s it propagates through the air or other medium. Alas, amplitude and power are different, and this creates more than a little confusion. The power of an electromagnetic (or audio) event is the energy per unit time. We are all familiar with power in units of watts or horsepower. In a car engine, for example, it is the driving force or thrust being applied multiplied by the speed at which it is being applied. In an electromagnetic event, power is the voltage multiplied by the magnetism 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 or current. In an audio event, power is the air pressure multiplied by the air velocity. In both radio and sound, this means that the power level is proportional to the square of the amplitude. To expand this idea a little more fully: In an electromagnetic wave, the amplitude is the voltage, and the current is proportional to that voltage. Power is voltage times cur- rent, so the power is proportional to the square of amplitude. In an audio event, am- plitude affects both the air pressure and air velocity, SO power is proportional to the square of amplitude there as well. Amplitude is measured in volts, which we almost never use in discussions of radio systems, and power is measured in watts, which we use constantly. A typical base-station radio transmitter in our mobile telephone world has about 100 W of effective radiated power (ERP). A typical broadcast radio or television (TV) sta- tion might have 1 million W ERP, so our mobile radio stations are in the lightweight division. The telephone itself is usually limited to 1 W, the bantamweight division. However, this comparatively puny signal reliably gets hundreds of millions of calls through every day. An effective medium for sending information, radio is not an efficient medium for sending energy. The mobile telephone