Microwave measurements with active systems

Possible practical applications of a novel technique for the measurement of dielectric constant, or analogous quantities, at microwave frequencies are considered. Although originally developed for microwave diagnostics in plasma-physics research, it is felt that the method may be of interest in certain problems of current scientific and technological interest. Conventional systems for the determination of microwave dielectric constant are "passive" in the sense that the quantities of interest are determined from a shift in phase, or frequency, in a passive element, such as a microwave bridge; the microwave energy required for the measurement is derived from an external source. In the method described here, use is made of an "active" system in which a positive feedback loop containing a microwave amplifier provides positive gain. The entire configuration, including the measurement element, then comprises a microwave oscillator, the frequency and amplitude of the oscillator containing information on the quantities being measured. It appears that the method can be attractive from the point of view of sensitivity, response time, linearity, dynamic range, and convenience of readout. In addition, an active system can function as its own telemetry unit. Aspects of the theoryof active systems are analyzed, and factors that enter into the design and construction of such systems are described. Experiments carried out to evaluate active systems in gas turbulence measurements and in the determination of size distributions of particles with dimensions of tens of microns are described.