A Self-Packaged Wideband Balanced Filtering Directional Coupler

This paper proposes a self-packaged wideband balanced filtering directional coupler (BFDC), which fuses the functions of differential-mode (DM) filtering on the through and coupled channels, DM directional coupling, and common-mode suppression (CMS) into one component, resulting in a reduced number of components, low loss and minimized size for the system. The stepped-impedance half-wavelength transmission lines not only provide in-band CMS and support filtering function but also contribute to wide through and isolation channels bandwidths due to their impedance characteristics. Furthermore, when the coupled lines with open-circuited termination interact with the stepped-impedance half-wavelength transmission lines, the wideband DM filtering and the out-of-band CMS can be greatly improved. For the four-port coupled line, it is the main factor determining the coupling level, and affects directivity and filtering function. Compared to state-of-the-art directional couplers, the proposed BFDC has the advantages of wide bandwidth, high frequency selectivity and multiple functions in one component. For demonstration, a prototype centered at 1.74 GHz with a size of 0.149λ_g² (where λ_g is the guided wavelength at the center frequency) was designed with the measured operating bandwidth of 69%, the DM coupling level of 19 dB, and the CMS level of 20 dB across the entire frequency band.