AbstractThe effective dielectric constant of the top grounded coplanar waveguide with a liquid crystal (LC) superstrate for phase shifting applications is investigated in the frequency range of 30–60 GHz. Two nematic LC mixtures, namely E7 and MDA‐00‐3506, are used as the superstrate. The measurements show that MDA‐00‐3506 offers higher values of phase shift per millimeter than its E7 counterpart. In particular, the MDA‐00‐3506 provides 3.14°/mm, whereas E7 gives 2.79°/mm at 60 GHz. The results of the dielectric constants from measurement and computer modeling are found to agree to within 5%. For the modeling, a comprehensive finite element package predicting the local alignment of LC molecules and effective dielectric constant at different bias voltages and frequencies are used. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1416–1418, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27564</jats:p

Bulja, Senad

Day, Sally E

Fernández, F Aníbal

James, Richard

Mirshekar‐Syahkal, Dariush

Senad Bulja

Dariush Mirshekar‐Syahkal

Richard James

Sally E. Day

F.Aníbal Fernández

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Microwave and Optical Technology Letters

Effective dielectric constant of top grounded coplanar waveguide on liquid crystal superstrate

University of Essex Research Repository

5. H. Arai and K. Kohzu, A bidirectional notch antenna, In: IEEEAntennas and Propagation Society International Symposium, Vol.1, Baltimore, MD, 1996, pp. 42–45.6. H. Arai, K. Kohzu, and T. Mukaiyama, Bi-directional notchantenna with parasitic elements for tunnel booster system, In:IEEE Antennas and Propagation Society International Symposium,Vol. 4, Montreal, Canada, 1997, pp. 2218–2221.7. J.D. Dyson, The equiangular spiral antenna, IRE Trans AntennasPropag 7 (1959), 181–187.8. K. Cho and T. Hori, Bidirectional rod antenna composed of narrowpatches, In: IEEE Antennas and Propagation Society InternationalSymposium, Vol. 1, Seattle, WA, 1994, 174–177.9. C. Phongcharoenpanich, T. Sroysuwan, P. Wounchum, S. Kosulvit,and M. Krairiksh, Theory and experiment of an antenna using a probeexcited rectangular ring, In: IEEE Antennas and Propagation SocietyInternational Symposium, Columbus, OH, Vol. 3, 2003, 737–740.VC 2013 Wiley Periodicals, Inc.EFFECTIVE DIELECTRIC CONSTANT OFTOP GROUNDED COPLANARWAVEGUIDE ON LIQUID CRYSTALSUPERSTRATESenad Bulja,1 Dariush Mirshekar-Syahkal,2 Richard James,3Sally E. Day,3 and F. Anı´bal Fernandez31 Bell Labs Ireland, Blanchardstown Industrial Park, Dublin 15,Ireland2 School of Computer Science and Electronic Engineering,University of Essex, Colchester CO4 3SQ, United Kingdom3Department of Electronic and Electrical Engineering, UniversityCollege London, Torrington Place, London WC1E 7JE, UnitedKingdom; Corresponding author: senad.bulja@alcatel-lucent.comReceived 26 September 2012ABSTRACT: The effective dielectric constant of the top groundedcoplanar waveguide with a liquid crystal (LC) superstrate for phaseshifting applications is investigated in the frequency range of 30–60 GHz.Two nematic LC mixtures, namely E7 and MDA-00-3506, are used as thesuperstrate. The measurements show that MDA-00-3506 offers highervalues of phase shift per millimeter than its E7 counterpart. In particular,the MDA-00-3506 provides 3.14/mm, whereas E7 gives 2.79/mm at 60GHz. The results of the dielectric constants from measurement andcomputer modeling are found to agree to within 5%. For the modeling, acomprehensive finite element package predicting the local alignment ofLC molecules and effective dielectric constant at different bias voltagesand frequencies are used. VC 2013 Wiley Periodicals, Inc. Microwave OptTechnol Lett 55:1416–1418, 2013; View this article online atwileyonlinelibrary.com. DOI 10.1002/mop.27564Key words: CPW; liquid crystal; finite element method; mm-wavedevices1. INTRODUCTIONThe abundance of a widely available spectrum between 30 and60 GHz (in mm-wave region) offers potential for the develop-ment of high data rate, short-range wireless communications [1].Nematic liquid crystals (LCs) are relatively cheap and theirdielectric constants, which have a tensor form, can be controlledby an external electric or magnetic field. This property of ne-matic LCs has already been exploited for the development oftunable antennas [2] and phase shifters [3].The choice of a suitable LC for a particular mm-wave appli-cation depends on many factors, however, by far the most influ-ential one is the dielectric anisotropy, De ¼ ek  e?, as itdirectly influences the tunability of an LC-based device.LC-based mm-wave devices can be developed