Antennas for meter and decimeter waves. For television and radio transmission at the meter and decimeter wavelengths, multiple-section (up to 30) turnstile (Figure 10), panel, slot, and other types of antennas are used; these have circular radiation patterns in the horizontal plane and narrow patterns in the vertical plane. The directivity factor of these antennas is proportional to the number of sections and is in the range from 6 to several dozen. In order to increase the coverage area of these antennas, they are mounted on towers or masts 100–300 m high and higher. The highest television tower in the world, 533 m, was built in Moscow. Television broadcasts are received with a symmetrical dipole, a Yagi antenna (Figure 11), and others, which are usually mounted on the roofs of houses or on high poles. In large buildings with many apartments, a community antenna is used, consisting of the antenna proper, a radio frequency amplifier, and a system of distribution feeders that pass the radio frequency energy from the amplifier output to the inputs of the television sets. The antennas used in community systems are of the Yagi and other types. The number of television sets served by one community antenna can be as high as several hundred. Major contributions to the development of transmitting and receiving television antennas have been made by the Soviet scientists B. V. Braude, V. D. Kuznetsov, and others and by foreign scientists such as N. Lindenblad (of the United States). For line-of-sight communication on meter wavelengths, symmetrical and asymmetric dipoles, Beverage, and other types of antennas are used; for ionospheric communication, the broadside multiple-dipole antenna arrays of the Yagi, rhombic, and other types are used; for meteor radio communication, the Yagi type is preferred.
Ultra-, super-, and extremely high frequency antennas (UHF, SHF, and EHF). In the UHF, SHF, and EHF ranges, which cover the decimeter, centimeter, and millimeter wavelengths, cophasal surface antennas are commonly used for radio relay communication links, radar, space communication links, radio astronomy, and so on. The operating principle of such antennas is similar to that of the broadside multiple-dipole arrays; it differs only in that they are not made up of discrete radiating elements (dipoles), but consist of a continuous flat surface on which a cophasal electromagnetic field is generated. The cophasal surface, like a broadside array, has a radiation maximum in a direction perpendicular to the surface, and the radiation pattern becomes narrower as the surface area is increased. The directivity of these antennas is given by the formula for a broadside antenna. The coefficient k in this case is called the surface utilization factor. In these bands, the effect of the earth is not taken into account when determining the antenna’s directivity factor. Hence, for an ideal plane having a cophasal and uniform excitation of the surface, the coefficient k is equal to 1. In actual antennas, the coefficient varies from 0.4 to 0.8 because of nonuniform excitation, departures from the cophasal state, and energy leakage past the main radiating surface. It follows from the formula that for a given area of cophasal radiating surface, the directivity of an antenna increases inversely as the square of the wavelength. As a result, antennas used in this range have large directivity factors, reaching as high as hundreds of thousands, and even millions. To create cophasally excited surfaces, technical methods have been widely borrowed from the fields of optics and electroacoustics. The simplest surface antenna is the horn antenna (Figure 12) in the form of a metallic wave guide with a smoothly expanding cross section. At the output of a horn with a relatively small flare angle, a plane surface passing through its rim achieves a nearly cophasal excitation. The surface utilization factor of such an antenna is between 0.5 and 0.8, and the directivity usually lies in the range from 10 to 100. Horn antennas are also used extensively to radiate for reflector and lens antennas.
The lens antenna (Figure 13) used at these frequencies has the same operating principle as an optical lens and consists of the lens proper and an irradiator mounted at its focus F. The lens transforms the spherical or cylindrical wave front from the radiator into a plane wave. In this way a plane surface that is cophasally excited by the electromagnetic field is achieved at the output of the lens. A special case of the lens antenna is the horn-lens antenna consisting of a horn with a large flare angle (60°-70°) that has a lens placed at its output which transforms the spherical or cylindrical wave front in the horn to a plane front. If the radiator for the lens is displaced from the focus in a plane passing through the focus and perpendicular to the lens axis, the wave front at its output is rotated through a certain angle. Correspondingly, the direction of maximum radiation is also rotated. This property of the lens antenna is utilized to scan the radiation pattern in radar (oscillation of the direction of maximum radiation). In ordinary lens antennas, the angle of rotation of maximum radiation is limited because as it is increased, the surface utilization factor is reduced. Exceptions are represented by aplanatic lens antennas, which are distinguished by the fact that over a wide sector of rotation of the direction of maximum radiation (by displacing the radiator), they have no essential attendant reduction in the surface utilization factor. High-quality lens antennas have a surface utilization factor of 0.5–0.6.
