Measurement of Resistance and Impedances at High Frequencies

Abstract

It is well known that the resistance of a conductor increases with frequency. At very high frequencies the usual method of measurement fail, especially if the unknown resistance is more than about one hundred ohms. In the following it is shown that the absolute value of the unknown impedance, when put across the end of a transmission line, is a simple function of the ratio of the currents, measured at the beginning and the end of a transmission line. This method has been tested out at a wavelength of 21.8 meters, measuring the resistance of a number of grid leaks and of a decade box. The a-c resistance was considerably higher than the labeled d-c value. At the same time, the shunted capacity across the resistors has been measured and valutes have been obtained, which agree with expectations. In general, this method lends itself to measurement of impedances of any kind; but it only gives the absolute value of the unknown impedance. However, by means of a known capacity or resistance, connected in series or in parallel with the impedance to be determined, the phase of the latter and, therefore, its real and imaginary components are found. At frequencies corresponding to wavelengths longer than 100 meters the line becomes rather long. In this case another method, as described in the second part of this paper, can be applied.