A new microstrip-to-microstrip vertical transition structure for ultra-wideband (UWB) applications is proposed in this paper. The transition consists of a low impedance microstrip ring stub and a couple of slotline square multiple-mode resonators (MMRs) on the common ground plane. The low impedance microstrip ring stubs are realized by the connection of three single stubs in parallel, and the high-impedance section of slotline SIR is realized by the connection of four single stubs in series. The simulated and measured results are in good agreement, showing good wideband filtering performance with ultra-wideband fractional bandwidth.
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