Abstract: A two-dimensional periodic conductive post array structure is used to suppress higher order wave propagation within a large microstrip housing assembly without attentuating the desired TEM mode of propagation. Substantially vertical conductive posts are spaced at appropriate wavelength periods in the X and Y directions on top of and through a microstrip substrate in a so called "waffle-wall" type configuration. The periodic post structure provides high microwave signal isolation by functioning as a band rejection filter both above the microstrip substrate and within the substrate to isolate between nearby active and passive circuit functions located on the microstrip substrate. The desired TEM mode microstrip transmission line is routed through the conductive post matrix to provide the signal, control and DC paths between circuit chips and other active and passive circuit functions.