An interposer that acts as a buffer zone between a transceiver IC and a dielectric waveguide interconnect is used to establish two well defined reference planes that can be optimized independently. The interposer includes a block of material having a first interface region to interface with an antenna coupled to an integrated circuit (IC) and a second interface region to interface to the dielectric waveguide. An interface waveguide is formed by a defined region positioned within the block of material between the first interface region and the second interface region.