An active antenna is an antenna that contains active electronic components such as transistors, as opposed to most antennas which only consist of passive components such as metal rods, capacitors and inductors. Active antenna designs allow antennas of limited size to have a wider frequency range (bandwidth) than passive antennas, and are primarily used in situations where a larger passive antenna is either impractical (inside a portable radio) or impossible (suburban residential area that disallows use of large outdoor low-frequency antennas).
Most active antennas consist of a short conventional antenna, such as a small whip antenna, connected to an active component (usually a FET). The signal attenuation caused by the antenna-size-to-wavelength mismatch is compensated by the active circuit. The active circuit consists of an impedance translating stage and an optional amplification stage. This arrangement is especially useful for constructing compact low frequency antennas which, due to budgetary, spatial, or practical requirements (e.g., installation in vehicles), must be downsized. Low frequency signal wavelengths range from one to ten kilometers.
Power for the active components may be supplied by batteries, a filtered power supply, or through the signal feeder itself (phantom power). Antennas containing active impedance translating and optionally amplifying stages are usually used only for receiving, since operation of such stages is unidirectional.
Ulrich L. Rohde: Active Antennas, RF Design, May/June 1981
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