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Microphone" to a 60 channel theme park system. It can evoke visions of freedom in prospective users and memories of ancient disaster in veteran sound engineers. In all its forms, wireless has become a fact of life for people who design and use audio systems. With increased use of wireless microphone systems has come the need for increased quantity and quality of information on the topic. The scope of this guide is limited to wireless microphone systems used in audio applications. The reader is presumed to be somewhat familiar with basic audio. However, since wireless microphone systems depend upon certain general principles of radio, some information on basic radio is included. While there are similarities between sound transmission and radio transmission, many of the characteristics of radio systems are neither analogous to audio systems nor intuitive. Still, though perhaps new, the key ideas are fairly straightforward. The purpose of this guide is to provide the interested reader with adequate information to select suitable wireless equipment for a given application and to use that equipment successfully. In addition, it is hoped that the fundamentals presented here will equip regular users of wireless with a framework to assist in their further understanding of this evolving technology. This guide is presented in two parts: how wireless microphone systems work and how to make wireless microphone systems work. The first part is a technical introduction to the basic principles of radio and to the characteristics of wireless transmitters and receivers. The second part discusses the practical selection and operation of wireless microphone systems for general and specific applications. The two parts are intended to be self-contained. The first part should be of interest to those who specify or integrate professional wireless equipment while the second part should be of use to anyone who regularly works with wireless microphone systems. RADIO WAVE TRANSMISSION Radio refers to a class of time-varying electromagnetic fields created by varying voltages and/or currents in certain physical sources. These sources may be "artificial," such as electrical power and electronic circuits, or "natural," such as the atmosphere (lightning) and stars (sunspots). The electromagnetic field variations radiate outward from the source forming a pattern called a radio wave. Thus, a radio wave is a series of electromagnetic field variations travelling through space. Although, technically, any varying source of voltage or current produces a varying field near the source, here the term "radio wave" describes field variations that propagate a significant distance away from the source. A sound wave has only a single "field" component (air pressure). Variations in this component create a pattern of air pressure changes along the direction the sound wave travels but otherwise have no particular orientation. In contrast, a radio wave includes both an electric field component and a magnetic field component. The variations in these components have the same relative pattern along the direction the radio wave travels but they are oriented at a 90 degree angle to each other as illustrated in Figure 1-1. In particular, it is the orientation of the electric field component which determines the angle of "polarization" of the radio wave. This becomes especially important in the design and operation of antennas. Like sound waves, a radio wave can be described by its frequency and its amplitude. The frequency of a radio wave is the time rate of the field variations measured in Hertz (Hz), where 1 Hz equals 1 cycle-per-second. The radio spectrum, or range of frequencies, extends from a few Hertz through the Kilohertz (KHz) and Megahertz (MHz) ranges, to beyond the Gigahertz (GHz) range. The suffixes KHz, MHz, and GHz refer to thousands, millions, and billions of cycles-per-second respectively. As far as is presently known, humans are directly sensitive to radio waves only at frequencies in the range of a few million GHz, which are perceived as visible light, and at those frequencies in the range just below visible light, which are perceived as heat (infrared radiation). The overall radio spectrum includes both natural and artificial sources as indicated by Figure 1-2. The amplitude of a radio wave is the magnitude of the field variations. It is the characteristic that determines the "strength" of the radio wave. Specifically, it is defined to be the amplitude of the electric field variation. It is measured in volts per unit length and ranges from nanovolts/meter (nV/m) to kilovolts/meter (KV/m), where nV refers to one billionth of a volt and KV refers to one thousand volts. The minimum level required for pickup by a typical radio receiver is only a few tens of microvolts (uV, a millionth of a volt) but much higher levels can be found near transmitters and other sources. The wide range of radio wave amplitudes that may be encountered in typical applica...