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

transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

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

capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...

s and

inductor An inductor, also called a coil, choke, or reactor, is a Passivity (engineering), passive two-terminal electronic component, electrical component that stores energy in a magnetic field when an electric current flows through it. An inductor typic ...

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 current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...

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

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

. The active circuit consists of an impedance translating stage and an optional amplification stage. This arrangement is especially useful for constructing

low frequency Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre waves. LF radio waves ...

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, electric current, current, and frequency to power ...

, or through the signal feeder itself (

phantom power Phantom power, in the context of Professional audio, professional audio equipment, is Direct current, DC electric power equally applied to both signal wires in Balanced line, balanced microphone cables, forming a phantom circuit, to power m ...

).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 Ulrich L. Rohde,

Magdalena Salazar Palma Magdalena Salazar Palma (born 1949) is a Spanish electrical engineer focusing on the computational modeling of wireless communications and Antenna (radio), antennas. She is a professor emeritus in the Department of Signal Theory and Communicati ...

, Tapan K. Sarkar: Electrically short antennas

Feb 11, 2016 {{reflist Antennas (radio) Radio frequency antenna types Radio electronics Television terminology

See also

References