An antenna is used to either transmit or receive electromagnetic waves. It serves as a transducer converting guided waves into free-space waves in the transmitting mode or vice versa in the receiving mode. Antennas, including aerials, can take many forms according to the radiation mechanism involved and can be divided into different categories. Some common types are wire antennas, aperture antennas, reflector antennas, lens antennas, traveling-wave antennas, frequency-independent antennas, horn antennas, printed and conformal antennas, etc. (see Antennas). When applications require radiation characteristics that cannot be met by a single radiating element, multiple elements are employed. Various configurations are utilized by suitably spacing the elements in one or two dimensions. These configurations, known as array antennas, can produce the desired radiation characteristics by appropriately feeding each individual element with different amplitudes and phases, which allows increasing the electrical size of the antenna. Furthermore, antenna arrays combined with signal processing lead to smart antennas (switched-beam or adaptive antennas), which offer more degrees of freedom in wireless system design (1). Moreover, active antenna elements or arrays incorporate solid-state components producing effective integrated antenna transmitters or receivers with many applications (see Antennas and Ref. 1).

Regardless of the antenna considered, there are some fundamental figures of merit that describe its performance. The response of an antenna as a function of direction is given by the antenna pattern. This pattern commonly consists of a number of lobes; the largest one is called the main lobe, and the others are called sidelobes, minor lobes, or back lobes. If the pattern is measured sufficiently far from the antenna so there is no change in the pattern with distance, the pattern is the so-called far-field pattern. Measurements at shorter distances yield near-field patterns, which are a function of both angle and distance. The pattern may be expressed in terms of the field intensity (field pattern) or in terms of the Poynting vector or radiation intensity (power pattern). If the pattern is symmetrical, a simple pattern is sufficient to completely specify the variation of the radiation with angle. Otherwise, a three-dimensional diagram or a contour map is required to show the pattern in its entirety. However, in practice two patterns, perpendicular to each other and to the main-lobe axis, may suffice. These are called the principal-plane patterns for the E plane and the H plane, containing the field vectors E and H, respectively.

Having established the radiation patterns of an antenna, some important parameters can now be considered, such as radiated power, radiation efficiency, directivity, gain, and antenna polarization. All of them will be considered in detail in this article.

Here scalar quantities are presented in lightface italics, while vector quantities are boldface, e. g., the electric field E (vector) of magnitude E (=|1?|) (scalar). Unit vectors are boldface with a circumflex over the

letter;х, У,Z and^ are the unit vectors in the x, y, z, and r directions, respectively. A dot over a symbol means that the quantity is harmonically time-varying or a phasor. For example, taking the electric field, /£ represents

^ jjr __ " ff1 jjt

a space vector and time phasor, butfi’.T, is a scalar phasor. The relations between them are — * , where 1 =

Eejmt.

The first section of this article introduces several antenna patterns, giving the necessary definitions and presenting the common types. The field regions of an antenna are also pointed out. The most common reference antennas are the ideal isotropic radiator and the very short dipole. Their fields are used to show the calculation and meaning of the different parameters of antennas covered in this article. The second section begins with a treatment of the Poynting vector and radiation power density, starting from the general case of an electromagnetic wave and extending the definitions to a radiating antenna. After this, radiation performance measures such as the beam solid angle, directivity, and gain of an antenna are defined. In the third section the concepts of wave and antenna polarization are discussed. Finally, in the fourth section, a general case of antenna pattern calculation is considered, and numerical solutions are suggested for radiation patterns that are not available in simple closed-form expressions.