antenna

An antenna is a transducer that converts electrical energy into electromagnetic waves for transmission or electromagnetic waves into electrical energy for reception. It radiates or receives radio waves and is designed to operate over a specific frequency or band. Performance depends on size relative to the wavelength and on how well the antenna is matched to the feed line.

Key concepts include resonance, impedance, and radiation patterns. A resonant antenna is typically sized to a fraction of the wavelength, such as a half-wave dipole or a quarter-wave monopole, to maximize efficiency. The input impedance should be matched to the transmission line to minimize reflections, which are quantified by the standing wave ratio. The radiation pattern describes how power is distributed in space; patterns can be omnidirectional or highly directional, as in Yagi or parabolic antennas. Antennas have parameters such as gain, bandwidth, polarization, and efficiency, which influence performance in a given application.

Polarization describes the orientation of the electric field of the radiated wave and can be linear, circular, or elliptical. Alignment between transmitting and receiving antennas is important for optimal reception.

Common types include wire antennas (dipoles, folded dipoles, monopoles), loop antennas, directional arrays (Yagi, log-periodic), patch antennas, and parabolic dishes. Antennas are used in broadcasting, cellular and Wi‑Fi networks, satellite links, radar, and various sensing and industrial applications.

Historically, the radiating properties of antennas were demonstrated in the late 19th century, with practical development in the early 20th century enabling modern wireless communication.