Single-Ended-to-Balanced Broadband/Filtering Planar Magic-Ts With High Common-Mode Suppression

In this article, a single-ended-to-balanced (SETB) broadband magic-T based on microstrip/slotline transitions and microstrip/slotline T-junctions is proposed. For the purpose of miniaturization and low interconnection loss, an SETB filtering magic-T with a sharp selectivity is also achieved by introducing a pair of microstrip stub-loaded resonators (SLRs) at the two output ports. For the first design, a U-type microstrip feed line integrated with a circular slot stub is adopted at each output port, which makes the differential-mode (DM) response independent of the common-mode (CM) one. Therefore, the design procedure is simplified significantly. Meanwhile, based on the microstrip/slotline T-junctions, in-phase and out-of-phase two-way equal power division networks can be realized, respectively. For the second design, a DM passband with two transmission zeros (TZs) is generated by introducing SLRs based on the first design. By employing the characteristic mode analysis method, it is easy to find that the center frequency and fractional bandwidth (FBW) can be controlled by changing the dimensions of SLR. To the best of the authors' knowledge, the SETB magic-T with broadband or filtering performances may be the first ever reported. To verify the feasibility of the designs, two prototype circuits of the proposed magic-Ts are fabricated and measured. The simulation results are in good agreement with the measured ones. In these two designs, the functions of in-phase and out-of-phase power dividing, broadband or frequency selectivity, isolation between output ports, and CM suppression can be realized at the same time.