Effect of Thick Film Dielectric Overlays on the Characteristics of a 3 db Branch Line Microstrip Directional Coupler

This paper reports the preliminary investigations on the effect of Bi2O3, Al2O3 and MgF2 thick film overlays on a symmetrical branch line microstrip directional coupler with a 3 db power split at 9.0 GHz. The changes in isolation and coupling with overlay thickness varying from 200 µm to 3000 µm were studied. It was observed that, for an overlay thickness of 200 µm to 600 µm, there was a 900 MHz bandwidth broadening with 3 db power split irrespective of the overlay material. The isolation also increased. Above 700 µm thickness an oscillatory behaviour was observed in the coupled ports with a decrease in coupling after about 10.5 GHz. The isolation characteristic of the 3 db directional coupler showed dependence on thickness as well as permittivity. It is felt that, with a suitable choice of dielectric material of appropriate thickness, an improved broadband coupling and high isolation may be realised by a simple overlay technique.