Wideband coupler of folded half mode substrate integrated waveguide design with combined microstrip and stripline feed

This article presents the 90° hybrid directional 3-dB coupler design with a wideband performance. The main part of the proposed coupler is built on a folded half-mode substrate integrated waveguide (FHMSIW) structure and the feed is composed of microstrip line and stripline. The FHMSIW is implemented by PCB technology which possesses the merits of compact size, low cost, light weight, low profile, high power handling capability, easy integration with planar transmission line, and possibly low insertion losses. In designing the circuit's feed, a quarter-wavelength stripline is placed between the coupler's main part and each microstrip I/O port. The stripline works as an impedance transformer, which increases circuit's bandwidth (BW) and reduces the reflection. The acquired fractional bandwidth (FBW) is 47.2% with the coupler's central frequency set to 4.5 GHz. The averaged transmission and coupling losses are around 1 dB and the isolation is better than 20 dB. Experiment is conducted to validate the circuit performance. The measured data are in good agreements with the simulated ones.