Diversity Antennas

Diversity Antennas

Diversity is a technique that improves reception of radio waves by taking advantage of the fact that signals that vary with time (e.g., fading) are not the same at separated locations. In other words, the fading of a signal may be quite different for two locations separated by as little as one wavelength. To take advantage of this, two antennas, separated by some distance, are used to receive the same signal. Of the two signals, the one with the highest signal level, at any given time, is automatically sent to the receiver. This process is only useful for reception. The electronics required for this kind of signal processing are sometimes part of the antenna system.

Adaptive antennas extend the concept of diversity another step further. These antennas usually incorporate more than just two elements (i.e., individual antennas) in the array. An adaptive antenna can modify its radiation pattern (within limits) in real time to ensure that the main lobe points in the direction of greatest signal level. Alternatively (or, possibly, simultaneously), the same technique can be used to point a null in the direction of an unwanted, interfering signal.

Antennas can be classified in several ways. One way is the frequency band of operation. Others include physical structure and electrical/electromagnetic design. The antennas commonly used for LMR—both at base stations and mobile units—represent only a very small portion of all the antenna types.

Most simple, nondirectional antennas are basic dipoles or monopoles. More complex, directional antennas consist of arrays of elements, such as dipoles, or use one active and several passive elements, as in the Yagi antenna.

New antenna technologies are being developed that allow an antenna to rapidly change its pattern in response to changes in direction of arrival of the received signal. These antennas and the supporting technology are called adaptive or “smart” antennas and may be used for the higherfrequency LMR bands in the future.

Table of Content Antennas :

• Dipoles and Monopoles
• Base-Station Applications
• Corner Reflector
• Yagi
• Log-Periodic
• Arrays
• Unusual Antennas
• Active Antennas
• Mobile Applications