In this paper, we show how metallic plate inclusions can be used to enhance the coupling values of microstrip directional couplers. The analysis and the design of symmetrical coupled microstrip lines with high values of forward-wave coupling with a coupling length very small are presented. A quasi-static circuit model of the structure is developed and physical insights on the operation of the proposed component and on the role of Epsilon NeGative (ENG) material slab loading are also given. Full wave numerical simulations are shown for different geometries.
Lauro, S., Toscano, A., L., V. (2009). Symmetrical Coupled Microstrip Lines With Epsilon Negative Metamaterial Loading. IEEE TRANSACTIONS ON MAGNETICS, 45(3), 1182-1185 [10.1109/TMAG.2009.2012547].