An active antenna is an antenna that contains active electronic components such as

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s, as opposed to most antennas which only consist of passive components such as metal rods,

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s and

inductor An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An inductor typically consists of an insulated wire wound into a c ...

s. Active antenna designs allow antennas of limited size to have a wider frequency range (

bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

) than passive antennas, and are primarily used in situations where a larger passive antenna is either impractical, such as inside a portable radio or on a vehicle, or impossible, such as in a suburban residential area with restrictions on large outdoor antennas. Most active antennas consist of a short conventional antenna, such as a small

whip antenna A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so ...

, connected to an active component (usually a

FET The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs contr ...

). The active circuit compensates for the signal attenuation caused by the mismatch between antenna size and signal

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

. The active circuit consists of an impedance translating stage and an optional amplification stage. This arrangement is especially useful for constructing low frequency antennas which, due to budgetary, spatial, or practical requirements, must be kept compact. Power for the active components may be supplied by batteries, a filtered

power supply A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a ...

, or through the signal feeder itself (

phantom power Phantom power, in the context of professional audio equipment, is DC electric power transmitted through microphone cables to operate microphones that contain active electronic circuitry. It is best known as a convenient power source for con ...

).Poole, Ian D. "Antenna Systems." Newnes Guide to Radio and Communications Technology. Amsterdam ; London: Newnes, 2003. Print. Antennas containing active impedance translating and (optionally) amplifying stages are usually used only for receiving, since operation of such stages is unidirectional.

References

Ulrich L. Rohde: Active Antennas, RF Design, May/June 1981 {{reflist Antennas (radio) Radio frequency antenna types Radio electronics Television terminology

See also

References