An active antenna is an antenna that contains active electronic components. This allows the construction of antennas of limited size and / or wide frequency range. Most active antennas use antenna parts of minimal conducting area, e.g., a small whip, connected to the active component (usually an FET). The signal attenuation caused by the antenna-size-to-wavelength mismatch is compensated by an 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, spacial, 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.