ty of transmitter and receiver applies to other radio terms as well. One might expect that a beam antenna would transmit a beam, and it does. However, a beam antenna also receives on the same narrow beam-a fact that is not obvious from the term. 6 In Chap. 4 we will discuss directional antennas, including phased arrays and smart adaptive antennas, more extensively. The mobile telephone antenna, however, is presently designed as an omnidirectional antenna, not just 360 degrees but a full 4TT sphere with no antenna gain (0 dB gain). This is essential because a subscriber may ro- tate or tilt the mobile telephone and its antenna in any direction. If the mobile telephone could have significant antenna gain, then the more efficient radio link would improve cov- erage in poor reception areas and allow the battery to function longer without recharg- ing. Also, if the mobile telephone could have some directional control of its radio signal path, then interference levels would be reduced and system capacity could be increased. There is one technology, available as a third-party add-on to many cell phones. It is an antenna extender that fits over the back of the phone and is said to improve signal quality. If this technology is indeed beneficial, it might be included in future cell phone design. 5This same two-way reciprocity of signal path characteristics is used in engineering sound sys- tems as well as radio systems. When the authors think of a beam, we think of a Star Trek phaser, the transmission of a nar- row beam. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The User Terminal The User Terminal Looking into the future, we might ask if there are any other technologies that would increase antenna gain and improve the quality of wireless telephone service. If a smart array antenna could be built into a mobile telephone, then it could constantly 