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The EM design of a coplanar RF cross with well isolated signal paths in a frequency range from DC up to 40 GHz is described. The cross is designed for a MEMS switch matrix where the different signal paths have to be isolated against each other. This allows one to have several 'on-state' signal paths at the same time in the matrix. One signal path is routed via an underpath metallisation while the other signal path is routed with an air bridge. A metal sheet connecting both coplanar ground metallisations is used as a shield to achieve good isolation. The optimisation of all elements from the cross is done with a full wave FDTD simulator. The achieved isolation between the two signal paths of the coplanar RF cross is above 40 dB up to 40 GHz. For both signal paths is in measurement and simulation for frequencies up to 20 GHz the return loss better than 18 dB and the insertion loss is below 0.7 dB.