Active Antenna Circuits
An active antenna is an antenna that has an amplifier built into it. This allows the amplifier to be extremely close to the antenna, which minimizes transmission line losses.
The signal attenuation caused by using an element that is many times shorter than the ideal half wavelength is lessened by the amplifier circuit, which may be tuned to provide maximal amplification on the desired frequency. Most active antennas consist of a small whip connected to an amplifier circuit (usually FET-based).
This arrangement is especially useful for constructing compact low frequency antennas, which may be difficult or prohibitively expensive for an individual to construct at an ideal size, since the wavelengths involved range from one to ten kilometers.
Power for the amplifier may be supplied by batteries, a filtered power supply, or through the coaxial cable itself. Antennas with built-in amplifiers are usually used only for receiving, since transmitters usually have their own amplifiers, which can handle the high power supplied by the transmitter.