Reflector antennas, consisting of a metallic reflector with a parabolic profile and a radiator, have become exceptionally popular in the UHF, SHF, and EHF ranges. The radiators are placed at the focus F of the paraboloid (Figure 14). The parabolic reflector transforms the spherical wave front of the radiator into a plane front at the aperture (on the plane surface circumscribed by the reflector’s rim). Thereby a plane surface is created which is excited cophasally by the electromagnetic field. The radiators used are moderately directive antennas (horns, dipoles with a small reflector, spirals, and so on). In the same way as in the lens antenna, a displacement of the radiator from the focus in a plane perpendicular to the antenna’s axis is accompanied by a rotation of the direction of maximum radiation. This property is also utilized in radar to scan the radiation pattern. In the ordinary parabolic antenna (Figure 14), the radiator is in the reflected wave field, thus causing distortion in the radiation pattern and a reduction of the directivity. A similar adverse effect is produced by the structural elements supporting the radiator. To avoid this, parabolic antennas often have an extended radiator; the reflector is a cutout from a paraboloid of rotation, with the radiator placed at its focus (Figure 15). In this case the electromagnetic energy flux reflected from the parabola misses the radiator and its supporting structural elements. In radio relay communications, extensive use is made of a horn parabolic antenna (Figure 16), which is a variant of a reflector antenna with an extended radiator. In this antenna the radiating horn and the parabolic reflector form an entity which practically eliminates energy leakage at the reflector’s edge. During the 1960’s, in radio relay communications, space radio communications, radioastronomy, and so on, extensive use was made of double reflector antennas (Figure 17) which consist of a main parabolic reflector, an auxiliary small reflector, and a radiator. The electromagnetic energy is fed to a radiator located at the apex of the paraboloid and radiated to the small reflector, which directs it to the main reflector. The auxiliary reflector makes it easier to obtain optimal distribution of the electromagnetic field at the aperture of the main reflector, thus ensuring the maximum directivity and permitting a reduction in the length of the line carrying energy to the radiator. The Soviet scientist L. D. Bakhrakh made a major contribution to the development of the theory and the technology of double reflector antennas. The surface utilization factor of well-constructed reflector antennas is between 0.5 and 0.7.
In addition to metallic reflectors with a parabolic profile, reflectors having the profile of a parabolic cylinder, a sphere (a spherical antenna), and other shapes are also in use. A distinctive feature of the spherical antenna is the possibility of controlling the direction of maximum radiation over a broad angular sector without significantly reducing the directivity. The Soviet scientists S. E. Khaikin and N. L. Kaidanovskii proposed the original reflector antenna for a radio telescope. Such an instrument was constructed at the Pulkovo Observatory. It consists of a movable radiator and a set of flat moving reflectors arranged on a broken line approximating a parabola. By shifting the radiator and adjusting the reflectors, the direction of maximum radiation can be controlled over a broad range.
A representative type of antenna in the UHF band is the slot antenna, in the form of a hollow metal duct with slots cut in it. Electromagnetic energy is fed into the duct and is radiated through the slots (slot dipoles). Wide use is made of a broadside antenna array of such dipoles. The array is often constructed in the form of a waveguide having a rectangular or a circular cross section (Figure 18) with slots ½ λ long cut in one wall in such a way that they are excited cophasally. The directivity of these antennas is approximately equal to three times the number of slots. The slot dipoles do not project from the metallic surface and are therefore widely used where this property is important—for example, in aircraft.
