Onur Hamza Karabey

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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An overview of liquid crystal technology for microwave and millimeter-wave frequencies is presented. The potential of liquid crystals as reconfigurable materials arises from their ability for continuous tuning with low power consumption, transparency, and possible integration with printed and flexible circuit technologies. This paper describes physical theory and fundamental electrical properties arising from the anisotropy of liquid crystals and overviews selected realized liquid crystal devices, throughout four main categories: resonators and filters, phase shifters and delay lines, antennas, and, finally, frequency-selective surfaces and metamaterials.

2 Elements in LC Display Technology

Based on a CMOS slow-wave coplanar-waveguide transmission-line topology, a novel compact millimeter-wave phase shifter is presented. The tunability is accomplished by using a liquid crystal (LC) material as a tunable dielectric between the coplanar signal strip and the shielding plane of the slow-wave transmission line. The device tunability is considerably enhanced by moving the free-standing signal strip with the application of a bias voltage. Combining the miniaturizing benefits of the slow-wave effect with the continuous tuning of LC material, the proposed device occupies only 0.38 mm2 and exhibits high performance. The phase shifter was characterized up to 45 GHz for a maximum bias voltage of 20 V without significant power consumption. The reproducible measurements show a figure-of-merit (ratio between the maximum phase shift and the maximum insertion loss) of 51°/dB at 45 GHz.

Electrically Tuned Microwave Devices Using 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.

Compact and Broadband Millimeter-Wave Electrically Tunable Phase Shifter Combining Slow-Wave Effect With Liquid Crystal Technology

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.

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

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 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.

Tunable loaded line phase shifters for microwave applications

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.

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

In this work the beam-scanning capability of a gradient index fishnet array, which is designed to operate at 22.5 GHz, is investigated. The results are discussed based on unit cell simulations and far-field measurements. The gradient is realized by changing the permittivity of a dielectric layer in each unit cell element resulting a tuning of the magnetic resonance frequency. Hence, an effective differential refractive index of 3.8 is achieved at 22.5 GHz yielding a theoretical beam-scanning angle of ±6.6°. Farfield measurements of a 5×5 array confirm the beam-scanning capability with a beam-scanning angle of ±3°. The deviation of the measured results is discussed.