using variousplanar transmission lines including the microstrip line and copla-nar waveguide (CPW). However, the implementation of an LClayer as a substrate in a microstrip line and integrating this linewith the rest of microstrip lines in a circuit are difficult, whilean LC superstrate can be realized in a part of a CPW circuitwith relative ease.In this article, the variation of the effective dielectric constantof the top grounded CPW loaded with an LC superstrate with fre-quency and LC bias voltage is investigated, and its use in phaseshifting applications is outlined. The frequency range consideredis 30–60 GHz. In the investigation, two different LCs, known asE7 and MDA-00-3506, are used. The measured effective dielec-tric constants are then compared with those obtained from a finiteelement modeling developed by the authors for the analysis anddesign of LC-based planar structures [4].2. MEASUREMENT DEVICE AND TECHNIQUEThe top grounded CPW with the LC cell as its superstrate isshown in Figure 1. For the dielectric characterization purpose, alength of the top grounded CPW with the LC cell (superstrate)is terminated into two short sections of ungrounded CPWs. Infact, the use of CPW electrodes allows for the measurements ofS-parameters using a probe station at different bias voltages sup-plied through a wideband bias tee. The rotation of the LC mole-cules in the LC cell is achieved by applying low frequency andlow voltage across the electrodes of the ungrounded CPW [4],which is in contrast to the magnetic field biasing method usedin Ref. 5.Figure 1 Device structure for measurement of effective dielectricconstant of top grounded CPW with LC superstrate: (a) perspective, (b)top, and (c) cross-section views (not to scale). [Color figure can beviewed in the online issue, which is available at wileyonlinelibrary.com]1416 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 6, June 2013 DOI 10.1002/mopAs shown in Figure 1, all the electrodes of the CPWs arepatterned on a continuous dielectric substrate. The dielectricconstant of the substrate is er1 ¼ 9.8 and its thickness is h1 ¼381 lm. The ungrounded CPW terminations are 50 X, and theelectrode dimensions of the CPWs are G ¼ 450 lm, S ¼ 80lm, and W ¼ 200 lm. The patterned U-shape conductor block,Figure 1, providing the housing for the LC superstrate is copperand is chemically etched to a depth of hc ¼ 80 lm.The lengths of the top grounded CPW and normal CPW sec-tions are LLC ¼ 12,750 lm and Lun ¼ 500 lm, respectively. Thedistance between the CPW center conductor to a CPW ground, S,is the same as the depth of the LC conductor housing, hc, whichensures approximately equal field distribution between the centerconductor and CPW grounds on the dielectric substrate andbetween the center conductor and the top ground plane. The smallpatterned periodical indentations with a spatial period of LS ¼1250 lm evident in Figure 1 along the edges of the U-shapeconductor housing prevent the build up of standing waves in thehousing within the frequency band of interest. The rest of thedimensions shown in Figure 1 are LW ¼ 250 lm, Wind ¼100 lm, Wf ¼ 100 lm, and Lf ¼ 250 lm. Additionally, thediameter of the registration holes is dr ¼ 500 lm, whereas thediameter of the LC filling holes is dLC ¼ 700 lm.To extract the parameters (effective dielectric constant, etc.)of the top grounded CPW with the LC superstrate, Figure 1, theeffects of the two transitions at the two ends of this CPW sec-tion must be accounted for first. This is done by using a modi-fied thru-line two tier de-embedding procedure developed by theauthors in Ref. 6. The extraction of the effective dielectric con-stants of nematic LCs under various voltages is then achievedusing the following matrix expressionT½ meas ¼ T½ tr  T½ LC  T½ revtr (1)where [T]meas is the overall measured transmission matrix of thedevice in Figure 1, [T]tr and [T]revir are the transmission matricesfor the transition in forward and reverse directions, respectively,and [T]LC is the transmission matrix of the CPW with LC super-strate, given byT½ LC ¼ ecLL 00 ecLL 	(2)In Eq. (2), c ¼ aeff þ jbeff is the complex propagation constant,obtained by solving Eq. (1). Considering the LCs used are oflow-loss, the effective dielectric constant, eeff, of the topgrounded CPW with the LC substrate is then found to be:eeff ¼ beffk02p 2(3)where k0 is the free space wavelength.3. MEASUREMENT RESULTSThe device in Figure 1 was fabricated using the standard photo-lithography process. In the fabrication, a commercially availabledielectric substrate, TMM 10i from Rogers Duroid [7], was usedFigure 2 Measured