Antenna

(electromagnetism) A device used for radiating or receiving radio waves. Also known as aerial; radio antenna.
(invertebrate zoology) Any one of the paired, segmented, and movable sensory appendages occurring on the heads of many arthropods.

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 also 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 also Radio telescope; Space communications; Transmission lines; Waveguide.

dielectric lens with a dipole-disk feed. (After D. J. Angelakos and T. E. Everhart, Microwave Communications, Krieger, 1983)">
Various types of antennas. (a) Top-loaded vertical mast; (b) center-fed horizontal antenna; (c) horn radiator; (d) paraboloidal reflector with a horn feed; (e) corrugated-surface wave system for end-fire radiation; (f) zoned dielectric lens with a dipole-disk feed. (After D. J. Angelakos and T. E. Everhart, Microwave Communications, Krieger, 1983)

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. See also Directivity; Radio broadcasting.

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. See also Polarization of waves.

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 also 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. See also Q (electricity).

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.

Equiangular spiral (after D. J. Angelakos and T. E. Everhart, Microwave Communications, Krieger, 1983). (b) Log-periodic structure.">
Frequency-independent antennas. (a) Equiangular spiral (after D. J. Angelakos and T. E. Everhart, Microwave Communications, Krieger, 1983). (b) Log-periodic structure.

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. See also Microwave optics.

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).

Cassegrain system.

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). See also Microwave.