M. Jost

Tunable dielectric delay line phase shifter based on liquid crystal technology for a SPDT in a radiometer calibration scheme at 100 GHz

This paper presents for the first time, an electrically tunable dielectric line based on fiber topology. A fiber segment is filled with liquid crystal (LC) for continuous tuning of the differential phase between 0° to 90° by an applied biasing voltage of up to ±500 V. This phase shifter is aimed to be implemented into a RF switch (Single-Pole Double-Throw, SPDT), to switch between the calibration loads and the antenna of a radiometer at 100 GHz. A subwavelength topology was chosen, where compared to classical dielectric waveguides, air is acting as cladding material, ensuring a low loss propagation comparable to hollow waveguides. The phase shifting section has a total length of 26mm and provides a maximum differential phase shift of more than 107° and 115° at 100GHz for electric and magnetic biasing, respectively. Accompanied by insertion losses between 2.5 dB to 3.0 dB, the phase shifter shows a figure of merit at 100GHz of 42 °/dB for electric and 44 °/dB for magnetic biasing.

Evolution of Microwave Nematic Liquid Crystal Mixtures and Development of Continuously Tuneable Micro- and Millimetre Wave Components

This work represents the evolution of micro- and millimetre wave optimized nematic liquid crystal mixtures and their applications. Starting with the well-known liquid crystal mixture K15 used in the display technology, microwave optimized LC mixtures have been developed. For that, novel characterization setups have been developed, which play an essential factor in the optimization process for the micro-, millimetre wave and THz regime. By using a specialized LC simulation tool, different tuneable waveguide topologies are compared in terms of tuneability, transmission loss and tuning speed. Based on that, a phase shifter for W-band frequencies has been fabricated, which reached a measured figure of merit for passive phase shifters of up to 148° /dB.

Comparison of hollow waveguide and dielectric fibre based SPDT switches for W-band

This work presents as a first approach, the comparison of two non-tuneable/switchable W-band single pole double throws (SPDTs), one realised in hollow waveguide and one in subwavelength dielectric fibre technology. For pre-investigations, both SPDTs are equipped with non-tuneable phase shifters, providing a fixed differential phase shift of 90° between the two paths. The waveguide SPDT shows a reflection better than -10 dB and an isolation between the two output ports of around 16 dB. The fibre SPDT shows reflections better than -10 dB over a wide frequency range and down to -25 dB at 100 GHz, but an isolation between 4 dB to 10 dB only. This deviation compared to simulations is due to the sensitivity of the system in terms of fabrication tolerances whereby the needed differential phase shift of 90° has been exceeded. Next, these phase shifters will be replaced by continuously tuneable phase shifters based on liquid crystal technology. At this point, dielectric fibres are a very promising alternative to hollow waveguides.

Tunable microwave component technologies for SatCom-platforms

Modern communication platforms require a huge amount of switched RF component banks especially made of different filters and antennas to cover all operating frequencies and bandwidth for the targeted services and application scenarios. In contrast, reconfigurable devices made of tunable components lead to a considerable reduction in complexity, size, weight, power consumption, and cost. This paper gives an overview of suitable technologies for tunable microwave components. Special attention is given to tunable components based on functional materials such as barium strontium titanate (BST) and liquid crystal (LC).

Interference based W-band single-pole double-throw with tunable liquid crystal based waveguide phase shifters

This work presents an interference based W-band single-pole double-throw (SPDT) in rectangular waveguide and liquid crystal technology. In radiometers, this kind of SPDT can be used e.g. for switching to the calibration load for power calibration. The SPDT is designed with an E-plane power divider, two different paths for the phase shifting regions, being separated by 30 mm to provide enough space for the used magnets for proof-of-concept, and a coupled line combiner, where the interference is taking place. Rexolite 1422 is serving as liquid crystal cavity. The matching is better than −12 dB between 88 GHz to 110 GHz, except a peak around 102 GHz. The insertion loss is less than 3 dB between 89 GHz to 105 GHz, while exhibiting an isolation of at least 9 dB in this frequency range. From 90 GHz to 100 GHz, isolation is even between 10 dB to 12 dB.

