Gap Waveguide: Low Loss Microwave Passive Components and MMIC Packaging Technique for High Frequency Applications
Doctoral thesis, 2013
The heavy congestion at the existing radio frequency spectrum allocated for the today’s wireless communications motivates and accelerates the research work at mmWave bands or even higher frequency range where more spectrum space is available for massive data rate delivery. The inclination of modern wireless system research and development is towards small size, reliable, high-performance and high-yield microwave multifunctional products. Interconnect problems, packaging problems and the mechanical assembly issues related to radio front-end components have been the major limitations towards using the mmWave technology for regular commercial applications.
There are some key issues to be considered while using conventional microwave technologies such as planar microstrip line or the metal waveguide for building up high frequency microwave modules or systems. At first, this thesis explains these factors briefly and put forward the existing performance gap between the planar transmission lines such as microstrip or coplanar waveguide and the non-planar metal waveguides in terms of losses, manufacturing flexibility and cost. The packaging problems in conventional microwave circuitry are also brought up from practical view point.
After that, the newly proposed gap waveguide technology is presented as a promising solution for high frequency microwave problems. Chapter 2 explains the operating principle of the proposed gap waveguide technology along with design of the parallel plate stop-band. Measurement results for the manufactured gap waveguide demonstrators are also provided with emphasis on loss analysis.
Chapter 3 presents mechanically flexible design of high Q resonators and bandpass filters based on groove gap waveguide. Narrowband filter design with Chebyshev response is presented at Ku-band and Ka-band. The filter in Ka band has been designed with commercial specifications in mind and this opens up the whole new idea of designing filters without problematic electrical contact between split blocks and sidewalls.
In chapter 4, ridge gap waveguide planar slot array has been described. One 4×1 element linear array and one 2×2 element array have been designed, manufactured and measured. Good agreement has been obtained between simulated and measured reflection coefficient for both slot array antennas. Obtained radiation patterns are also in agreement with the simulated patterns.
Chapter 5 shows how the parallel plate stop-band obtained from PMC surface and smooth metal surface can be utilized as a new packaging solution for high frequency RF circuitry. The basics of new PMC packaging along with some experimental verification for passive structures as well as active MMIC amplifier chain are detailed in this chapter.
Chapter 6 deals with crucial transition design for gap waveguide structures. One microstrip to ridge gap waveguide transition has been designed and measured at Ka-band. This transition is compact and utilizes only mechanical pressure contact between the microstrip line and ridge gap waveguide line instead of soldering or epoxy gluing process. The ridge waveguide to rectangular waveguide transition is also designed in two different approaches. In one approach, the ridge height is reduced in several steps to match the height of rectangular waveguide. In the other approach, the ridge gap waveguide is fed from the bottom by a rectangular waveguide and the width of the ridge section is tuned at the rectangular waveguide opening.
Thus, the topic of this doctoral dissertation is concerned with research tasks to validate the concept of gap waveguide technology and to investigate its potentials for high frequency microwave applications. All the studies presented in this thesis are proof of concepts and accomplished mainly in Ku-band or Ka- band. Nevertheless, the gap waveguide technology is quite suitable for mmWave frequency or even higher frequency applications.