I. Introduction
COPLANAR waveguide (CPW), coplanar stripline (CPS), and slotline (SL) are widely used as the building blocks for uniplanar monolithic microwave integrated circuits (MMICs) [1]. Many circuit components based on these uniplanar lines such as filters, couplers, mixers, and antenna feeding structures have also been proposed in recent years. To fully utilize the distinct features of CPWs, CPSs, and SLs, effective transitions or interconnections between them is of practical importance. Various CPW-to-CPS and CPW-to-SL transition structures were proposed and investigated [2]–[10]. Applications of these uniplanar transitions in balanced mixers, antenna feeding structures, frequency doublers, and CPW-to-waveguide transitions were also widely documented in the literature. Most of the previous studies on uniplanar transition design were focused on bandwidth improvement [2]–[5] or size reduction [4], [7]–[10]. In certain applications, the transition output is connected to other circuit components like mixer diodes [11] or antennas [12], [13] with different impedance values so that additional output matching or impedance transformation is required. However, previous studies on the impedance transformation of uniplanar transitions are rather limited.