CHARACTERISTICS OF THE SYMMETRICAL N-PORT WAVEGUDIE JUNCTION

Abstract

Tue six-port technique first proposed by Hoer and Engen in 1972 is an alternative to the automated network analyzer (ANA) and has an important advantage over the ANA in that it can determine both magnitude and phase of the reflection coefficient of DUT with only four scalar power measurements. Over the past decades, many different implementations of the six-port network have been reported. The network comprising a symmetrical five-port junction together with a directional coupler (to provide the additional sixth port) was proposed and extensively studied by some researchers. In the ideal case when both the five-port junction and the directional coupler are perfect and matched, the six-po1t network does indeed satisfy the optimum specifications of the six-port reflectometer. Currently available designs for the symmetrical five-port waveguide junction have residual mismatches as high as -14 dB over parts of the waveguide bandwidth. In this project, we concentrate on the design of matched symmetrical five-port waveguide junction over the entire waveguide bandwidth. First, we describe the development of a computer model that is able to predict the scattering characteristics of the symmetrical N-port (E-plane coupled) step waveguide junction loaded with metallic post and dielectric sleeve in its central cavity. Comparison between computational and experimental results shows that the resultant software package can yield accuracies of ±1 % for the scattering parameters of the junction. The computer model has then been used to search for a new five-port junction design (based on a combination of metallic post, dielectric sleeve and inductive diaphragms). The residual mismatch of the new design does not exceed -20 dB over the entire waveguide bandwidth. An improved version of a four-standard calibration procedure for six-port reflectometer is also reported in the thesis. Simulation and experimental tests for our sixp01t reflectometer set-up indicate that the calibration software is capable of yielding reasonable good accuracies even in the presence of measurement noise. Measurement accuracies of ±1 % in magnitude and ±1 ° in phase can be obtained from the six-port reflectometer developed in our laboratory. As an extension, it was found that the computer model could be adapted to study the scattering characteristics of the waveguide cross junction and the symmetrical six-port waveguide junction as well. An interesting feature was obtained for the cross junction in that its four scattering parameters are almost constants of 0.5 in magnitude over the entire waveguide bandwidth. For the six-port junction, the residual mismatch can be reduced, by adjusting the radii of metallic post and dielecric sleeve, to - 14 dB over the entire waveguide bandwidth.