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Wideband modeling of complex loss-free isotropic RF structures is a challenging task in electrical engineering. This paper presents a new formalism to create compact wideband equivalent models of complex RF structures. In a first step, the complex structure is partitioned into segments. On the basis of the segment's eigenmodes with either vanishing tangential electric or magnetic fields on the boundary and a correction term, systems of ordinary differential equations (ODEs) are derived. In consequence, real eigenvalue problems need to be solved for each segment in addition to the actual field distribution in the segment, which only needs to be computed for a small number of discrete frequency samples for the correction term. Linking the established ODE systems of the segments with a suitable concatenation scheme leads to an ODE system for the entire structure. This system allows the computation of complex structure responses because of transient port excitation and the determination of transient 3-D fields in the structure. As a side product, the frequency-domain transfer function of the complex structure is available. Besides the theoretical derivations, two validation examples for the time-domain scheme are presented. These examples show that the method provides a good approximation of the transient processes in the structures under consideration.