A critical study of a new double bridge method for the measurement of impedance at ultra high frequencies using a system of Lecher wires

Abstract

The fundamental theory of transmission lines shows that the terminating impedance of a pair of lines may be expressed in terms of the current at any point in the lines, the potential difference across the lines at this point, and certain other characteristics of the lines. Therefore, if it were possible to measure the currents and voltages at various points along the lines, this would provide a method of measuring impedance. Unfortunately, when a bridge containing a current measuring instrument is placed across the lines, the current through the terminating impedance and the current through the bridge assume complicated forms. Also, the movement of the bridge along the lines changes the input impedance, and the input current varies with the position of the bridge. These difficulties render the direct method of measuring current and voltage at any point by means of a single bridge impracticable. Flint and Williams (1941) eliminated the effect of variations in the input by measuring the ratio of the currents flowing in two bridges. One bridge is fixed at the end of a pair of Lecher wires, and contains the unknown impedance in series with a current meter; the other bridge is movable, and contains a current meter only. Graphs of the current ratio plotted against certain functions of the distance between the two bridges give values of two constants, from which the terminating impedance may be calculated. Since the terminating impedance consists of the unknown impedance and the impedance of the current meter, a subsidiary experiment must be performed to determine the impedance of the current meter. Also, the exact way in which the two impedances are combined is not known.