Rolf Jakoby

Investigation of barium strontium titanate thick films for tunable phase shifters

Abstract The influence of the microstructure of Ba0.6Sr0.4TiO3 (BST) bulk ceramics and thick films on the dielectric properties have been studied. Thick films have been prepared by screen printing technique on Al2O3 substrates. The powder has been prepared by using the common mixed oxide technique. In comparison to dense bulk ceramics, the permittivity of thick films is approximately 10 times less. The effect of temperature on the permittivity and the tunability (change of the dielectric constant with applied voltage) has also been investigated at low frequency (1 kHz). At microwave frequencies, BST thick films have been characterized by measuring coplanar waveguides (CPW) at room temperature and utilizing an enhanced quasi-static CPW model for multilayer dielectric substrates.

Metamaterial Inspired Microwave Sensors

Cheap and ubiquitous sensor systems will shape the coming decades. There is an emerging class of small high-performance electronic devices such as mobile phones, electronic toys, home appliances, monitoring and control systems in industrial facilities, and medical diagnosis systems, which are or will be equipped with pill box sized microprocessors or computers as well as sensors. These “smart sensors” with limited power and processing capabilities are often wirelessly interconnected. An assembly of many of them spread throughout the physical world will form sensor networks able to identify, localize, and monitor physical, environmental, and industrial processes, biological and health conditions, goods, vehicles, factories, stores, or even people.

Broad-band microwave characterization of liquid crystals using a temperature-controlled coaxial transmission line

Liquid crystals (LCs) promise to be suitable passive tunable material for microwave devices with excellent features concerning tunability and losses. In order to optimize the synthesis of LCs and the design of tunable microwave devices based on them, LCs have to be characterized at microwaves. For a microwave analysis between 360 MHz-23 GHz, use was made of a broad-band characterization method with a two-port coaxial line, temperature controlled within -7/spl deg/C and 115/spl deg/C. Two LCs are investigated: K15, as a quasi-standard, and MDA-03-2838, as a novel mixture with increased dielectric anisotropy and reduced microwave losses tan/spl delta/.

Design of compact planar antennas using LH-transmission lines

This contribution demonstrates the possibility to reduce the size of planar antennas by using LH-transmission lines. Two different types of radiators are investigated - a microstrip patch and a slotted cavity. A transmission line model is employed to design and compare these two approaches and their realization with a varying number of L-C loaded unit cells. Three representative antenna configurations have been selected and subsequently optimized with full wave electromagnetic analysis. Return loss and radiation pattern measurements of these antennas proof the developed concept.

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.

Beam Steering Transmitarray Using Tunable Frequency Selective Surface With Integrated Ferroelectric Varactors

A tunable frequency selective surface (FSS) with beam steering capability is presented. The FSS is used as a transmitarray with a bandpass characteristic in Ku-band. The periodic sub-wavelength (λ0/25) unit-cells are composed of capacitive and inductive structures creating a bandpass for an incident wave. By patterning the capacitive elements on a screen-printed barium-strontium-titanate (BST) thick-film ceramic, the resonant frequency of the FSS can be tuned. This technology offers a simple and cost effective way for integrating varactors into the FSS and is particularly attractive for microwave circuits with a high varactor density. A prototype is fabricated including two capacitive layers with an overall size of 40 mm × 40 mm. Each layer includes a total of 1600 integrated BST varactors that are fabricated by using a patterning and metallization process. A transmitted wave passing through the FSS will experience a phase shift, which can be tuned by tuning the passband. This allows steering the transmitted wave to a certain direction by applying a phase gradient along the FSS interface. A phase shift range of 360° can be covered by cascading several FSS panels. The prototyped tunable FSS demonstrates the feasibility of the proposed technology and its potential for beam steering.

Tunable passive phase shifter for microwave applications using highly anisotropic liquid crystals

A tunable broadband inverted microstrip line phase shifter filled with Liquid Crystals (LCs) is investigated between 1.125 GHz and 35 GHz at room temperature. The effective dielectric anisotropy is tuned by a DC-voltage of up to 30 V. In addition to standard LCs like K15 (5CB), a novel highly anisotropic LC mixture is characterized by a resonator method at 8.5 GHz, showing a very high dielectric anisotropy /spl Delta/n of 0.32 for the novel mixture compared to 0.13 for K15. These LCs are filled into two inverted microstrip line phase shifter devices with different polyimide films and heights. With a physical length of 50 mm, the insertion losses are about 4 dB for the novel mixture compared to 6 dB for K15 at 24 GHz. A differential phase shift of 360/spl deg/ can be achieved at 30 GHz with the novel mixture. The figure-of-merit of the phase shifter exceeds 110/spl deg//dB for the novel mixture compared to 21/spl deg//dB for K15 at 24 GHz. To our knowledge, this is the best value above 20 GHz at room temperature demonstrated for a tunable phase shifter based on nonlinear dielectrics up to now. This substantial progress opens up totally new low-cost LC applications beyond optics.

Low-Cost Phased-Array Antenna Using Compact Tunable Phase Shifters Based on Ferroelectric Ceramics

A low-cost phased-array antenna at 10 GHz is presented for a scan angle of ±50°. The array employs continuously tunable phase shifters based on a screen printed barium-strontium-titanate thick-film ceramic. Due to the use of artificial transmission line topology, the proposed phase-shifter design has a very compact size (3 mm × 2.8 mm) for 342° total phase shift. In the frequency range from 8 to 10 GHz, it exhibits a figure of merit >;52°/dB, which is among the best of phase shifters based on ferroelectric thick films. In a prototyped phased array, the RF circuit consists of a feeding network, phase shifters, and antenna elements, which are integrated into one planar metallization layer. Furthermore, a simple way for routing bias lines for phase shifters is demonstrated using high resistive electrodes. Using screen printed thick films and applying a simplified fabrication process for the RF and bias circuitry can reduce the total expense of phased arrays considerably.

Tunable inverted-microstrip phase shifter device using nematic liquid crystals

This paper introduces an alternative low-cost planar integrated, tunable liquid crystal phase shifter device for microwave applications, using the dielectric anisotropy of a nematic liquid crystal in conjunction with inverted-microstrip technology and a DC control voltage. With a standard nematic liquid crystal, a differential phase shift of 53/spl deg/ was achieved at 18 GHz for a physical line length of 49 mm, indicating a figure-of-merit of up to 12/spl deg//dB with a control voltage of only 40 V and a very low power consumption of 0.1 mW. On this basis, the phase shifter performance can be considerably improved by miniaturizing the device and by optimizing liquid crystals in the microwave region.

Sensor array based on split ring resonators for analysis of organic tissues

A sensor array concept has been developed using microstrip-line-excited split ring resonators (SRR). With the proposed structure it is possible to spatially resolve the dielectric properties of a Material Under Test (MUT). The split rings are designed to have different resonant frequencies and are decoupled from each other to allow a spatial distribution where a frequency shift of one individual resonant peak will indicate the dielectric properties of the MUT and its location within the array. Several prototype sensors have been realized and tested with different MUT such as dielectric bricks and pig lung tissue to prove the concept.