Alexander Gaebler

Liquid Crystal-Reconfigurable Antenna Concepts for Space Applications at Microwave and Millimeter Waves

Novel approaches of tunable devices for millimeter wave applications based on liquid crystal (LC) are presented. In the first part of the paper, a novel concept of a tunable LC phase shifter realized in Low Temperature Cofired Ceramics technology is shown while the second part of the paper deals with a tunable high-gain antenna based on an LC tunable reflectarray. The reflectarray features continuously beam scanning in between . Also first investigations on radiation hardness of LCs are carried out, indicating that LCs might be suitable for space applications.

Continuously Polarization Agile Antenna by Using Liquid Crystal-Based Tunable Variable Delay Lines

In this paper, a polarization agile planar antenna is presented by using tunable liquid crystal (LC) material. The antenna includes a 2 × 2 dual-fed microstrip patch array and two separate feeding networks for each feeding of the dual-fed antenna. The polarization state of the antenna can be controlled continuously between dual linear and dual circular polarizations depending on a differential phase shift between the antenna feedings. The feeding networks are implemented in inverted microstrip line topology with the liquid crystal material as a tunable dielectric. A desired differential phase shift is obtained between the feeding networks by tuning the LC material. Thus, no additional tunable components are required according to proposed antenna topology. Additionally, owing to the continuous tuning of the LC material, any polarization state between the circular and linear ones are achievable. The antenna prototype is designed at 13.75 GHz. The measured return losses are greater than 10 dB in a frequency range of 13.5 to 15 GHz for different polarization states. Far-field pattern measurements are performed, which confirm the continuous tuning of the antenna polarization. Specifically, the measured antenna patterns are presented for ±45° linear and right-handed circular polarizations. The prototype can be fabricated in a larger size with more radiating elements and can be efficiently scaled for higher operating frequencies at the Ka-or W-band since the LC material features even lower dielectric losses at higher frequencies.

A 2-D Electronically Steered Phased-Array Antenna With 2

For the first time, a 2-D electronically steered phased-array antenna with a liquid-crystal (LC)-based variable delay line is presented. The structure, which is designed at 17.5 GHz, consists of a 2 × 2 microstrip patch antenna array, continuously variable delay lines with a novel geometry, RF feeding, and biasing networks. The expected insertion loss of the variable delay line is less than 4 dB with a maximum differential phase shift of 300°. During the measurements, the antenna is steered by applying an appropriate dc biasing in the range of 0-15 V to the variable delay lines. It is also shown that the return loss is always better than 15 dB at the operating frequency when the antenna is steered.

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This paper present the numerical modeling of transmission lines taking into account the special material properties when liquid crystals (LC) are used as steerable anisotropic dielectrics. At first a short introduction of the properties of LC is given as well as the theoretical models used for the physical simulation of LCs in terms of molecular dynamics. Then, a full wave method to analyze the electrical behavior of LC filled waveguide structures is described. The combination of molecular dynamic and full wave simulation allows finally a systematic investigation of these structures.

2 Elements in LC Display 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.

Modeling of electrically tunable transmission line phase shifter based on liquid crystal

We have studied the dynamics of dielectric microparticles dispersed in a nematic liquid crystal (NLC) in the presence of an external AC electric field. Investigations were performed using optical trapping technique in the cell with in-plane electrodes. It was shown that the main driving force in the bulk of the material has electrophoretic nature. It was demonstrated that the microparticle behavior strongly depends on the distance with respect to the electrode and is influenced by the dielectrophoretic force. The model, which enables estimation of the electrokinetic forces, is proposed. The forces are found from the balance with the optical trapping force. The microparticle surface charge q≈2.1×10−17 C, linear electrophoretic mobilities μ∥≈10−11 m2/(V⋅s),μ⊥≈7×10−12 m2/(V⋅s), and the NLC viscosity η≈(21.2±4.7)×10−3 Pa⋅s at T=40 °C are evaluated.

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

This paper describes the characterization of Liquid Crystals (LCs) at W-Band using the standard WR-10 rectangular waveguide. The reflection parameter of a 10 mm long waveguide - terminated by a short-circuit - is measured. The LC is oriented with a homogeneous magnetostatic field strength of 400 A/mm, generated with an air-cooled coil. The extracted complex permittivity of the LC K15 from Merck KgaA (also known as 5CB) is presented, showing permittivities well below 3 with a dielectric anisotropy of Deltaepsiv<0.19 (equivalent to tau=Deltaepsiv/epsivmax<8.5%) and dielectric losses less than tandelta<0.023, all at W-Band.

Measurements of the electrokinetic forces on dielectric microparticles in nematic liquid crystals using optical trapping

A tunable substrate-integrated waveguide phase shifter using low-temperature co-fired ceramic (LTCC)-technology is presented in this paper. By changing the effective permittivity in the liquid crystal (LC)-filled waveguide, the differential phase can be tuned continuously. This is achieved by means of an analog signal applied to the electrodes, surrounding the LC. The design allows for precise tuning of the differential phase, which is proven with a Monte Carlo measurement resulting in phase errors of less than 3° at 28 GHz. Besides that, the ambient temperature dependency of the module is shown. The phase shifter has a high integration level and can be included into a complete and lightweight single-phased array antenna module. The phase shifter is realized with a high level of integration which is available through the multilayer process of the LTCC. It has a length of 50 and provides a differential phase shift of more than 360° at 28 GHz. The figure of merit for tunable phase shifters is >40°/dB.

W-Band Characterization of Anisotropic Liquid Crystals at Room Temperature

This paper presents the design and realization of a tunable periodically loaded slot line phase shifters. The tunability is achieved by using liquid crystal (LC) as a tunable dielectric. Two prototypes are fabricated based on a printed circuit board (PCB) - LC - PCB structure for 12 GHz and a glass - LC - glass structure for 18 GHz. Measurements are performed and the figure of merits of the phase shifters are determined as 47 °/dB and 42 °/dB, respectively.

Reliability study of a tunable Ka-band SIW-phase shifter based on liquid crystal in LTCC-technology

This paper presents a new method for the extraction of material parameters, in this case the permittivity, by formulating the Maxwell equations as an eigen-susceptibility problem of the considered sample. This offers a direct solution of the desired material parameters by utilizing adequate and well proven numerical techniques. Hence, it is very useful if analytical approaches do not provide the aimed accuracy or even fail completely. The procedure will be demonstrated by applying the variational approach to a triple mode cavity perturbation method for the characterization of the complex permittivity tensor of general biaxial anisotropic media.