Design of a continuously tunable W-band phase shifter in dielectric waveguide topology

This work presents a liquid crystal (LC) based phase shifter in a dielectric waveguide (DW) topology consisting of core and cladding for the W-band. For continuous tunability, a part of the core material is replaced by liquid crystal. Furthermore, suggestions of materials for designing such a DW, i.e. for core and cladding, are given in this paper. In comparison to other topologies, the advantage of this topology is that the necessary electric biasing can be realized easily, by placing electrodes directly on the cladding. With an electric biasing of ±550 V, a maximum differential phase shift of 430°, accompanied with insertion losses between 2.8 to 5.5 dB with standard WR10 connections, could be achieved. The maximum figure of merit is around 100 °/dB at 102 GHz.

In-plane hollow waveguide crossover based on dielectric insets for millimeter-wave applications

This paper presents an in-plane hollow waveguide crossover for W-band frequencies. This kind of crossover can be implemented e.g. into a Butler matrix, to simplify the fabrication process significantly. It is based on a partially dielectric filling of the waveguide, focussing the field in the center. The dielectric is placed in the center of a hollow waveguide crossing and has a star-shape. Inside the dielectric filled region, a higher order mode propagation is possible, which has no significant influence on the overall performance of the crossover. It shows an insertion loss between 0.8 dB to 1.0 dB in the frequency range of 101.5 GHz to 108.0 GHz, while the matching is better than −10 dB and even down to −20 dB between 105 GHz to 108 GHz. The isolated ports show transmission coefficients better than −20 dB over the whole frequency range and even down to −50 dB in the best performing frequency range.

Liquid crystal technology for reconfigurable satcom applications

Modern satellite communication scenarios require a steerable antenna pattern e.g. to continuously align the antenna of a low earth orbit satellite toward a geostationary relay satellite. One promising solution especially in the higher frequency bands starting from Ku-band is based on liquid crystal as functional material. Based on the anisotropy of this material tunable components and systems like phase shifters and array antennas can be built. This paper shows the potential of specially synthesized liquid crystals for the use in phased array antennas for SatCom applications.

Electrical biasing scheme for Liquid-Crystal-based tunable Substrate Integrated Waveguide structures

In this paper, an electrical biasing structure is proposed for a tunable Substrate Integrated Waveguide (SIW) device with Liquid Crystal (LC) as the tuning component. The biasing circuits or electrodes are made of chromium and gold layers. Silicon nitride layer, which has a thickness of 1 μm, is employed to isolate the electrodes against ground. Since voltage as high as ±200V may be applied, high breakdown voltages are required. As a proof of concept, a simple LC-SIW phase shifter based on a tunable delay line is manufactured and sealed with the fabricated biasing structure. Simulation results exhibit a Figure of Merit (FoM) of 89°/dB at 20GHz. The fabricated phase shifter give FoM of 32°/dB with switching time of 5s.

Tuneable hollow waveguide devices for space applications based on liquid crystal

This paper presents an overview in the field of passive, continuously tuneable liquid crystal (LC) devices in hollow waveguide topology. In particular, the designs and measurements of a Ka-band phase shifter as well as a K-band band-pass filter based on an LC filled waveguide resonator are shown. Both demonstrators are designated to be space qualified as their field of application is in satellite communications. While the high performance phase shifter will be integrated in a phased array antenna for beam scanning purposes, the high quality (Q) factor band-pass filter will be used to change the operating frequency and band allocation of a satellite.The key feature of the LC based hollow waveguide phase shifter is its high efhciency, dehned by the maximum differential phase shift divided by the maximum insertion loss in all tuning states, which is measured in the design frequency range of 23 GHz to 27 GHz to more than 130°/dB by means of electric biasing. The key feature of the band-pass hlter is its high Q-factor of up to 484, resulting in a comparatively small bandwidth compared to common hlters based on planar topologies. For the presented hlter, a relative bandwidth of 1% is measured at 20 GHz.