Antennas

What Are Antennas?

Antennas are transducers that convert a guided electromagnetic wave traveling on a transmission line into a wave radiated into free space, and that perform the reverse conversion on reception. Every radio link terminates in one at each end, which places antenna design at the intersection of electromagnetics, microwave circuit theory, and materials engineering. The subject is normally studied together with wave propagation, because the performance a designer can claim for an antenna depends on the medium the radiated field must cross.

An antenna is characterized by a compact set of quantities. The radiation pattern maps radiated power against direction. Gain and directivity measure how sharply that pattern concentrates energy relative to an isotropic reference. Input impedance governs how much power the feed delivers rather than reflects, and polarization must be matched at both ends of a link or signal is lost. All of these vary with frequency, so usable bandwidth is a first-order design constraint rather than an afterthought.

Radiating Elements and Feeds

The element family runs from wire structures such as dipoles and monopoles through apertures such as horns and slots to reflector systems in which a small feed illuminates a shaped parabolic surface. Microstrip antennas, etched as copper patches on a grounded dielectric substrate, dominate where a low profile and photolithographic fabrication matter, and microstrip antenna arrays extend the approach to many elements on a single board. Fractal geometries such as Sierpinski gaskets and Koch curves are used to pack electrical length into a small footprint and to produce the repeated resonances that give multiband behavior. Feeding arrangements range from a coaxial probe or a microstrip line to aperture coupling and, at millimeter wavelengths, to rectangular waveguide, where waveguide theory sets the cutoff frequencies and mode structure the feed can support. Shrinking an element below roughly a tenth of a wavelength runs into physical bounds relating size, bandwidth, and loss, a subject treated in work on fundamental limitations for antenna radiation efficiency.

Arrays and Beam Steering

Placing many elements in a regular lattice and controlling the relative phase of their excitations steers the composite beam without moving any hardware. Element spacing is typically held near half a wavelength, since wider spacing admits grating lobes that waste power in unintended directions. Analog beamforming networks build the required phase progression passively: a Butler matrix combines hybrid couplers and fixed phase shifters so that N input ports produce N mutually orthogonal beams, which is the basis of switched-beam systems. Spacing elements far apart serves a different purpose, spatial diversity, in which independently fading copies of a signal are combined to improve link reliability. Multiple-antenna operation of this kind was standardized for wireless local area networks in IEEE 802.11n and remains central to MIMO radio.

Antenna Measurements

Because analysis and fabrication both introduce error, measured performance is the accepted record, and the quantities reported carry the meanings fixed by IEEE Std 145, the standard for definitions of terms for antennas, a document whose lineage runs back to a 1948 IRE publication and which settles terms such as gain, directivity, radiation pattern, and polarization. Test ranges include outdoor far-field ranges, indoor anechoic chambers lined with absorber, compact ranges that use a reflector to synthesize a plane wave, and near-field scanners that sample amplitude and phase on a surface and transform the result to the far field. Gain is commonly established by comparison against a calibrated standard gain horn. Procedures and reporting conventions are set out in IEEE Std 149-2021, the recommended practice for antenna measurements, which replaced the 1979 edition of the same document.

Applications

Antennas have applications in a wide range of fields, including: