Precision Coaxial VSWR Measurements by Coupled Sliding-Load Technique

Abstract
The method described employs a slotted line for VSWR measurement and a precision coaxial line as an impedance standard. A sliding load terminates the standard line during the process of transforming the impedance of the slotted line to that of the standard line. This load is coupled mechanically to the slotted-line probe so that the two move together. The VSWR seen by the probe is then, to first approximation, only that due to the reflection from the junction of the two lines. It is simple, therefore, to tune out the residual reflection without requiring a perfect load. The slotted line is then effectively transformed to the impedance of the standard line, and subsequent measurements made with it at the same frequency indicate VSWR with respect to the standard line. In order to minimize the effect of amplitude fluctuations of the RF source and to expand the scale, the detected probe output is compared with the detected output of a directional coupler monitoring the incident power. The difference is recorded with a strip recorder; the paper transport of the recorder is synchronized with the probe travel. In describing the method, considerable background material is presented on other precision impedance-measuring methods and techniques and on the general state-of-the-art. Theory, accuracy, system details, and results are given. The system appears capable of making absolute impedance measurements on a low VSWR termination to a maximum possible error in reflection coefficient of 0.0012.

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