Wideband bended CPS-to-microstrip transition for millimeter-wave antenna-detector module

In this article, the design of a new compact transition with two bended structures for millimeter-wave (W/F-band) antenna-detector module is presented. The millimeter-wave antenna is a dual dipole antenna fed by a coplanar stripline (CPS). Due to small size of the antenna-detector, the CPS line impedance tends to be very high (~160 Ω). The down-converted broadband microwave signal in the CPS is channeled to a 50 Ω microstrip line using a CPS-to-microstrip line transition (balun). If the start frequency of the transition is low, the conventional straight balun tends to be very long. In order to reduce transition length, the transition with bended parts while optimizing the impedance tapering based on the analytical formulas (conformal mapping) is chosen. In the proposed transition design, a transition with two bends in 120° is designed and implemented. The measured results show the maximum insertion loss of 1.5 dB in the frequency range of 1.8 to 17.8 GHz. Also, the return loss of the proposed transition is greater than 10 dB from 1.8 to 20 GHz.