High-performance differentially fed coupled-line directional couplers realised in inhomogeneous medium

In this study, a generalised approach for the design of high-performance differentially fed coupled-line directional couplers in inhomogeneous medium is presented for the first time. Conditions for the realisation of ideal coupled-line section under differential excitation are provided and it is shown that generally in such medium couplers feature different capacitive and inductive coupling coefficients, which significantly deteriorate circuits' performance. Therefore, the compensation technique is proposed allowing for equalisation of the differential coupling coefficients by adding appropriate compensating elements to the per-unit-length capacitance and inductance matrices of the coupled-line section. The proposed approach allows for realisation of quasi-ideal differential coupled-line couplers designed in arbitrarily chosen dielectric stack-up, and almost for arbitrarily chosen coupling. The provided theoretical analysis and resulting design methodology are verified experimentally with a 3-dB differential coupler operating at f(0) = 1.5 GHz and realised in suspended microstrip technique. Moreover, a combination of laminate and additive manufacturing technologies was used for higher design flexibility and low-cost manufacturing.