Local, High-Frequency Analysis of the Fields in a Mode-Stirred Chamber

Abstmct-The fields inside a mode-stirred chamber are analyzed by a local plane-wave method. This approach has the advantage of being applicable to nonseparable geometries, for which the more standard modal method of analysis is not possible, and gives insight into the nature of the various contributions to the total field. Results are given for the quality factor and for the fields near the walls of the chamber. The expression for the quality factor agrees with previously published work. I. INTRODUCTION ODE-STIRRED chambers are presently under considM eration at a number of research laboratories as an attractive alternative to more conventional anechoic chambers for use in the testing of objects in various electromagnetic environments [ 11-[5]. Typically, the mode-stirred chamber consists of a rectangular test chamber with metal walls and a stirrer, usually in the form of a large paddle or fan blade, near the ceiling of the chamber. The object under test is placed in the chamber and exposed to an electromagnetic field during which time the stirrer slowly revolves. The average response of the object to the field is found by integrating the response over the time period of one revolution of the stirrer. The metal walls of the cavity allow a large field to be built up inside the chamber. At the same time, the stirrer smooths out the sharp nulls of the field; these nulls are usually present in such a resonant structure. The object under test can be exposed to a high field level consisting of several different polarizations. It is, therefore, much quicker to test the object in this way when compared with more conventional methods. It is extremely difficult to theoretically obtain the fields for such a chamber by solving Maxwell’s equations. Instead, most researchers have attempted to come up with expressions for average values of interest. Furthermore, it is usually assumed that the fields are completely random. The most common figure of merit when discussing such chambers is the Q of the chamber, which is defined to be the energy in the chamber divided by the power that must be injected into the chamber, multiplied by the angular frequency of operation. It is desirable to make a chamber with as large a Q as possible in order to give a large field. The theoretical expression available for the Q of such a rectangular chamber is [5]