Christian Damm

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/.

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.

Compact tunable periodically LC loaded microstrip line for phase shifting applications

The properties of periodically LC loaded lines with tuneable capacitors are investigated for phase shifting applications. Basic equations are derived describing the losses due to the tuning mismatch and the quality of the tuneable material. The performance of microstrip line phase shifting devices with right- and lefthanded wave propagation is compared and evaluated in terms of a defined figure of merit. As a demonstrator, a phase shifter circuit using the principle of periodically LC reactive loading is realized with varactor diodes. The comparison with other circuit topologies confirms the idea to realize compact devices with this circuit concept.

Performance evaluation of left-handed delay lines for RFID backscatter applications

The operation principle of a passive RFID tag on the basis of delay lines is presented and the performance of left- and right-handed lines for this application is compared. Additionally, limitations due to the dispersion properties of left-handed lines for the maximum number of usable cells are discussed. In conclusion, an expression relating the maximum number of usable bits for the tag impulse response to system parameters like bandwidth, centre frequency etc. is given. A proof of concept is realized with a 100 cell four bit backscatter tag. The set up of the tag is described and the performance analysed.

Artificial Line Phase Shifter with separately tunable Phase and Line Impedance

Based on the theory of metamaterial transmission lines, a phase shifter with independently controllable phase shift and line impedance is developed. The possibility of separate Deltaphi and Z L tuning reduces mismatch losses and leads to an increased performance, or can be applied for a combined phase shifter attenuator circuit. The presented phase shifter operates at a frequency of 6.5GHz with a FoM of 60 to 70deg/dB over a 30% bandwidth region. The second operation mode shows a 0deg-90deg differential phase shift with 0dB-10dB differential magnitude control with 10% bandwidth around 5GHz

Tunable composite right/left-handed leaky wave antenna based on a rectangular waveguide using liquid crystals

A leaky wave antenna based on a composite right/left-handed waveguide structure is presented. The antenna features beam steering capabilities for a fixed operation frequency of 7.6GHz. The frequency independent tunability is obtained by inclusion of liquid crystal (LC) material whose dielectric properties can be tuned by static external electric or magnetic fields. Simulations as well as measurements of a built prototype are shown for scattering parameters and far field patterns. The maximum obtained beam tilt amounts to ±10° around broadside.

LH phase shifter using ferroelectric varactors

A tunable left handed (LH) transmission line used as continuously tunable phase shifter is presented. Using varactors based on barium-strontium-titanate (BST) thick films, a figure of merit FoM = 29deg /dB has been measured at 2.8 GHz. The bandwidth of the realized circuit using 3 unit cells reached 10%. The agreement of the presented result from simulation and measurement is good, the differences can be explained by tolerances of the used components. Additionally the simulations of a 180deg phase shifter based on the same components for the unit cell is presented. Simulated results of this phase shifter shows a bandwidth of 27% and a FoMmax = 31.5deg /dB

Transmission lines loaded with pairs of magnetically coupled stepped impedance resonators (SIRs): Modeling and application to microwave sensors

This paper is focused on the analysis and modeling of transmission lines loaded with pairs of shunt-connected stepped impedance resonators (SIRs), and their application to differential sensors for dielectric characterization, and for diagnosis and quality control of material samples by comparison to a reference. It is demonstrated that by placing the SIR junctions in the same position of the line, the SIRs are magnetically coupled. Such coupling has significant influence on the sensitivity of the sensor, determined by the split in frequency caused by an asymmetric dielectric loading of the SIRs. The circuit model of the structure, including magnetic coupling between SIRs, is proposed and validated through electromagnetic simulations and measurements. Finally, the principle of sensing is experimentally validated by a proof-of-concept demonstrator.

Compact Tunable Phase Shifters on Screen-Printed BST for Balanced Phased Arrays

Two designs for compact continuously tunable phase shifters are presented employing integrated ferroelectric varactors based on screen-printed barium-strontium-titanate thick film. The circuits are based on left-handed transmission-line topologies that allow to realize very compact modules. The proposed designs have total lengths of 2.1 and 3.8 mm and provide a differential phase shift of more than 360° and a figure of merit of 51°/dB at 10 GHz. Furthermore, the phase shifters are in coplanar strip configuration, making them attractive for systems composed of balanced components.

Artificial transmission lines for high sensitive microwave sensors

Two common and two new unit cells for artificial lines made of four reactive elements are studied concerning their sensitivity to capacitive changes and their applicability as capacitive sensors. It is shown that selection of the unit cell is very important and can highly influence the sensitivity. The shown principle is very powerful and can be scaled up to THz frequencies, only limited by available fabrication technology. A built planar differential microwave sensor based on artificial lines working at 2.3 GHz is presented. It features a simple output power evaluation instead of a classic but more complicated phase difference measurement. With power variation of up to 60 dB in dependence of small environmental dielectric changes it shows a very high sensitivity and dynamic range. Functional layer coatings can be applied easily due to its planarity and open wide application areas for biomedical sensing.