What is an antenna?
An antenna is, in the words of IEEE Std 145, “that part of a transmitting or receiving system that is designed to radiate or to receive electromagnetic waves.”[1] It converts the guided energy on a feed line into a radiated wave (and back again when receiving), and its usefulness is described by a handful of parameters: radiation pattern, directivity, gain, efficiency, impedance, bandwidth and polarization.[2]
How does an antenna turn current into radiation?
Radiation is produced by time-varying current, that is, by accelerating charge. A conductor carrying an alternating current with a significant fraction of a wavelength in extent creates electric and magnetic fields whose outward-travelling part detaches from the structure and propagates as a spherical wave.[2] The antenna is designed so that this radiated power is large compared with the power stored in its reactive near field and lost as heat. See how antennas radiate.
What parameters describe an antenna?
| Parameter | What it describes | Typical unit |
|---|---|---|
| Radiation pattern | Angular distribution of radiated power | dB vs angle |
| Directivity | Peak intensity relative to an isotropic source | dBi |
| Gain | Directivity times radiation efficiency | dBi |
| Radiation efficiency | Radiated power / accepted power | % or dB |
| Beamwidth | Angular width of the main lobe | degrees |
| Input impedance | Voltage/current ratio at the terminals | Ω |
| VSWR / return loss | Match to the feed line | ratio / dB |
| Polarization | Orientation of the radiated E-field | linear, circular, axial ratio |
| Bandwidth | Frequency range meeting a specification | % or Hz |
What are the main families of antennas?
Textbooks group antennas as wire antennas (dipoles, monopoles, loops, helices), aperture antennas (horns, slots), printed antennas (patches, IFAs and PIFAs), reflector antennas (reflectors and dishes), frequency-independent antennas (spirals, log-periodics) and arrays.[2][3]
Is a receiving antenna different from a transmitting antenna?
For linear, passive, isotropic materials the reciprocity theorem guarantees that an antenna has the same pattern, gain and impedance whether it transmits or receives.[2] Practical differences come from power handling, non-reciprocal components such as ferrites, and the fact that receive performance at low frequencies is often limited by external noise rather than efficiency.[4]
Frequently asked questions
What is the simplest antenna?
The short (Hertzian) dipole is the simplest analytical model; the half-wave dipole is the simplest practical resonant antenna.
Does an antenna amplify a signal?
No. Gain is a redistribution of power in angle relative to an isotropic radiator; a passive antenna never radiates more power than it accepts.
Why are antennas often about half a wavelength long?
A half-wave dipole is resonant, so its input impedance is nearly resistive (about 73 Ω) and easy to match.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “IEEE Standard for Definitions of Terms for Antennas (IEEE Std 145-2013)”. IEEE Antennas and Propagation Society. 2014.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
- “Antenna Theory and Design, 3rd ed.”. W. L. Stutzman and G. A. Thiele, Wiley. 2012.
- “Recommendation ITU-R P.372: Radio noise”. ITU-R. current edition.