Characterization of losses in ridge, groove and microstrip gap waveguides

Licentiate thesis, 2011

The gap waveguide is a new technology that recently has been presented as an attractive candidate for applications at millimeter and sub millimeter waves. The waveguide is generated in the gap between a smooth upper metal plate and a ridge (or a groove) in a textured lower metal plate. The difference compared to standard technologies, such as waveguides and microstrip transmission lines, is that the gap waveguide can be made of only metal and can support propagating TEM or TE/TM modes without need of metal contact between the assembled blocks. This is possible because the ridge is surrounded by an artificial magnetic conductor (AMC) that creates a parallel plate cut-off region and forces the field to be confined soley along the ridge. The AMC can simply be realized with a textured surface of metal pins, but other realizations are also possible. This thesis is focused on two main tasks. Firstly, to present an experimental study of losses in ridge and groove gap waveguides, and secondly to show the design, experimental validation and loss study of a new type of gap waveguide, called microstrip gap waveguide, for low frequency applications. The losses are determined by studying the unloaded Q-factor of resonators made in gap waveguide technology. The Q-factor is known to be a measure of the loss in a resonant circuit, and it can therefore also be used to determine loss per length unit of waveguide resonators. In this thesis different resonator designs are presented for ridge, groove and microstrip gap waveguides. The Q-factor is directly calculated from the simulated or measured transmission coefficient of the resonators. The attenuation per length unit can be easily obtained from the Q-factor for TEM-type waveguides, and the ridge and microstrip gap waveguides are quasi-TEM. The simulation results have been validated by measurements. The obtained Q-factors approach values of 4200 for the ridge gap resonator and 6000 for the groove gap resonator, compared to an ideally realized rectangular waveguide that has a Q of 8000. The ideally realized rectangular waveguide has no air gaps between joining metal parts and this cannot easily be achieved at high frequency. Any gap, even if it is very small, destroys performance of the rectangular waveguide, and its Q-factor. The advantage of the gap waveguides is that the AMC pin surface totally removes any leakage from gaps between the two metal plates. We also propose a new geometry made in microstrip gap waveguide, using a textured surface with mushroom-type electromagnetic bandgap (EBG) structure, to create the parallel-plate cut-off. This circuit represents a compact, low loss and already packaged solution, that can suppress cavity modes and radiations generated when packaging standard microstrip lines. Keywords: Gap waveguides, Artificial Magnetic Conductors, Mushroom-Type EBG, Losses, Quality Factor, Waveguide Resonators, Packaging of Microwave Components, Microstrip Lines.