A 2-to-67 GHz Balun Based on the SISL Platform

This article proposes an ultra-wideband (UWB) balun based on the substrate-integrated suspended line (SISL) platform. The proposed balun consists of a 180° phase shifter and a power divider. The phase shifter based on an inverter is composed of a transition structure from the suspended microstrip line (SML) to a substrate-integrated suspended parallel strip line (SISPSL). A shielding enclosure composed of multilayer metamaterials is adopted to eliminate the cavity resonance caused by the phase shifter. The power divider could be combined with the SISPSL phase shifter to form an UWB balun based on the SISL platform. To achieve wider bandwidth and better performance of the SISL balun, a pair of transition structures from SISL to SML are designed and inserted into the balun. As a demonstration, an UWB balun is fabricated by the printed circuit board (PCB) process and measured. From 2 to 67 GHz, that is, a 188.4<sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</small>% fractional bandwidth, the measured return loss is better than 11 dB. The output phase imbalance is within ±5° from 2 to 67 GHz. The proposed SISL balun has the advantage of low loss and is well packaged, making it suitable for applications in UWB communication systems.