The Soviet scientists M. S. Neiman, A. A. Pistol’kors, Ia. N. Fel’d, and others made a valuable contribution to the theoretical development of slot antennas.
In addition to the broadside antenna, use is made in the UHF and EHF ranges of the traveling-wave antenna, which consists of a system of radiators that are excited in accordance with the law of traveling waves and have maximum radiation in the direction of propagation. This type includes the spiral antenna, the Yagi, the dielectric antenna, the surface wave antenna (impedance antenna), and others. The impedance antenna (generally consists of a ribbed surface and a radiator. In the antenna shown in Figure 19, the radiator is a horn. With a rib height of less than ¼λ, a traveling wave is produced along the ribbed surface which is propagated at a velocity less than that of light. This kind of antenna, like the slot type, can easily be made nonprojecting. The directivity of the traveling-wave antennas used at ultrahigh frequencies is not usually higher than 100. In the development of the theory and technology for impedance antennas, a major role was played by the work of the Soviet scientists L. D. Bakhrakh, L. D. Deriugin, M. A. Miller, V. I. Talanov, O. N. Tereshin, and others; the American scientist G. Bolljahn; and other scientists.
In the 1950’s and 1960’s, aperiodic antennas became common for the short, meter, and centimeter wavelength ranges. These differed from the other antenna types in that for a wide range (10 to 20 times or more) they had almost constant characteristics (the shape of the radiation pattern, the directivity factor, the input impedance, and so on). One of the popular types is the log-periodic antenna, one version of which is shown in Figure 20. The electromagnetic energy supplied to the antenna generates large currents only in three to five of the dipoles which have a length close to half that of the operating wave. This group of dipoles forms a so-called active area of the antenna. As the operating wavelength is varied, the antenna’s active area is shifted accordingly. Thus, the ratio of the linear dimensions for this part of the antenna to the operating wavelength does not change with frequency. This is the reason for the small dependence of the electrical characteristics on frequency. The directivity of log-periodic antennas is equal to 30–50.
Possible developments. In the 1960’s a number of possible directions were being contemplated for theoretical and technical antenna development. Most important among these were the following:
(1) The creation of antenna arrays with a large number of radiating elements (electrical dipoles, horns, and so on), each supplied from a separate transmitter output unit with an adjustable phase shifter. By controlling the relationship of the phases of the fields of the individual radiating elements, it is possible to change quickly the direction of maximum radiation, as well as the shape of the radiation pattern, for the antenna. In the same fashion, receiving antenna arrays are created with a large number of slightly directional antennas connected to separate input units of a receiver.
(2) The creation of antennas based on an aperture synthesis method, which consists particularly in shifting one or several small antennas and recording the amplitudes and phases of the received signals sequentially in a memory unit. By the proper addition of these signals, the same effect can be obtained as from a large antenna having linear dimensions equal to the path lengths of the small antennas’ displacements.
(3) The design of economical, easily erected antennas (reflector antennas, antenna towers and masts, and so on) based on the use of metallized films and pneumatics to impart the required shape to the antenna.
(4) The extensive introduction of rigorous analysis and synthesis techniques (designing for specified antenna characteristics) by using electronic computers.
(5) The development of statistical methods for antenna analysis.
REFERENCESPistol’kors, A. A.Antenny. Moscow, 1947.
Aizenberg, G. Z. Antenny ul’trakorotkikh voln. Moscow, 1957.
Markov, G. T. Antenny. Moscow, 1960.
Drabkin, A. L., and V. L. Zuzenko. Antenno-fidernye ustroistva. Moscow, 1961.
Aizenberg, G. Z. Korotkovolnovye antenny. Moscow, 1962.
