2015 IEEE. Developing radio-frequency (RF), microwave, and millimeter-wave systems requires low-cost, mass-producible, high-performance, and high-yield technologies for passive and active sections. Dealing with radio coexistence and strong coupling between different subsystems while maintaining or even reducing the size and weight of the final system is a very challenging task. This is more important when designing passive circuits such as resonators, antennas, highly selective filters, couplers, power dividers, and circulators [1], [2]. Classical waveguide technology was, and in some cases still is, the mainstream for designing high-performance passive circuits and systems [3], [4]. However, these bulky and heavy concomitants are not suitable for integration and low-cost mass production. Via posts were first used under the name laminated waveguides [5] in 1998 and as a subset of substrate integrated circuits [6] in 2001 to form a planar waveguide structure. Then, by the introduction of the substrate integrated waveguide (SIW) technology as we know it today by Deslandes and Wu [7], a remedy for the full integration of active and passive circuits in a planar fashion was found [7]-[11]. Using this method, a nonplanar rectangular waveguide can be made in a planar form that is compatible with the existing printed circuit board (PCB) and lowtemperature cofired ceramic techniques [12]. Conductive walls in classical waveguides are replaced with planar-compatible PCBs via posts in SIW structures. Similar to classical metallic waveguides, by shortening the two openings of the SIW structure at both ends, SIW cavities can also be formed.
