Dr. Senad Bulja, PhD, FIET, SMIEEE https://drbulja.com Mon, 16 Sep 2024 03:59:17 +0000 en-US hourly 1 Characterisation and application of nematic liquid crystals in microwave devices https://drbulja.com/characterisation-and-application-of-nematic-liquid-crystals-in-microwave-devices-1643/ https://drbulja.com/characterisation-and-application-of-nematic-liquid-crystals-in-microwave-devices-1643/#respond Mon, 16 Sep 2024 03:59:17 +0000 https://drbulja.com/?p=1643 The abundance of a widely available spectrum at a frequency band of around 60 GHz (mm-wave region) displays potential to support high data rate, short range wireless communications. This has led to an increased demand for cost effective solutions for the RF front end, such as antennas, phase shifters and filters. Preferably, these mm-wave devices need to be reconfigurable and compact.

Liquid Crystals (LCs) have become attractive substrates for microwave devices. They possess a significant tuneable dielectric constant in the mm-waveband, which can be exploited in compact and reconfigurable devices such as phase shifters and antennas. When designing such devices two main problems are normally encountered. Firstly, the dielectric properties of few LCs have been fully characterised in this waveband. Secondly, design tools fail to account fully for the spatial dependence of the liquid crystal orientation and its effect on the electromagnetic fields. We address the problem of characterisation using a microstrip line fabricated with a layer of liquid crystal as its substrate. Standard microwave substrates are employed resulting in a practical and cost-effective characterisation device. A network analyser is used to measure the scattering parameters prior to and after filling with liquid crystal. Accurate models of the director and microwave fields are then used to set up an inverse problem that allows for the recovery of a number of liquid crystal material properties, including permittivities, loss tangents and elastic constants. Results of the characterisation are presented for a number of liquid crystalline materials.

Fig. 1 Perspective view of the structure of measurement LC cell
Fig. 1 Perspective view of the structure of measurement LC cell
]]> https://drbulja.com/characterisation-and-application-of-nematic-liquid-crystals-in-microwave-devices-1643/feed/ 0 Planar transmission line method for measurement of dielectric constants of liquid crystals in the 60 GHz band https://drbulja.com/planar-transmission-line-method-for-measurement-of-dielectric-constants-of-liquid-crystals-in-the-60-ghz-band-343/ https://drbulja.com/planar-transmission-line-method-for-measurement-of-dielectric-constants-of-liquid-crystals-in-the-60-ghz-band-343/#respond Mon, 06 Feb 2023 08:33:06 +0000 https://drbulja.com/?p=343 The availability of several licensed and unlicensed frequency spectra in the millimeter (mm)-wave frequency range has created new opportunities in the area of high data rate applications. This has led to demands for mm-wave devices, especially high gain antennas, which are adaptive (reconfigurable), compact and cheap to manufacture.

In our previous publications on the matter of Liquid Crystals (LCs), we presented an accurate analytical technique based on the Finite Element Method (FEM), which can be used for the extraction of unknown dielectric properties of anisotropic media, including LCs [1]. In addition, we have also presented how such a technique can be used in the context or well-known LC mixtures, such as K-15.  

In this paper, a new method, a planar transmission line method, for the characterization of nematic LCs in the frequency range of 57- 62 GHz is presented.  The LC cell used for the dielectric characterization consists of a finite length balanced stripline where a layer of LC is sandwiched between the two strips of the line.  Unlike the previous methods where the LC is magnetically biased, one of the novel features of the present characterization is the direct application of an electric field to the LC through the strips using a low voltage source, Fig. 1.  This feature allows gradual variation of the bias voltage, and hence, partial switching of the LC.  With a detailed account of the spatial variation of the dielectric properties through the LC layer, which can be obtained through accurate LC modeling [1] a precise determination of the anisotropic dielectric constants is possible.  In the present measurement device, a new special transition from the balanced stripline to the finite ground coplanar waveguide (FGCPW) is used, Fig. 1.  This transition facilitates the probe-station measurement and eases the application of an external electric field to the LC under test. The LC investigated in this paper is the E7 mixture.

Fig. 1 (Left) Perspective view of the measurement structure and (Right) extracted eff of E7 LC mixture for two different voltages: (a) – 0 Volts and (b) – 9 Volts
Fig. 1 (Left) Perspective view of the measurement structure and (Right) extracted eff of E7 LC mixture for two different voltages: (a) – 0 Volts and (b) – 9 Volts.

[1] R. James, S. E. Day, F. A. Fernandez, S. Bulja, M. Yazdanpanahi and D. Mirshekar-Syahkal, “Finite-element analysis of a balanced microstrip line filled with nematic liquid crystal”, IEEE IMS 2009, Boston, USA, 133-136, June 2009.

 

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