Accurate Analysis and Design of Microstrip Interdigitated Couplers

The electrical behaviour of interdigitated directional couplers in an inhomogeneous dielectric medium is analyzed. Thanks to the symmetry properties of the device, the concepts of even and odd networks can be applied, leading to simple, closed-form expressions for coupler analysis which compare favourably with experimental results. The same model allows the most important parasitic effects to be understood and quantitatively described. It is shown that the discrepancies between computed and experimental data can be accurately explained in terms of the parasitics arising from junction discontinuities. Finally, design data for alumina-microstrip interdigitated couplers are presented in graphic form.