G. Z. AIZENBERG and O. N. TERESHIN
The device that couples the transmitter or receiver network of a radio system to space. Radio waves are used to transmit signals from a source through space. The information is received at a destination which in some cases, such as radar, can be located at the transmitting source. Thus, antennas are used for both transmission and reception. See Radar
To be highly efficient, an antenna must have dimensions that are comparable with the wavelength of the radiation of interest. At long wavelengths such as the part of the spectrum used in broadcasting (a frequency of 1 MHz corresponds to a free-space wavelength λ of 300 m), the requirement on size poses severe structural problems, and it is consequently necessary to use structures that are portions of a wavelength in size (such as 0.1 λ or 0.25 λ). Such antennas can be described as being little more than quasielectrostatic probes protruding from the Earth's surface.
In order to control the spread of the energy, it is possible to combine antennas into arrays. As the wavelength gets shorter, it is possible to increase the size of the antenna relative to the wavelength; proportionately larger arrays are also possible, and techniques that are familiar in acoustics and optics can be employed (Fig. 1). For example, horns can be constructed with apertures that are large compared with the wavelength. The horn can be designed to make a gradual transition from the transmission line, usually in this case a single-conductor waveguide, to free space. The result is broadband impedance characteristics as well as directivity in the distribution of energy in space. Another technique is to use an elemental antenna such as a horn or dipole together with a reflector or lens. The elemental antenna is essentially a point source, and the elementary design problem is the optical one of taking the rays from a point source and converting them into a beam of parallel rays. Thus a radio searchlight is constructed by using a paraboloidal reflector or a lens. A very large scale structure of this basic form used as a receiving antenna (together with suitably designed receivers) serves as a radio telescope. Antennas used for communicating with space vehicles or satellites are generally large (compared to wavelength) structures as well. See Space communications
A small electric or magnetic dipole radiates no energy along its axis, the contour of constant energy being a toroid. The most basic requirements of an antenna usually involve this contour in space, called the radiation pattern. The purpose of a transmitting antenna is to direct power into a specified region, whereas the purpose of a receiving antenna is to accept signals from a specified direction. In the case of a vehicle, such as an automobile with a car radio, the receiving antenna needs a nondirectional pattern so that it can accept signals from variously located stations, and from any one station, as the automobile moves. The antenna of a broadcast station may be directional; for example, a station in a coastal city would have an antenna that concentrated most of the power over the populated land. The antenna for transmission to or from a communication satellite should have a narrow radiation pattern directed toward the satellite for efficient operation, preferably radiating essentially zero power in other directions to avoid interference.
The plane of the electric field of the radiated electromagnetic wave depends on the direction in which the current flows on the antenna. The electric field is in a plane orthogonal to the axis of a magnetic dipole. This dependence of the plane of the radiated electromagnetic wave on the orientation and type of antenna is termed polarization. A receiving antenna requires the same polarization as the wave that it is to intercept. By combining fields from electric and magnetic dipoles that have a common center, the radiated field can be elliptically polarized; by control of the contribution from each dipole, any ellipticity from plane polarization to circular polarization can be produced.
The input impedance of an antenna is the ratio of the voltage to current at the terminals connecting the transmission line and transmitter or receiver to the antenna. The impedance can be real for an antenna tuned at one frequency but generally would have a reactive part at another frequency.
An array of antennas is an arrangement of several individual antennas so spaced and phased that their individual contributions add in the preferred direction and cancel in other directions. One practical objective is to increase the signal-to-noise ratio in the desired direction. Another objective may be to protect the service area of other radio stations, such as broadcast stations. See Signal-to-noise ratio
The simplest array consists of two antennas. It makes possible a wide variety of radiation patterns, from nearly uniform radiation in azimuth to a concentration of most of the energy into one hemisphere, or from energy in two or more equal lobes to radiation into symmetrical but unequal lobes.
For further control over the radiation pattern a preferred arrangement is the broadside box array. In this array, antennas are placed in a line perpendicular to the bidirectional beam. Individual antenna currents are identical in magnitude and phase. The array can be made unidirectional by placing an identical array 90° to the rear and holding its phase at 90°. The directivity of such a box array increases with the length or aperture of the array.
