Large Microwave Birefringence Liquid-Crystal Characterization for Phase-Shifter Applications

Abstract

This work is concerned with the improvement of a microwave liquid-crystal phase shifter using a large birefringence nematic liquid crystal. This material is a eutectic mixture of isothiocyanatotolane molecules. Microwave dielectric properties are reported and compared to the data obtained with the 5CB cyanobiphenyl material in the 26 -40 GHz frequency range using a rectangular waveguide. The phase-shifter design consists of a central cavity, where a liquid crystal is inserted, and two coplanar strip lines accesses. Its dimensions were calculated by electromagnetic simulation, using measured dielectric permittivities of the liquid crystal. The measurements were performed with a commercial Wiltron 3680 K probe test fixture. Phase-shift variations with and without bias voltage versus frequency are presented. As expected, the large-birefringence nematic liquid crystal exhibits a higher microwave dielectric anisotropy (Á" 0 ¼ 1:06 against 0.34) and the tunability of the phase shifter strongly increases (1.8 degÁcm À1 ÁGHz À1 against 0.8 degÁcm À1 ÁGHz À1 ).

High Birefringence Liquid Crystals

“…The ability for destabilizing the smectic A phase is strongly related to the position of its substituents. For double substituted compounds, the following relation is observed: 3,3′ (33) < 3,5(32) < 2′,3′ (34).…”

“…They are connected with telecommunication devices needs or nondestructive microwaves apparatus to control goods and people [19], cholesteric lasers [20,21], manufacturing tunable metamaterial [22][23][24][25]. Different devices such as tunable spatial modulators for light and laser beam steering and for holography [26][27][28][29][30][31][32][33], switchable phase shifters [34][35][36], various types of tunable filters which select or remove bands [37][38][39][40], tunable capacitors [41], antennas [42], lenses [43,44] were manufactured, and others such as photonic fiber [45] (see also, for more information, reviewed papers [46,47]). For these purposes, different high birefringence materials from 0.2 to more than 0.5 are now available [48][49][50][51][52], but further development and improvement of their properties is still necessary.…”

Tuning of resonator by control of nematic liquid crystal properties

“…Reconfigurable microwave varactors , filters, phase inverters for phase array antennas are described in [2][3][4] for the first time. It was noticed that the permittivity change r under a change of an external field strength can be used for a phase control which can achieve 12 º/dB.…”

Note: Phase-locked loop with a voltage controlled oscillator based on a liquid crystal cell as variable capacitance

“…[5][6][7][8] However, only a few applications for non-optical applications have been described until now and mainly related to microwave region. [9][10][11][12] As it is well known, the capacitance value for a plane capacitor is given by the following expression:…”

“…The VCO implemented is based on relaxation configuration and may generate a square wave. Thus, the operational 220 k R 3 1 k R 4 1 k R 5 1 k R 6 1 k R 7 50 k R 8 10 k R 9 2 k R 10 2 k R 11 1 k C 2 1 nF amplifier OP 2 , as shown in Figure 2, operates as a comparator, with no negative feedback. Resistors R 1 , R 2 , and R 3 and the capacitance of the TN LC cell determine the frequency of the output signal.…”