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A passive antenna is a radiating element composed entirely of passive components. Typically, this means that a passive antenna system is one with at least a passive radiator (antenna element), passive impedance matching, passive balun, passive tuning (capacitive or inductive), and passive interconnect (commonly 50 Ohm or 75 Ohm impedance). A circulator or isolator may also be considered part of a passive antenna, depending on if the antenna system is packaged as a unit or separate parts. Ambiguously, the term antenna may be used to describe an antenna element or structure as well as an antenna system, in some cases.
An active antenna system uses some method of active electrical enhancement or control to provide improved antenna performance for a given application. Active antenna components may consist of amplifiers, such as low-noise amplifiers (LNAs) or power amplifiers (PAs), active filtering (tunable filters or switched filter banks), or potentially even switched antenna radiator sections. Active antennas enable control and performance enhancement which can either be manually engaged/configured, or can be enabled automatically by software or analog/digital control systems. Active antennas are crucial for many applications that require additional degrees of freedom, tunability, increased signal strength to/from the antenna radiator, or configurable antenna. Active antenna can provide better rejection of unwanted signals, lower signal-to-noise ratio (SNR) using LNAs close to the antenna, high transmit power, better impedance match to the antenna, and other performance benefits depending on the active antenna technology used.
A more advanced version of an active antenna is an active antenna system (AAS) which may employ a complete transmit/receive (TRX) component along with phase and amplitude adjustment to enable beamforming. Other forms of AAS include multi-input multi-output (MIMO) capability that includes digital processing that enables multiple spatial streams from a multi-element antenna array. Antenna arrays can use analog beamforming, digital beamforming, or hybrid-beamforming depending on the technology employed to drive the AAS beamforming antenna array. Even more advanced AAS may include all of the antenna array, TRX, modulation/demodulation, digital-to-analog conversion (DAC), analog-to-digital conversion (ADC), digital processing, beamforming, MIMO, carrier aggregation (CA), and networking communication hardware in a single module, as is done with the latest 4G LTE AAS modules and is planned for 5G infrastructure.
Active antennas may also be part of a smart antenna, or cognitive radio, and include some awareness of the electromagnetic (EM) spectrum in their environment or include the ability to externally provide intelligence. In this way a smart antenna could reconfigure itself for best performance in its current environment. A cognitive radio system may even be able to learn about its environment and the dynamics of wireless communications in its environment and develop strategies to enhance its service using active antenna technology or other radio technology.
Typically, active antennas are more expensive and complex than passive antennas, hence there are less available options for suppliers of active antennas. Also, active antennas may be more difficult to troubleshoot and service than passive antennas, though some active antennas and AAS may also include built-in-self test (BIST) and built-in-diagnostic (BID) technology that aids with the troubleshooting process. In many cases, an active antenna or AAS is part of a much larger communications infrastructure and must perform in a very specific manner, making the active antenna or AAS only useful for that given application. With passive antenna, often a variety of different antenna may be used given the need or changing requirements.
