Integrated two-pass reflectometer: low cost seven-port circuit

A novel realisation of a 7-port reflectometer of the type initially suggested by Engen and Hoer, has been proposed and constructed. The integrated 2-pass reflectometer (ITPR) is implemented using a forward-coupling mechanism between nearest-neighbour transmission lines, or guides, which are terminated in a set of reflecting terminations at one end and power-measurement devices at the other end, excepting the input port. Over a wide range of microwave frequencies, the ITPR can be conveniently constructed using a single multiline combline directional coupler which has a planar structure that is relatively tolerant to the dimensions of the conductors, and has a low cost. A mathematical analysis of the ITPR is presented and its characteristics are related to parameters describing the coupling network and its associated reflecting terminations. A numerical optimisation technique, varying one of the normal-mode eigenvectors and the set of mode-propagation constants, is outlined and an example ITPR circuit has been designed, built and tested in the 1.8¿¿2.7 GHz range. The measured and theoretical characteristics are compared, and the constraints imposed on the optimisation algorithm are considered. In addition, the ITPR has been calibrated to an accuracy of about 0.01 in magnitude and 1¿¿ in phase by the method of multiple 6¿¿4-port reduction using the technique of sliding terminations.