An interconnection plate apparatus provides high frequency signal paths for communication among multiple pc boards and/or high frequency system components. The apparatus includes an interconnection plate which is a conductor having cross-sectional channels at which transmission line structures are formed. The transmission line structure includes a through-plate conductor concentrically surrounded by a through-plate insulator concentrically surrounded by the channel walls. An RF ground ring is positioned between the interconnection plate and a pc board at the transmission line structure. The interconnection plate provides a common ground reference and a common ground path for each board mounted to the interconnection plate. The ground ring structures assure a stable ground connection between the interconnection plate and the pc board ground planes. Different transmission line structures implement different signal transformation functions. The transmission line structure may simply pass the signal. In one embodiment, however, the transmission line structure serves as a low-pass filter.