Gap Waveguide Technology- Electromagnetic Packaging and Transitions

Doctoral thesis, 2015

The scarcity of available frequency bands for new wireless communication systems, as well as the growing need for higher capacity, motivates the current interest in moving upwards in frequency. The millimeter wave frequency range is very attractive to allocate potential commercial opportunities such as short range wireless communications that allow high-speed data/video transmission, and automotive radars for both driver assistance and increased safety. The shift towards higher frequencies involves significant challenges. For instance, the mechanical tolerance requirements become much stricter, which provokes the need for high precision fabrication and assembly processes. Moreover, in order to achieve highly functional and densely packed electronic modules, new waveguide technologies that can meet the demands of high interconnectivity and ensure optimum electromagnetic packaging, need to be developed. The gap waveguide technology constitutes a new type of guiding structure that shows strong potential to become a suitable millimeter wave approach, and overcome the limitations of conventional planar transmission lines and hollow waveguides. This thesis presents recent investigations on gap waveguide technology, concerning packaging aspects and design of millimeter wave transitions. The first part of the thesis deals with an investigation of the electromagnetic packaging capabilities of the gap waveguide. This study addresses the performance of a Ku-band microstrip coupled line filter when it is packaged in different ways: unpackaged situation, and packaged by using a: i) metal cover, ii) metal cover with absorbing material (i.e. the conventional approach), iii) Perfect Magnetic Conductor (PMC), and iv) gap waveguide packaging realized by using a lid of nails. The obtained results show that the best filter performance is achieved when the microstrip circuit is packaged with a PMC, and a lid of nails. The PMC packaging condition is an ideal situation and can only be simulated, but it is still of great interest since it enables us to reduce simulation time compared to other packaging approaches. Also, the PMC represents a good approximation of the practical lid of nails. The lid of nails solution has been properly validated by both simulations and measurements. The second part of the thesis focuses on the design of millimeter wave transitions between different types of gap waveguide and classical waveguide or transmission lines. In order to achieve high integration between passive components, active components and antennas in a gap waveguide module, good compatibility between gap waveguides and other technologies is critical. One of the reasons is that for integrating gap waveguide circuits with Monolithic Microwave Integrated Circuits (MMIC), we need to develop suitable interconnections to microstrip and coplanar waveguide since the MMICs are substrate based. On the other hand, the interfaces of millimeter wave measurement instruments are either standard coaxial connectors, coplanar waveguide probes or rectangular waveguides. The transitions are therefore necessary in order to enable measurements of gap waveguide prototypes at high frequencies. We present two different F-band transition designs between ridge gap waveguide and coplanar waveguide/microstrip, and a V-band transition from inverted microstrip gap waveguide to rectangular waveguide. During the experimental validation of these transitions, we have identified different mechanical limitations involved in the assembly process of the gap waveguide prototypes. The assembly has an impact on the gap waveguide performance, and we have analyzed methods to mitigate the effect on the measurements.