Wideband Compact-Size 3-dB Backward Directional Coupler Using Slotted-Microstrip Based Unit-Cells

This article presents a novel coupled-line unit cell that has different loadings for the even and odd modes. The proposed unit cell consists of two coupled lines loaded with slotted-microstrip stubs and a lumped capacitor. The strip loading affects both the even and odd modes leading to a size reduction, while the capacitor controls the electrical properties of the odd mode, which enables the realization of a 3-dB directional coupler. The proposed coupler operates at 1.5 GHz with 61% size reduction and 61.3% fractional bandwidth. Measurements, which are in a good agreement with EM simulations, confirm the theoretical predictions.