Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts Radio Engineering Concepts Figure 1.4 Antenna with no gain and with 10 dB of gain. Consider a perfectly omnidirectional radio transmitter. The surface area of a sphere is 4 , so we sometimes call this spherical symmetry a 4TT radiator. If we focus the en- ergy in a circular pattern out of the sides of the antenna system, then we have a hori- zontally omnidirectional transmitter with much less energy coming out its top and bot- tom. This is illustrated in Fig. 1.4. By squeezing nearly all the energy into one-tenth Sector +20dB +10dB +20dB +10dB Figure 1.5 Antenna radiation pattern. 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. Radio Engineering Concepts Key Radio Concepts the surrounding spherical area, we can achieve a factor of 10 increase in ERP, a 10-dB antenna gain. This is shown in the figure. Narrowing the field of this 10-dB gain antenna to a 120-degree horizontal sector gives us another factor of 3, that is, 5 dB, for a total antenna gain of 15 dB. There are two rea- sons to use such an antenna. First is the radio gain issue: Higher-gain antennas mean that we can reach further (coverage) using lower-power amplifiers (efficiency). However, a second reason to use a narrower beam is that we can be selective in our coverage area. If we use a 120-degree sector with one antenna, then the area behind that antenna, which we call the backlobe, can be served by another antenna. We call these separate ser- vice areas sectors. The lower the cross-interference between the two antennas, the more subscribers we can serve with those antennas. Antennas have different radiation pat- terns typically drawn on a circular scale in decibels, as shown in Fig. 1.5. More selective antennas are more expen