The term “passive antenna” refers to a radiation-emitting device made completely of inert materials. In most cases, an Iridium passive antenna system consists of a passive radiator (antenna element), passive impedance matching, passive balun, passive tuning (capacitive or inductive), and passive interconnects (commonly 50 Ohm or 75 Ohm impedance). Depending on whether the antenna system is packed as a whole or divided into individual components, a circulator or isolator may be included in the definition of passive antenna. In some circumstances, the term “antenna” can be used to designate both an antenna part or structure and a whole antenna system.
In order to optimize the Iridium antenna performance for a specific application, an active system makes use of some type of active electrical enhancement or control. Low-noise or power amplifiers, active filtering, or even switched antenna radiator sections are examples of active antenna components, as are tunable filters or switched filter banks. It is possible to use software or analog/digital control systems to activate active antennas automatically or to engage/configure them manually. Any application that calls for more degrees of freedom, tunability, greater signal intensity to and from the antenna radiator, or a configurable antenna, will benefit greatly from using Iridium active antennas as part of the overall system design. When utilizing LNAs adjacent to the antenna, an Iridium active antenna can provide greater signal rejection, lower signal-to-noise ratios (SNRs), higher transmit power, better antenna impedance matching, and other performance advantages.
For example, an Iridium active antenna system (AAS) may include a complete transmit/receive component (TRX) as well as a phase and amplitude adjustment mechanism to facilitate beamforming. Digital processing allows for many spatial streams to be generated from a multi-element antenna array in other types of AAS, such as MIMO. If the AAS beamforming antenna array is driven by a digital or hybrid technology, it can use analog beamforming or hybrid beamforming. As with the latest 4G LTE AAS modules and planned for 5G infrastructure, an even more advanced AAS may include all of the antenna array, TRX, modulation/demodulation, DAC, analog-to-digital conversion (ADC), digital processing, beamforming and MIMO carrier aggregation (CA) and networking communication hardware in a single module.
It is also possible to have an Iridium active antenna as part of a “smart antenna,” or “cognitive radio,” which is able to sense the electromagnetic (EM) spectrum in its surroundings or to deliver intelligence to the environment. In this approach, a smart antenna may adapt to its current environment and optimize its performance. An Iridium active antenna or other radio technology may potentially be used in a cognitive radio system to learn about its environment and the dynamics of wireless communications in that context and develop ways to improve service.
There are fewer possibilities for suppliers of active antennas because they are more expensive and sophisticated than Iridium passive antennas. It is also possible that active antennas are more difficult to troubleshoot and maintain than passive antennas, but some AAS and active antennas may feature built-in test and diagnostic technology (BIST and BID) to help with the troubleshooting procedure. Many times an active antenna or AAS is only usable for a specific application because it is part of a wider communications infrastructure and must work in a very precise manner. When the need arises or the needs change, a number of different antennas can be employed with passive antennas.