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This paper addresses the topic of high-speed interconnects in high density systems [systems on chip (SoCs), systems in package (SiPs), systems on package (SoPs), and multichip modules (MCMs)]. These microstrip or coplanar lines have, often, an underlayer of orthogonal metal grids liable to affect transmission characteristics. The characterization proposed in this paper relies on S -parameter measurements and electromagnetic simulations. The grids under study are of two kinds: grounded (CC) and floating (CO). In both cases, the signal is distorted in the time domain further to the occurrence of transmission zeroes whose position depend mainly on the grid length and, of course, on the grid charge, i.e., CC or CO. In order to easily estimate this position, we developed a simple equivalent circuit model and validated it by measurements and electromagnetic simulations. Then it was used to develop a set of expressions enabling one to analytically pinpoint the location of transmission zeroes in the frequency domain, while remaining valid for any underlayer of orthogonal metal lines or grids.