Gap Waveguide Technology for Millimeter Wave Applications and Integration with Antennas

Doctoral thesis, 2013

The increasing advance of short range wireless communications requiring high data rates and the lack of available spectrum have driven researchers and industry to move towards higher frequencies, in particular to the millimeter wave range. The opening of this wide portion of free spectrum has risen a large interest in developing mm-wave communication systems for commercial applications. Higher frequencies lead to smaller sizes of RF components including antennas, thus introducing a new trend of realizing single compact modules, where active, passive components and antennas are integrated in the same package/chip. However, the implementation of passive components and interconnected transmission lines on these modules is difficult at millimeter waves with classical technologies, such as microstrip transmission lines and waveguides. Microstrip transmission lines suffer from high dielectric and conductive losses and waveguides are difficult to combine with integrated circuits and need accurate assembly process to assure good electrical contacts when made in different blocks. Therefore, new technologies are needed in order to face the challenges of the next generation systems. The new gap waveguide technology has been recently introduced as a promising candidate to address some of the problems faced at millimeter waves with conventional technologies. The key idea behind this circuit is based on the possibility to guide the electromagnetic field along desired directions in the gap between metal plates and to avoid any propagation along undesired directions. In this way, any leakage occurring in between the split blocks of a circuit with poor quality metal contacts as well as unwanted radiations are avoided. This condition is achieved by surrounding a metal ridge/strip, or groove, with a so-called Artificial Magnetic Conductor (AMC). When this textured layer is placed below an upper metal lid, all parallel-plate modes will be in cut-off, thus letting the propagation only in the air gap between the ridge/strip and the upper plate. This thesis presents the development of the gap waveguide in terms of packaging capability, losses, and integration with antennas. The losses study is performed by calculating the unloaded Q-factors of resonators made in ridge and groove gap waveguides. Experimental validations show that the gap waveguide has very low losses, and in particular the groove gap waveguide has similar measured unloaded Q-factors as rectangular waveguides when they are made in different blocks. The advantage is that the AMC surface in the gap waveguide can remove any leakage from the tiny gaps between two metal plates, which is a benefit at high frequency. The losses study is also provided for micromachined gap waveguides above 200 GHz. In addition, the idea of a contactless waveguide flange made by bed of nails is presented for high frequency measurements. This new design can avoid typical mismatch and unwanted radiations which can occur in standard waveguide flanges when they are not well tightened to the circuit under test. We also present the packaging capabilities of the gap waveguide for low frequency applications, since the metal pins become too thick, by introducing a compact periodic surface made of printed zigzag wires. In particular, this thesis analyzes the microstrip gap waveguide, which becomes attractive for both low and high frequency applications because made in printed technology, but still allowing propagation in the air, thus being low loss. 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. Finally, the microstrip gap waveguide is applied as low loss feed network for horn antenna array. This thesis shows the design and experimental validation of a sixteen-element planar dual-mode horn array excited by a microstrip gap waveguide corporate feed network, which can be an advantageous solution for 60 GHz antenna applications.