Determining the high-frequency resistivity of slightly lossy coaxial air lines

Abstract

Coaxial air lines are used as primary standards of characteristic impedance at microwave and millimetre-wave frequencies. A key property of these lines that needs to be known in order to define the characteristic impedance is the resistivity of the conductors of the line at these high frequencies. This paper presents a simple, reliable method for determining the high-frequency resistivity, and subsequently the characteristic impedance, of air lines. The method is based on measuring the S-parameters of the line. The method is supported by a detailed uncertainty analysis, and is also verified using a second method which uses measured values for the line's propagation constant. Throughout the paper, measurements of a typical air line are used to illustrate both methods.