Antenna array (electromagnetic)

Mammut phased array radar World War II

A multiband television reception antenna in the United States

An antenna array is a set of individual antennas used for transmitting and/or receiving radio waves, connected together in such a way that their individual currents are in a specified amplitude and phase relationship. This allows the array to act as a single antenna, generally with improved directional characteristics (thus higher antenna gain) than would be obtained from the individual elements. The resulting array in fact is often referred to and treated as "an antenna," particularly when the elements are in rigid arrangement with respect to each other, and when the ratio of currents (and their phase relationships) are fixed. On the other hand, a steerable array may be fixed physically but has electronic control over the relationship between those currents, allowing for adjustment of the antenna's directionality without requiring physical motion.

The array uses electromagnetic wave interference to enhance the radiative signal in one desired direction at the expense of other directions. It may also be used to null the radiation pattern in one particular direction, especially for a receiving antenna in the face of a particular interfering source.

Classifications

An antenna array may be classified as Parasitic or Driven.^[1] The best known parasitic array is the Yagi-Uda antenna, consisting of several elements but only one of which has an electrical connection to the transmitter or receiver. The other elements are electromagnetically coupled to that element (and to each other) through proximity, and are tuned so that their currents will be in the appropriate phases to enhance the directionality of the resulting array.

A driven array implies that all of the elements have an electrical connection to the transmitter or receiver, through circuitry that tailors their respective currents to the same end. One common example is the log-periodic dipole array frequently used as a rooftop TV antenna. Driven arrays, in which all the radiating elements are connected to the energy source, have smaller losses than parasitic arrays.

Driven arrays often (but not always) consist of elements which are identical; frequently these are just half-wave dipole or quarter-wave monopole (vertical) elements. The resulting array inherits the characteristics of that basic element, but the resulting array behaviour is determined more by the geometrical arrangement of the elements and the phases (and sometimes amplitudes) which each is assigned. Typically there will be a number of identical elements equispaced along one direction. In that case, an end-fire array is one in which the intended directionality is in the direction of the array. A broadside array has its intended directionality at right angles to the array direction. Of course there are also more complex arrays possible in which these classifications do not apply.

Sometimes a broadside array consists of identical elements arranged vertically. In this case, the directional pattern of the resulting antenna in the horizontal plane may be unchanged from that of a single such element. However by adding those additional elements, the antenna pattern in the vertical plane is altered in order to supply a greater amount of power in horizontal directions (in order to communicate with terrestrial locations) at the expense of directions aimed more toward the sky or ground which are not useful. Consequently the gain of the antenna is increased without sacrificing directionality towards any intended stations.^[2]

Characteristics

The main purpose of creating a fixed antenna array is an increase in the antenna's directional gain; an array consisting of N identical elements can achieve an increase in gain of up to a factor of N if optimally fed. A parasitic array does not achieve such an increase since there are more constraints on the distribution of currents attainable through passive coupling.

Use of arrays for increasing antenna gain are an alternative to so-called aperture antennas, in which the gain is achieved using a single geometric structure, more akin to an optical focusing device. Designs such as the horn antenna and parabolic dish antenna become more practical at higher frequencies (shorter wavelengths) whereas antenna arrays may be practical at any wavelength. At lower frequencies, where rigidly turning a large structure may be unfeasible, an array may be made steerable (or may just be tuned to optimize a particular radiation pattern) through electronic adjustment of the elements' relative phases. This principle is often used in AM broadcast, where two or more large fixed towers (vertical monopoles) may be "phased up" to create a desired radiation pattern; obviously such structures cannot be moved once erected!

References