Characterising beadless air lines as reference artefacts for S-parameter measurements at RF and microwave frequencies

Beadless coaxial air dielectric transmission lines can be used as fundamental reference devices for S-parameter measurements at RF and microwave frequencies. However, for these lines to be used for such applications, they must first be characterised accurately in terms of their impedance and propagation characteristics. This includes assessment of the small amount of loss in the line and the associated impact of this loss on other characteristics. A simple method is presented whereby the loss is determined experimentally using a vector network analyser (VNA). The determined loss can then be used to characterise the line so that it can be used for a range of high-precision metrology applications. Two such applications are given: determination of the actual frequency-dependent characteristic impedance of a line; and use of the line´s phase constant to verify VNA phase measurements