Branch Line Coupler on an Artificial Dielectric Substrate

This paper presents a possible application of an artificial dielectric substrate. The substrate is fabricated as a 2D array of metallic posts separated from the top conductor by a thin dielectric slab. The effective permittivity of a microstrip line fabricated on this substrate depends on the dimensions of the posts, the permittivity of a dielectric material filling the space between the posts, and on the thickness and permittivity of the top dielectric slab. The thinner this slab is and the higher its permittivity is the higher the effective permittivity will be. Due to the high effective permittivity, a substantial reduction in the dimensions of the microwave circuit elements is possible. Two 3 dB branch line couplers were designed, fabricated and measured by applying the microstrip line on this artificial dielectric substrate. They have considerably smaller dimensions than a coupler on a standard substrate.