C. Weickhmann

Continuously tunable phase shifters for phased arrays based on liquid crystal technology

This paper presents a novel liquid crystal technology for tunable microwave devices, primarily developed by the TU Darmstadt and Merck consortium over the last 10 years. This paper focuses on quasi powerless continuous (analog) tuning of phase shifters implemented in different transmission line topologies, which are used for electronically-controlled phased arrays aimed for various applications and frequency ranges. It consists of phase shifters and antenna arrays in planar topologies, in LTCC microstrip line and substrate integrated waveguide and rectangular waveguide topologies using liquid crystal filled cavities to tune the velocity of the traveling waves along the various line phase shifters. The measured figure of merit of the various phase shifter values range from 45° dB-1 to 72° dB-1 @ 30 GHz for LTCC integrated phase shifters and 62.5°dB-1 @ 17.5 GHz for planar phase shifters as well as 120° dB-1 to 200° dB-1 for waveguide phase shifters. Besides the phase shifter performances, far field measurements are presented as well to demonstrate the scanning capabilities of a 2 × 2 and 4 × 1 array antenna built with different topologies.

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.

Simulation of an electronically steerable horn antenna array with liquid crystal phase shifters

This paper presents the simulation results for a phased antenna array with novel liquid crystal phase shifters (LCPSs). This work investigated the usability of this electronically steerable antenna technology in an inter-satellite link using a geostationary satellite. Due to the design of the LCPS and the aperture of the antenna, communication over the desired steering range of ±11° can only be accomplished if the manipulated phase number of the radio wave changes during the steering process. This results in the need for ideally instantaneous phase changes of 360° (jump) of some of the LCPSs during beam steering. Tests on first prototypes of the LCPS reveal that the currently employed high frequency liquid crystals (LC) require 44 s to 150 s to complete such a 360° phase jump, resulting in a severe drop in antenna gain during a portion of the time of these phase jumps. This study investigates the influence of these jumps on the communication link and compares different LCPS control techniques, used to optimize the distribution of phase jumps over time, by a figure of merit (FoM) for the link. Losses caused by the LCPSs within the waveguide are not considered in this paper. Mitigation methods to minimize the effects of out-of-phase LCPSs could aim at reducing the magnitude (optimized, staggered jumps) and/or duration of losses (faster LCPSs through faster LC or stronger E-field). None of these methods can avoid the loss in gain entirely, only minimize the duration or magnitude of loss. Therefore, a parametric study on phase shift speed vs. mean loss for various phase jump strategies was performed. Despite the slow alignment of the LCs and the resulting loss of gain, usable links could be identified with the investigated mitigation methods.

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).

Manufacturing and testing of liquid crystal phase shifters for an electronically steerable array

In 2011 TU-Munich together with TU-Darmstadt started to develop a novel phased array antenna with liquid crystal phase shifters for space applications (Lisa-ES). The antenna design and manufacturing is based on the precursor Lisa-MS (mechanically steered). Both antennas are designed for inter-satellite links in the Ka-Band (23/27 GHz). The novel part in this antenna is a phase shifter that uses liquid crystals that are placed within a waveguide. Several of these liquid crystal phase shifters are than placed in-between the distribution network of the array antenna and each horn. The phase shift is controlled by the orientation of the liquid crystals and thus by the variation of the relative permittivity. In this case the orientation of the liquid crystals is controlled by an electric field. The liquid crystal phase shifters design is very sensitive to air gaps and air bubbles within the waveguide. Therefore, the non-conductive polystyrene container is vapor-metallized (400 nm silver) and then electroplated with 600 μm copper to achieve a gap-free conductive surface (= waveguide). The process for the antenna was well established from a precursor project. Finding the appropriate integrated manufacturing sequence and processes was much more time-consuming and challenging than originally expected at the start of this project. The process remains complex and error-prone. Environmental tests in addition to further high frequency performance tests are designed to prove the viability of the selected design. Despite the complexity and unavoidable losses within the phase shifter, the concept has advantages over alternate designs and could become a viable option for selected applications.

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.