Gap waveguide for packaging microstrip filters & investigation of transitions from planar technologies to ridge gap waveguide

Licentiate thesis, 2013

Gap waveguide technology has proved to constitute an effective alternative for the design of microwave passive components, due to its advantages with respect to traditional planar technologies and standard waveguides. There is no requirement for conductive contact between the two different metal pieces making up a gap waveguide prototype, since one of these pieces makes use of a textured surface that eliminates any possible leakage of the field through the gap to the surface of the other metal piece above it. The textured surface provides together with the opposite surface a cutoff of parallel-plate modes, so cavity modes are suppressed for any extent of the gap between the two surfaces. It is also less lossy than microstrip and coplanar waveguide, because there is no need to use dielectric in the design of gap waveguide components. The packaging of electronic circuits has become a critical factor, and it is important to study the effects on the performance when packaged. The first part of this thesis is focused on the study of packaging using gap waveguide technology, as a promising packaging method of microstrip passive devices such as filters. The gap waveguide packaging is in this thesis realized by using a lid of nails. Planar technologies, such as microstrip lines, are open structures that need to be electrically shielded and physically protected. One of the main drawbacks of the traditional packaging in metal boxes with a smooth metal lid, is the appearance of cavity resonances when two of the dimensions of the box are larger than half wavelength. It is possible to dampen these resonances by attaching absorbers to the lid of the metal cavity. The problem with this is the uncertainties in locating the absorbers in the package, and the additional losses introduced by the absorbing material. The present thesis investigates a microstrip coupled line filter by employing different types of packaging, including gap waveguide technology in Ku-band. Numerical results are also presented for the ideal case of using a Perfect Magnetic Conductor (PMC) as a lid. There is a stronger potential for advantages of the gap waveguides at higher frequencies, approaching and including the THz range. Therefore, the second part of this report deals with gap waveguide components that have been numerically analyzed at 100 GHz. At these frequencies, there is in particular a need for appropriate transitions that can ensure compatibility between the gap waveguide circuits and existing vector network analyzer ports and probe stations. For this reason, we have designed a transition from coplanar waveguide to ridge gap waveguide and another transition from microstrip to ridge gap waveguide. The integration of active components (MMIC) into the gap waveguides is challenging and can only be achieved with good transitions.