High-Performance TE10-Waveguide Phase Shifters for Millimeter Waves - Generic Concepts Towards Liquid Crystal (LC) filled MultiGap WaveGuides (MGWG)

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This proposal aims to create a new generic concept towards voltage-controlled Liquid Crystal (LC)-based TE10-groove MultiGap WaveGuide (MGWG) phase shifters at Ka-band. It will be a first low-loss tunable RF building block specifically-designed for high-performance applications, using the unique feature of microwave LCs with low losses, slightly decreasing with frequency, and the key advantages of Gap WaveGuides (GWGs), that are its galvanic-isolated walls and its low-loss nature comparable with TE10-rectangular waveguides. One technical goal is to achieve the highest phase shifter Figure-of-Merit (FoM) possible, in the range of 200°/dB at 30 GHz, compared to around 120°/dB of rectangular waveguide phase shifters in split-block design with the same LC and electrode foils at the top and bottom walls to control the LC orientation by a bias voltage. This exceptional goal is feasible, since the FoM of the rectangular waveguide phase shifters was more than 200°/dB for magnetic biasing (without electrodes), where the difference is accounted to the losses caused by the electrodes, including coupling into stripline modes, the field leakage from the small gaps for the electrode foils to be led out, and because the tuning efficiency by electric fields could not fully be exploited as for magnetic fields. The issues above can be overcome by using GWGs, having two electrically isolated metal plates at the top and bottom, where one has an artificial magnetic conductor surface, which can be realized by a Bed of Nails (BoN). These two DC-decupled metal plates can be used as biasing electrodes. Moreover, since no joining process is necessary, no appreciable field leakage occurs. However, for a phase shifter, more than two electrodes are required to fully control the LC orientation. For this, multiple GWG elements with BoN are stacked together without any galvanic coupling. Based on this new MGWG concept, the envisaged lab-scale demonstrator consists of four groove GWG-elements to build up TE10-waveguide phase shifters with LC-filled Rexolite containers, using them at the same time as electrodes to fully control the LC orientation continuously from parallel to perpendicular with respect to the RF-field, thus, controlling its phase shift. Far beyond state-of-the-art, this new electrical scheme could achieve similar tuning efficiency as for magnetic biasing. To connect this MGWG phase shifter to other standard waveguide components, WR-28 flanges are used with adapter flanges and special DC-block flanges in-between to avoid the galvanic connection of the biasing electrodes. These flanges are also part of this project, using three different approaches relying on the BoN principle, a mushroom-type structure and on photonic crystals. The most promising one will be implemented into the final demonstrator to evaluate the potential of this new MGWG concept with inherently galvanic-decoupled electrodes as a platform for reconfigurable millimeter-wave systems.