Balanced-to-unbalanced filtering magic-T based on circular patch resonator with high selectivity

This Letter presents a new design of balanced-to-unbalanced (B2U) filtering magic-T based on circular patch resonator. Without additional circuit, the prescribed in-phase and out-of-phase specifications of the magic-T are achieved by elaborately studying and exploring the electric field distributions of the degenerate resonant modes in the circular patch resonators. Meanwhile, the desired filtering responses are attained by virtue of the coupling extent between the two designed patch resonators in the back-to-back form. In the proposed design, a pair of TM11 modes at first make up passband. Meanwhile, nice common-mode suppression can be achieved at the balanced ports, and high isolation is achieved at the single-end ports, respectively. For demonstration, a prototype B2U filtering magic-T working at 4.3 GHz with 3 dB fractional bandwidth of 16.3% is implemented. Measured results coincide well with simulated ones, verifying the proposed design concept.