How does antenna diversity work?

　　One of the more difficult aspects of radio communications to understand is that radio link quality is dependent on a variety of external factors. These variables intertwine to create a complex transmission environment that is often difficult to explain. However, understanding the fundamental concepts often helps to understand the variability of radio link quality. Once these fundamental concepts are understood, many of these issues can be addressed through a low-cost, easily implemented technique called antenna diversity.

　　Environmental Considerations

　　The primary environmental factors that affect the sustained stability of radio communication links are phenomena known as multipath/fading and antenna polarization/diversity. These phenomena can have either constructive or destructive effects on link quality, depending on the specific environment. With so many possible scenarios, understanding how specific environmental conditions will affect a radio link at any given moment and what the resulting link quality will be can be challenging.

　　Antenna Polarization/Diversity

　　The phenomenon known as antenna polarization is caused by the directional properties of a given antenna. While antenna polarization is sometimes interpreted as a degradation in the quality of certain radio communication links, some radio communication designers often exploit this property to adjust antennas to achieve desired results by restricting the transmitted and received signals to a specific range of directions. This is possible because antennas radiate unevenly in all directions, and this property can be exploited to shield RF noise from sources in other directions.

　　Simply put, antennas are categorized as omnidirectional and directional. Omnidirectional antennas transmit and receive signals with equal strength in all directions, while directional antennas transmit and receive signals in a limited range of directions. To build a highly reliable link, it's important to first understand the application. For example, if a link's signals originate only from a specific direction, choosing a directional antenna is more beneficial. A receiver equipped with a directional antenna receives signals from transmitters within the line of sight determined by the antenna's directional properties, while signals from transmitters outside this range are blocked.

　　A transmitter equipped with a directional antenna radiates most of its energy in a predetermined direction, rather than transmitting in all directions, without compromising its coverage. To simplify the understanding of antenna profiles, antenna manufacturers provide antenna radiation diagrams.