brations stemming from life in the trunk of a moving vehicle, caused both physical and electronic problems that were so bad that in 1927 the Detroit police department shut down the station out of sheer frustra- tion. It wasn't until 1928, when Purdue University student Robert Batts developed a super heterodyne radio that was somewhat resistant to buf- feting and vibration, that Detroit reinaugurated their radio system. Other cities followed in Detroit's footsteps. By 1934, there were nearly 200 radio-equipped city police departments communicating with nearly 5,000 roving vehicles. Radio had become commonplace, and in 1937 the FCC stepped in and allocated 19 additional radio channels to the 11 that they had already made available for police department use. No one real- ized it, but the spectrum battles had begun. Early radio systems were based on amplitude modulation (AM) tech- nology. One drawback of AM radio is that the strength of the transmit- ted signal diminishes according to the square of the distance. A signal measured at 20 watts, one mile from the transmitter will be one quar- ter that strength, or 5 watts, two miles from the transmitter. When Edwin Armstrong (see Chapter 1) announced and demonstrated his invention of frequency-modulated (FM) radio in 1935, he turned the broadcast industry on its ear. He offered a technology that not only eliminated the random noise and static that plagued AM radio, but also lowered transmitter power requirements and improved the ability of receivers to lock onto a weak signal. 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. Access Technologies Access Technologies World War II provided an ideal testing ground for FM. AM radio, used universally by the Axis powers during the war, was easily jammed by powerful Allied transmitters. FM, on the other hand, used only by Allied forces, was unaffected by AM 