180° hybrid using a novel planar balun on suspended substrate for beam forming network applications

Abstract

In this paper, a 180° hybrid consisting of a broadband balun and a T-junction in-phase power divider is proposed. The 180° hybrid is intended for operation in the C-band. The balun resembles a center tapped magnetic transformer where the coils are replaced by broadside-coupled lines. This new technique allows magnetic couplers to be realized by planar structures at microwave frequencies. The planar balun is broadband, and the size reduction is considerable. The broadside-coupled lines are on a suspended substrate. The proposed structure can be used in the beam forming networks of the multiple-beam antennas. It also prevents cross connections in the conventional beam forming networks. The planar balun is analyzed by the signal flow graph method. The scattering parameters of the 180° hybrid are calculated by the multiport connection method. Some design formulas for the construction of the balun are presented. The 3-dimensional model of the planar 180° hybrid is simulated by a full-wave simulator. Measurement results of the fabricated hybrid approve the simulation results. The 180° hybrid exhibits a relative bandwidth of 45% centered at 4.5 GHz. In the operating band, the amplitude and phase imbalance of the output ports are <0.6 dB and <10°, respectively.