Further use of array concepts has enabled improvements in communications. By introducing a network for each antenna element, it is possible to receive a signal from a source direction and to return a signal in the direction of the source. The returned signal can be modulated or amplified or have its frequency changed. Such an array is called a retrodirective array. Basically, the array seeks out the incoming signal and returns one of useful characteristics, such as that which is needed for the communication between a moving vehicle and a stationary or slowly moving source.
The bandwidth of an antenna may be limited by pattern shape, polarization characteristics, and impedance performance. Bandwidth is critically dependent on the value of Q; hence the larger the amount of stored reactive energy relative to radiated resistive energy, the less will be the bandwidth.
Antennas whose mechanical dimensions are short compared to their operating wavelengths are usually characterized by low radiation resistance and large reactance. This combination results in a high Q and consequently a narrow bandwidth. Current distribution on a short conductor is sinusoidal with zero current at the free end, but because the conductor is so short electrically, typically less than 30° of a sine wave, current distribution will be essentially linear. By end loading to give a constant current distribution, the radiation resistance is increased four times, thus greatly improving the efficiency but not noticeably altering the pattern.
Long-wire antennas, or traveling-wave antennas, are usually one or more wavelengths long and are untuned or nonresonant.
There are two principal approaches to constructing frequency-independent antennas. The first is to shape the antenna so that it can be specified entirely by angles; hence when dimensions are expressed in wavelengths, they are the same at every frequency. Planar and conical equiangular spiral antennas adhere to this principle (Fig. 2a). The second approach depends upon complementary shapes. According to this principle, which is used in constructing log-periodic antennas, before the structure shape changes very much, when measured in wavelengths, the structure repeats itself (Fig. 2b). By combining periodicity and angle concepts, antenna structures of very large bandwidths become feasible.
When they are to be used at short wavelengths, antennas can be built as horns, mirrors, or lenses. Such antennas use conductors and dielectrics as surfaces or solids.
By using reflectors it is possible to achieve high gain, modify patterns, and eliminate backward radiation. A low-gain dipole, a slot, or a horn, called the primary aperture, radiates toward a larger reflector called the secondary aperture. The large reflector further shapes the radiated wave to produce the desired pattern.
A beam can be formed in a limited space by a two-reflector system. The commonest two-reflector antenna, the Cassegrain system, consists of a large paraboloidal reflector. It is illuminated by a hyperbolic reflector, which in turn is illuminated by the primary feed (Fig. 3).
A series of antennas are useful in situations which require a low profile. Slot antennas constitute a large portion of this group. In essence, replacing a wire (metal) by a slot (space), which is a complement of the wire, yields radiation characteristics that are basically the same as those of the wire antenna except that the electric and magnetic fields are interchanged.
Because flush-mounted antennas present a low profile and consequently low wind resistance, slot-type antennas have had considerable use in aircraft, space-launching rockets, missiles, and satellites. They have good radiation properties and are capable of being energized so as to take advantage of all the properties of arrays, such as scanning, being adaptive, and being retrodirective. These characteristics are obtained without physical motion of the antenna structures. Huge slot antenna arrays are commonly found on superstructures of aircraft carriers and other naval ships, and slot antennas are designed as integral parts of the structure of aircraft, such as the tail or wing.
The patch antenna consists of a thin metallic film which is attached to a dielectric substrate mounted on a metallic base. Depending on its use, the patch can be of different shapes and can be driven in various fashions. Driven at one end, the radiated electric field at this end has a polarization that is in phase with the radiated electric field at the farther end of the patch antenna.
Planar antennas are designed as integral parts of monolithic microwave integrated circuits (MMICs). Coupling can be effected through the use of planar (flush-mounted) antennas fabricated directly on the microelectronics chips (integrated circuits). This arrangement eliminates the need for coaxial lines, which at these microwave frequencies exhibit considerable losses. As is the case with other planar antennas, it is possible to design circuitry so as to obtain many, if not all, the properties of arrays mentioned above. The elements of these arrays can take on the form of slot antennas or patch antennas (of course with suitable modification for use on the MMICs).