arXiv:2610.22624v1 Announce Type: cross Abstract: This paper presents a data-driven Loewner framework for constructing reduced-order models (ROMs) from time-domain electromagnetic (EM) simulation data. High-fidelity simulation of microwave devices can be computationally expensive, particularly when fine spatial discretization and long observation windows are required. The proposed method uses transient input-output data to construct Loewner and shifted Loewner matrices, from which a reduced descriptor realization is obtained through singular value decomposition (SVD). The resulting models are identified from a limited training window and used to predict responses over substantially longer time intervals, with prediction windows up to 50 times the duration of the training window. The method is formalized as an explicit, reproducible algorithm and validated against full-order simulations for four microwave structures: a substrate-integrated-waveguide filter, a frequency-selective surface, an evanescent-mode coaxial comb-line filter, and a diplexer. The results demonstrate the potential of Loewner-based identification to provide compact models of electromagnetic responses while reducing the need for repeated full-order simulation.