Dielectric properties of microstrip‐line adaptive liquid crystal devices

Abstract

AbstractThe dielectric constant of liquid crystals at microwave and millimeter‐wave frequencies is measured directly with an inductively coupled ring resonator of a microstrip line configuration, which is widely used for the development of many adaptive devices such as variable delay lines. By comparing the electromagnetic simulation value and the measured value of the resonant frequency of the ring resonator on a liquid crystal as a dielectric substrate, we can determine the relative permittivity $\varepsilon'_{//}$ for the major axis of the liquid crystal molecules parallel to the RF electric field and the relative permittivity $\varepsilon'_{\perp}$ from the major axis normal to the field, as well as the dielectric birefringence Δε′. Although it is dependent on the microstrip line configuration and viscosity of the liquid crystal, the orientation of the liquid crystal molecules cannot be controlled completely. Therefore, $\varepsilon'_{//}$ becomes smaller than the permittivity of the liquid crystal material. This paper studies the cause. The physical meaning of the effective orientation coefficient ηa is elucidated from photographs indicating imperfections of the liquid crystal molecular orientation and from calculations of the electrostatic energy immediately below the conductor and elsewhere. In a microstrip line device using liquid crystal, the effective value of $\varepsilon'_{//}$ to be used in circuit design should be the value obtained by multiplying ηa by the permittivity of the material itself. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 87(10): 13–24, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.20082