Paul Laurent

Patch antenna adjustable in frequency using liquid crystal

The topic considered here is the design, achievement and test of a liquid crystal-based patch antenna. This device was realised in planar technology with low-cost foam substrate. Liquid crystal dielectric anisotropy permits frequency agility. Owing to technological difficulties of the foam, we used this dielectric property to adjust the shift of the antenna resonance frequency whenever required. The comparison of simulation and measurement data evidenced that frequency can be adjusted.

KTa/sub 0.6/Nb/sub 0.4/O/sub 3/ ferroelectric thin film behavior at microwave frequencies for tunable applications

In this study about the relationships between structural and microwave electrical properties of KTa1-xNbxO3 (KTN) ferroelectric materials, a KTN thin film was deposited on different substrates to investigate how KTN growth affects the microwave behavior. Interdigital capacitors and stubs were made on these films through a simple engraving process. Microwave measurements under a static electric field showed the importance of the substrate on the circuit behavior and, notably, on the tuning factor

Highly tunable microwave stub resonator on ferroelectric KTa0.5Nb0.5O3 thin film

A coplanar waveguide (CPW) stub resonator has been fabricated on a pulsed-laser deposited KTa(0.5)Nb(0.5)O(3) (KTN) thin film (600 nm-thick) onto a r-plane sapphire substrate. It was designed to operate at 10 GHz when the applied bias voltage is zero. We show experimentally that the resonance frequency is shifted by 44% under a 70 kV/cm DC applied electric field. In addition, the dielectric characteristics of the KTN film have been assessed through post-processed measurements of CPW 50-Omega transmission lines using the conformal mapping method.

Improvement of an inverted microstrip line-based microwave tunable phase-shifter using liquid crystal

This paper deals with the study of electrically tunable phase-shifters based on the use of liquid crystals. First, the dielectric characterization of the commercial liquid crystal involved in our investigations allowed us to determine its dielectric anisotropy. Secondly, two phase-shifter configurations were studied. Both structures propose inverted microstrip line with coplanar accesses, but they are based on a different design of the transmission line. The measurements carried out with the first one evidenced an efficient phase shift. The second one showed considerable improvement in performances.

KTN Dielectric Properties at Microwave Frequencies: Substrate Influence

To determine the dielectric properties of ferroelectric thin-films over a wide range of frequencies we developed a simple and efficient method based on the measurement of coplanar waveguide realized on ferroelectric/dielectric heterostructures by using a Spectral Domain Approach. We applied this method to KTa 1−x Nb x O 3 (KTN) thin-films deposited on different substrates. The strong differences observed in the dielectric behavior of KTNs according to the growth substrate suggest that it is affected not only via the resulting microstructure, but also by the induced strains. Accuracy of the method was estimated to 15 and 10% on permittivity and loss tangent, respectively.

Liquid Crystal Tunable Filter Based On DBR Topology

The use of liquid crystals for tunable devices has already been investigated for phase-shifters and antennas in planar technology. Here, the focus is, first, on a half-wavelength open-circuited stub resonator, then on a second-order dual behavior resonator filter. To have the benefit of liquid crystal anisotropy and thus obtain agility, a bias voltage is applied to the device under study. In order to avoid the shift observed between simulated and measured results of the half-wavelength open-circuited stub for the first structure, the technological process has been improved for the second one in the aim to reduce mismatching and LC leaking

Influence of design liquid crystal-based devices on the agility capability

This article deals with liquid-crystal-based tunable components: phase-shifters and patch-antennas. It reports on investigations about the influence of design on their characteristics. For phase-shifters based on an inverted microstrip line, the line design is paramount for improvement of phase-shift and figure-of-merit. Moreover, the use of liquid crystal in the design of patch antennas allows adjustment of the resonance frequency, and slot insertion results in size reduction with no decrease of performances. Both cases highlight the interest of liquid crystal in millimeter frequency devices.

KTa0.5Nb0.5O3 ferroelectric thin films: processing, characterization and application to microwave agile devices

This paper is aimed at demonstrating the high capabilities of KTa0.5Nb0.5O3 thin films for microwave agility applications. Tunable capacitors, resonators and phase shifters were realized with KTN materials. Their high frequency measurements showed, in particular, the very high tuning factors of these films under quite low electric field.

Epitaxially grown ferroelectric thin films for agile devices

Agile devices based on ferroelectric material thin films provide a promising approach to solve the problem of multi-standard communication systems. As an alternative to BaxSr1− x TiO3, the potentialities of two other materials, namely SrBi2Nb2O9 and KTa1− x Nb x O3 have been explored. This review gets an insight into the growth of thin films of these materials, including the control of stoichiometry and of their orientation on various substrates usable in the microwave range. Dielectric measurements show that the Curie temperature is close to the value reported on bulk materials and that the transition is of first order, while the Curie-Weiss temperature is frequency dependant in the example of KNbO3 films. Microwave test devices, patterned on such films, show promising agility levels, while it appears that further efforts have to be done to reduce the dielectric losses.

Tunable DBR resonators using KTN ferroelectric thin-films

Tunable dual behavior resonators were realized on using KTN ferroelectric thin-films. Their measurements demonstrated an agility of more than 21% in the 6.2-7.6 GHz frequency range under an applied field of 80 kV/cm. At the same time, insertion losses at the center frequency decreased from 3.27 to 2.06 dB; reflection losses were better than 11.5 dB at the same frequency. This study evidenced the potential of the DBR structure for tuning applications as well as the high tuning factor of KTN thin-films at microwave frequencies.