Abstract: A six-port reflectometer for measuring the electrical thickness of radome and permittivity of dielectric material is introduced, which consists of a symmetrical five-port waveguide junction and a directional coupler. The key part of this system is the five-port junction, it has been simulated and designed by HFSS. The reflection coefficient of sliding shorts measured by this system has been given and the result shows that the system is stable and accurate. A measurement method of the reflection coefficient of the radome electrical thickness has been introduced. Through measuring the phase of reflection coefficients of radome wall, the relative change of electrical thickness of radome wall can be got. Short circuits waveguide method to measure the permittivity of materials has been given, t