A Low-Cost Substrate Integrated Suspended Line Platform With Multiple Inner Boards and Its Applications in Coupled-Line Circuits

This article proposes a novel substrate integrated suspended line (SISL) platform with multiple inner boards, and it uses all FR4 material with very low cost. A new coupling scheme which significantly eliminates the dielectric loss is proposed. When compared with the conventional multilayer coupled-line circuits where the coupling is from one metal layer to another metal layer on the same board, the proposed coupling scheme is between different inner boards, and there is almost no lossy substrate in the coupling region. As demonstration cases, a broadside coupled-line coupler and a coupled-line Marchand balun are designed, fabricated, and measured. The obtained performances are excellent when compared with the state of art, though very low-cost FR4 materials are used. The proposed coupled-line circuits based on multiple inner board SISL using FR4 material have advantages of low loss, low cost, and self-packaging.