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A novel, broadband, vialess, and vertical microstrip-to-microstrip transition is proposed in this paper. The transition consists of two open-circuited microstrip resonators and a U-shaped resonant-slot on the common ground plane. A physics-based equivalent-circuit model is developed for interpreting its working mechanism and facilitating the design process. The transition is analogous to a three-pole resonator filter. Based on the equivalent-circuit model, the coupling coefficients of the physical circuit can be calculated from the group delay information of two segregated electromagnetic models. To effectively control the couplings, a modified configuration is also proposed. A prototype transition is designed using the proposed design formulas. The fabricated circuit is measured to validate the proposed transition and the equivalent-circuit model. Good agreement is obtained between not only the measured and the simulated performance, but also the designed and the extracted-circuit model. In addition to the wide bandwidth, the features of vialess and easy fabrication make the novel transition very attractive for system-on-package applications.