A Semi-Automatic Method for the Precision Measurement of Microwave Impedance

A system is described for the measurement of the magnitude and phase of the reflection coefficient over wide frequency bands. The method consists of recording the voltage at several points along the standing-wave pattern in a coaxial line. A fixed probe is used and the distance between the probe and the load is altered by successively inserting several lengths of precision coaxial line. Knowing these lengths and the frequency of the signal, a curve representing the standing-wave pattern is fitted to the experimental points by means of a computer program performing a least-squares minimization. To cover a given frequency band, the frequency is changed in discrete steps and the corresponding probe voltage recorded automatically. The measured impedance is referred directly to the characteristic impedance of air-dielectric coaxial lines.