This paper addresses high-speed interconnects in high density systems (Systems on Chip (SoC) in package (SiP) ...). These lines (of miscostrip or coplanar type) often have an underlayer of orthogonal metal grids which can affect transmission characteristics. We subsequently present a characterization through S-parameter measurements and electromagnetic simulations. Two kinds of grid are studied; grounded (CC) and floating grid (CO). In both cases, transmission zeroes appear. The position of these transmission zeroes in the frequency domain depends mainly on the grid length and, of course, on the grid charge CC or CO. In order to easily estimate it, we propose a simple equivalent circuit model which we validate by measurements and electromagnetic simulations. We then determine a set of expressions based on this model enabling us to analytically pinpoint the location transmission zero in the frequency domain, valid for any underlayer of orthogonal metal lines or grids.