Gap waveguide components for millimetre-wave systems: Couplers, filters, antennas, MMIC packaging

Abstract

Gap waveguides were first presented in [1] as an alternative guiding technology especially attractive for frequencies over 30 GHz up to THz. At those frequencies, the current technologies show some deficiencies regarding to the performance, integration ability, or product cost. Planar technologies, such as microstrip and coplanar, are often chosen due to their good integration ability and manufacture simplicity, but they suffer from higher losses with increasing frequency as well as from the presence of cavity resonances when encapsulated. Hence, hollow waveguides are usually resorted for low-loss applications, in spite of their difficulty for integration with active components and a high manufacturing cost. The need of new transmission line technologies for mm- and sub mm-wave systems is leading to the apparition of alternative technologies. Substrate Integrated Waveguide (SIW) technology has been widely used for high-frequency applications [2], but it exhibits significant losses at increasing frequencies due to wave propagation in substrate. Gap waveguides, on the contrary, support waves in the air gap between two metal plates. One of the plates is provided with a texture, in the form of a bed of nails, to create a high impedance condition at the surface, which in turn forces a cut-off for the parallel-plate modes [3]. On the same plate, there are metal ridges in between the nails providing a path to the waves so that fields are confined to the air gap between the ridges and the metal plate on top. This propagation path can alternatively be provided by a microstrip line lying on the bed of nails, or by a groove in between the nails. An interesting application using similar technology can be found in [4] where a multi-layered phased array antenna developed in Japan was presented.