Real Time Tracking Measurements of a Time-Varying Comlex Impedance at K-Band Using Three Fixed Probes

Three-probe microwave automatic impedance measuring system has been designed and used in many places. The principle of this system is similar to that of the six-port systems. But because of the simplicity of the three-probe theories. It is possible to realized the design of the three-probe system by using only analog circuits as reported in IEEE Trasactions on MTT as well as in the ARFTG conferences. The main advantage of this analog system is its real time measurement capacity. That is, it can measure and track in real time the variations of the real and the imaginary parts of the unknown complex impedance when this impedance is varying in real time. In this article, the author wish to report to the audience some experimental results of real-time tracking comparison measurements. These results were obtained by using simultaneously an HP 8410 automatic network analyzer and the current three-probe system. The 8410 is used with data taken directly from its analog output without going through its digital processing part. The quantitative difference between these two measured results is seen to be within a few percents while the qualitative variations of the two are exactly the same. The digitally treated outputs from any commercial network analyzer are usually more accurate but they are not measured continuously in real time. That is, it may not capture many important features of the varying unknown impedance when the impedance is varying very fast in real time.