An active antenna is a device that consists of both a relatively short whip antenna and an amplifier. Active antennas are able to counteract signal attenuation due to the close proximity of the components, and are usually tuned to amplify a specific frequency. One common way to accomplish this is through the use of a field-effect transistor (FET) based amplifying circuit. Such a configuration can allow an active antenna to pick up much lower frequencies than would otherwise be possible. This has made active antennas somewhat popular with the amateur radio operating community, since they are substantially more portable than the large roof-mounted antennas that are otherwise necessary.
Due to the way that antennas interact with radio waves, there are specific formulas that can be used to determine how long an antenna should be in addition to the optimal configuration. According to these formulas, a traditional dipole antenna needs to be as long as one half of the wavelength it is meant to receive. Monopole whip antennas can be one quarter of a wavelength, but for low frequencies this can still mean very large antennas. Since amateur radio operators are allocated frequencies that have wavelengths of between 10 and 160 meters (about 28 to 525 feet), the necessary antennas can be quite long. They may range anywhere from 2.5 meters (8 feet) for a monopole in the 28Mhz range to 80 meters (262 feet) for a dipole in the 1.8Mhz range.
It is possible for shorter antennas to operate with lower frequencies, though additional components can be required to receive quality signals. This is where an active antenna can be of use. In most cases, active antennas are combinations of FET-based amplifiers and monopole whip antennas. An FET circuit can be tuned to amplify a specific frequency, allowing a short monopole antenna to operate with a wavelength greater than four times its length. The quality of a signal provided by an active antenna will typically be lower than that of a properly sized dipole, but the portability is much greater.
Portability is one of the reasons for having the whip antenna and amplifier connected directly together, though signal attenuation is another important factor. Due to attenuation, the strength of a signal can be lost as it passes through any type of wiring. Since short whip antennas are typically not properly sized to resonate with medium and high frequencies, the signals they receive can degrade or even be lost during transition to a remotely located amplifier. By placing the amplifier directly on the active antenna, the chance for attenuation can be greatly decreased.