The Effect Of Dielectric Loss On Impulse Response Of Microstrip Structures J´ ulio L. R. da Silva and C´ assio G. Rego GAPTEM – Research Group on Antennas, Propagation and Applied Electromagnetic Theory Department of Electronics Engineering Federal University of Minas Gerais, Belo Horizonte, Brazil juliolana@gmail.com , cassio@cpdee.ufmg.br Abstract: A three-dimensional FDTD algorithm is applied to the analysis of microstrip structures comprising lossless and lossy dielectric substrates. A well-posed perfect matching layer (WP-PML) designed for general media is employed here to truncate the domains under study. Microstrip cir- cuits are studied in both time and frequency domains yielding an analysis of the influence of using lossy materials in their impulse response. Keywords: FDTD, WP-PML, microstrip circuits, lossless and lossy substrates, impulse response. 1. Introduction Since its initial proposal the FDTD technique has been widely applied on microwave circuits’ modeling [1]. The numerical model employed in this technique allows a wide-band approach as well as a time response analysis. However, the limitation of computational resources has always limited the application leading to another research field on absorbing boundary conditions (ABCs). Considerably effort has been applied on different boundary conditions in order to achieve a perfect ABC that would absorb electromagnetic energy. Some of those efforts have as a main proposal the right description of the terminations on simulated regions, including lossy material. This is an interesting approach to microwave circuits, and to other kind of structures as well, since perfect materials are only a mathematical description. Wittwer and Ziolkowski described the effect of the loss on this structures using a proposed lossy two-time derivative Lorentzian material (L2TDLM) [2]. Also, Fan and Liu also proposed the well-posed perfectly matched layer (WP-PML) applying it on the pseudospectral time-domain method (PSTD) reaching good results [3]. The use of special boundary conditions to treat domains that include lossy material allows the study of impulse response of such structures. This kind of approach has been addressed to the design of highly complex microwave systems. Focusing on this, Marrocco and Bardati showed the application of moment expansion to obtain the impulse response of microstrip structures with loss- less materials [4]. Such process has shown to be simple as it has no need of a Fourier deconvolution. This paper describes the application of the moment expansion technique in lossy microwave structures simulated via FDTD using WP-PML to define the limits of the domains under study.