The essential dielectric properties, the basic alignment techniques, and the common measurement methods of the nematic liquid crystal (LC) at RF are briefly reviewed. A new device for the broadband measurement of the dielectric constants and loss tangents of nematic LCs at microwave and millimeter-wave frequencies is presented. This device whose specification and fabrication are outlined is essentially a two dielectric layer microstrip structure with coplanar-waveguide terminals, which is easy to fabricate. Compared to previous structures, the proposed device is extremely broadband with 15-65-GHz bandwidth, benefits from a solid exposed ground plane for easy temperature test, and operates under bias voltage. The technique for the extraction of the dielectric parameters of the nematic LC analyzed by this device is explained and the sources imposing the frequency limits on the device performance are identified. Two different nematic LCs, MDA-00-3506 and GT3-23001, are characterized and the results are shown to compare well with those available in the literature. In the comparisons, the maximum difference found for the dielectric constants for MDA-00-3506 is 5% and for GT3-23001 is 5.3%.
Induced bulk orientation of nematic liquid crystal in contact with micron-scale patterned surfaces is investigated using the Landau-de Gennes theory by means of three-dimensional simulations. The effect of the size and spacing of square cross-sectional well and post patterns is investigated and shown to influence the orientation of the liquid crystal bulk, far removed from the surface. Additionally, the effective anchoring strength of the induced alignment is estimated using a modified version of the torque balance method. Both azimuthal and zenithal multistability are shown to exist within unique ranges of feature sizes.
A patch antenna array with beam steering capability in one dimension for 60 GHz applications is presented. In this array the phase shifters are liquid crystal based meander lines, each providing a maximum differential phase shift of 38°. The array elements (the patch antennas) are coupled to the microstrip feed line through coupling gaps. These gaps are adjusted in order to uniformly excite the antenna patches. The final design comprises of a 4 × 1 patch array, which is able to scan up to 9° in the E-plane as the bias on the liquid crystal changes from zero to saturation voltage.Index Terms -Liquid crystal (LC), phased arrays, millimeter-wave phase shifters.
Liquid crystal substrates have been shown to provide the means to develop low-cost, reconfigurable, adaptive and tuneable microwave and MM-wave devices for mobile and wireless communication systems. However, techniques for the characterisation of materials, device fabrication and design are necessary in order to take maximum advantage of the possibilities that these materials offer. This includes appropriate modelling methods to simulate accurately the switching behaviour of the liquid crystal and the characteristics of the wave propagation through the devices, taking full consideration of the point-by-point variation of the material tensor permittivity. We describe these techniques here and show their application in the design of a meander-line phase shifter.
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