Variable coupler for Butler beam-forming matrix with low sidelobe level

A Butler beam-forming network with low sidelobe level (SLL) is proposed and demonstrated on the basis of variable couplers. A set of couplers with 0.75, 1.26, 3, 6 and 8 dB coupling levels are required to achieve an SLL of 13.6, 20 and 30 dB. Such variable coupling is achieved by creating a movable mirror structure in the common region of perpendicularly or cross-coupled waveguide building blocks. This mirror is composed of a 5 7 cylindrical rod array. The coupling and directivity characteristics depend on the position of this proposed fringe structure. In this work, WR28 is used to demonstrate the capability of this type of coupler over 26 30 GHz bandwidth. Structure usefulness and design procedure are validated by electromagnetic simulation high frequency structural simulator (HFSS) and prototype experimentation. Very good results are observed, that have also demonstrated attractive performances of the coupler. Measured results of couplers are used to assess the properties of Butler matrix with excitation coefficients corresponding to 13.6, 20 and 30 dB of SLLs. Radiation patterns obtained by this investigation show a promising performance. This proposed technique allows flexibility in the choice of excitation coefficients without degradation on the efficiency.