Antennas are electromagnetic devices that convert electrical signals into radiated electromagnetic waves during transmission and convert incident electromagnetic waves into electrical signals during reception. They provide the interface between guided electrical energy and electromagnetic propagation through space. Antennas are characterised by properties including radiation pattern, gain, directivity, efficiency, impedance, bandwidth, polarisation and physical aperture. They can range from tiny structures integrated into consumer electronics to enormous reflector systems used for radio astronomy and satellite communication. Although an antenna can often transmit and receive, its precise behaviour depends on frequency, geometry, surrounding materials, orientation and the electromagnetic environment in which it operates.
The physical behaviour of an antenna is fundamentally related to wavelength. A structure whose dimensions are a substantial fraction of the operating wavelength can support useful electromagnetic current or field distributions and radiate efficiently. Common antenna types include dipoles, monopoles, loops, patches, horns, slots, helices and reflector antennas. A half-wave dipole is a classic resonant structure whose length is approximately half a wavelength after accounting for practical electrical effects. A monopole can provide comparable behaviour above a suitable ground plane with approximately half the physical length of a corresponding dipole. Patch antennas use conductive regions over a dielectric substrate and are particularly useful when a low-profile planar structure is required.
Antenna radiation pattern describes how radiated power varies with direction. An isotropic radiator is an idealised reference that radiates equally in every direction, although no practical antenna is perfectly isotropic. Real antennas have main lobes, sidelobes and sometimes nulls. Beamwidth describes the angular extent of a principal lobe according to a specified criterion, commonly the half-power or −3 dB points. Directivity measures how concentrated radiation is relative to an isotropic radiator, while gain additionally accounts for efficiency. High-gain antennas concentrate energy into narrower angular regions, increasing useful signal strength in the intended direction while reducing coverage elsewhere. This trade-off is central to antenna selection because high gain does not mean uniformly stronger transmission in all directions.
Antenna impedance determines how effectively an antenna interfaces with a transmission line or RF circuit. If the antenna input impedance does not appropriately match the source impedance, part of the incident energy is reflected rather than delivered to the radiating structure. Return loss and reflection coefficient quantify this behaviour. A matching network can transform the antenna impedance to a more suitable value at a particular frequency or across a desired bandwidth.
However, matching an antenna does not necessarily make it an efficient radiator; conductor loss, dielectric loss, surface-wave effects and other mechanisms can still dissipate power. Antenna efficiency therefore needs to be distinguished from impedance matching. A poorly matched but inherently efficient antenna may radiate little power because of reflection, while a well-matched antenna may still have significant internal losses.
Polarisation describes the orientation and time variation of the electric-field vector of a radiated electromagnetic wave. Common forms include linear, circular and elliptical polarisation. Two linearly polarised antennas can experience substantial polarisation mismatch when their orientations differ, reducing received power. Circular polarisation can be advantageous in systems where antenna orientation changes or where reflections may alter polarisation. Satellite navigation commonly uses controlled polarisation characteristics to support signals received from different orientations. Polarisation is therefore a design variable alongside frequency, gain and radiation pattern. In dual-polarised systems, two orthogonal polarisation channels can also be used to increase capacity or support MIMO processing.
Antenna arrays combine multiple radiating elements so that their electromagnetic fields interact in a controlled manner. By adjusting amplitude and phase, an array can shape its radiation pattern, increase gain or steer a beam. Phased-array Antennas extend this principle by controlling element phases electronically. Multiple antennas are also fundamental to MIMO Technology, where spatially separated elements provide multiple propagation channels for transmitting or receiving independent streams or improving reliability. Array performance depends on element spacing, mutual coupling, phase accuracy, amplitude balance, calibration and the surrounding physical structure. At high frequencies, many elements can fit into a small aperture because the wavelength is short, but manufacturing tolerances and packaging effects can become increasingly important.
Antenna performance is strongly affected by the environment. Nearby conductive or dielectric objects can detune an antenna, alter its radiation pattern and change its efficiency. A smartphone antenna, for example, must operate near batteries, displays, circuit boards and the user's body, all of which influence electromagnetic behaviour. Vehicle antennas interact with the vehicle body, while building-mounted antennas are affected by structures and mounting hardware. Antenna designers therefore use electromagnetic simulation, prototypes and measurement systems such as anechoic chambers and antenna ranges. Measured characteristics may differ from isolated simulation because cables, enclosures, connectors and environmental objects form part of the actual electromagnetic system.
Antenna engineering has developed from relatively simple wire structures into highly integrated electromagnetic systems. Modern antennas can incorporate dielectric materials, metamaterial-inspired structures, electronically controlled phase shifters, integrated RF electronics and adaptive beamforming. Satellite and radar applications may use large reflector or array antennas requiring extremely precise mechanical and electrical control. Consumer devices prioritise compactness, cost, efficiency and operation across multiple frequency bands. There is no universally superior antenna type because requirements differ among applications. Antenna design is ultimately an optimisation problem involving frequency, bandwidth, size, gain, efficiency, polarisation, radiation pattern, cost, mechanical constraints and the surrounding electromagnetic environment.
