A MICROWAVE OSCILLATION LOOP FOR DIELECTRIC-CONSTANT MEASUREMENT

We designed and analyzed a microwave oscillation loop formed by a diel ectric loaded cavity, amplifiers and transmission lines for the dielec tric constant measurement of samples at both room and very high temper ature. An oscillation condition for an arbitrary loop is derived in S- parameter notation, by which the commonly used oscillation condition i n loop phase and gain notation is proved to be valid only in the speci al case when either S11 = S21 = 0 or S22 = S12 = 0. Based on the S-par ameter oscillation condition, a theoretical model is established and v erified with a discrepancy of less than 0.041% between the calculated and the measured oscillation frequencies. With this model, the loop ch aracteristics are investigated. From the measured loop oscillation fre quency, the cavity resonant frequency, and thereby the dielectric cons tant of the sample in the cavity, can be predicted. Based on this anal ysis, an active dielectrometer is constructed with resultant errors of less than 4% for epsilon' < 20 and less than 11% for epsilon' < 80. T his dielectrometer requires no tuning and no external microwave power source. Moreover, a high power (> 100W) oscillation loop for the diele ctric constant measurement of a microwave heated sample (1000-degrees- C) is developed.