Six-Port Reflectometer

Abstract

The six-port reflectometer is an alternative to the network analyzer, it is simple device that does not need high precision components and requires only power meters in order to measure complex impedances. The six-port reflectometer is a device which allows the phase and magnitude of an unknown complex impedance to be measured directly at the operating :frequency and in terms of power measurement alone using a standard impedances and a calibrating procedure. The calibrating procedure is carried out in order to take into consideration the imperfections of the reflectometer hardware and imperfect matching and isolation between different ports. There are different configurations of six-port reflectometers such as three lines coupler, four directional couplers, quadrature hybrids, five-port junction plus a coupler, ... etc.

In this thesis, the analysis and design of the three-section backward couplers is carried out by taking the advantages of the four fold symmetry of the structures and the chosen excitations. The three-section coupler has two symmetry planes corresponding to an electric wall or a magnetic wall, and consequently one has to determine two even-mode and two odd-mode impedances. Then its S-parameters and physicai dimensions can be determined. From the analysis it was found that, the S-parameters are not uniform around the center frequency. Changes of the electrical lengths for the center and outer sections lead to uniform performance keeping the overall coupling lengths equal to 3/4..t at the center frequency. Accordingly, some shift in the center frequency occurs. A curve fitting process was carried out to determine this shift with an error not exceeding 2%.