effective dielectric constants of top groundedCPW with E7 superstrate biased at 0 V for four probe positions andresult from modelingFigure 3 Measured effective dielectric constants of top groundedCPW with E7 superstrate biased at 11 V for four probe positions andresult from modelingFigure 4 Measured effective dielectric constants of top groundedCPW with MDA-00-3506 superstrate biased at 0 V for four probe posi-tions and result from modelingFigure 5 Measured effective dielectric constants of top groundedCPW with MDA-00-3506 superstrate biased at 11 V for four probe posi-tions and result from modelingDOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 6, June 2013 1417on which the CPW electrodes were patterned. The internal sur-face of the U-shape ground conductor block housing the LC,Figure 1, and the surface of the dielectric substrate with pat-terned CPW electrodes were spin coated with a thin passivationlayer of polyimide to achieve surface planarization. The polyi-mide covered surfaces were then mechanically rubbed in thedirection of wave propagation in the antiparallel fashion. Thisprocedure ensures anchoring of the LC molecules in the direc-tion of rubbing, defining the so-called ‘‘ground state’’ of the LCin the absence of an external voltage bias. Two devices asshown in Figure 1 were fabricated and each filled with one ofthe LC mixtures, namely, E7 and MDA-00-3506. In the meas-urements, a slow varying bias signal of frequency 200 Hz wasapplied to the LC superstrate through a wideband bias tee. TheS-parameters of the devices were measured at a bias signalamplitudes of 0 and 11 V at a temperature of about 23C (roomtemperature). Due to the uncertainty associated with the landingpositions of the CPW probes on the CPW terminals of the de-vice in the measurements, Figure 1, the measurements wererepeated four times for each device, and the effective dielectricconstants are extracted using Eqs. (1)–(3), Figures 2–5.The results presented in Figures 2–5 are for the two deviceswith the LC biased at 0 V (ground state) and at 11 V (fullyswitched-on state). Examining the results, the following twoobservations are noteworthy. First, there is some influence ofthe uncertainty in the landing position of the CPW probes onthe measured eeff. If it can be assumed that the correct value ofthe measured eeff is the average of all eeff measured for differentlanding positions of the CPW probes, then the maximum errorof just over 1% can be attributed to the probe landing position.The second observation relates to some disagreements betweenthe measured eeff and those obtained by the finite element mod-eling [4] over the concerned frequency range (30–60 GHz). Thefinite element modeling program uses the electrical properties ofthe two LCs obtained from a broadband measurement techniquedeveloped by the authors [4]. These properties are believed tobe accurate to within 1% in the frequency range of 30–60 GHz.Therefore, the maximum error of 5% observed between themeasured and the modeled results is believed to be due toimperfections in the fabrication process.An important parameter in the design of LC-based devicesincluding phase shifters is the amount of differential phase shiftper unit length, DU. Table 1 summarizes this value for theinvestigated LC filled CPW devices across the 30–60 GHzrange. It shows that MDA-00-3506 provides larger phase shift.4. CONCLUSIONThis article investigated the variation of the effective dielectricconstant of the top grounded CPW using an LC superstrate atthe frequency range of 30–60 GHz. The effective dielectric con-stants of two LC filled top grounded CPW are extracted fromthe S-parameters measurements for two different LC mixtures,E7 and MDA-00-3506 at different bias voltages. The modelingand measured results follow each other closely, to within 5% ac-curacy. Based on the extracted values of the effective dielectricconstants, the amount of phase shift per unit length achieved bythe top grounded CPW for each LC mixture was calculated. Itwas found that the top grounded CPW including MDA-00-3506offers a superior phase shifting performance of 3.14/mm com-pared to that of E7, which offers 2.79/mm at 60 GHz.REFERENCES1. Federal Communications Commission, Amendment of parts 2, 15and 97 of the commission’s rules to permit use of radio frequenciesabove 40 GHz for new radio applications, Federal CommunicationsCommission, Washington, DC (1995). Available at ftp://ftp.fcc.gov/pub/Bureaus/Engineering_Technology/Orders/1995/fcc95499.txt.2. N. Martin, P. Laurent, C. Person, P. Gelin, and F. Huret, Sizereduction of a liquid crystal-based, frequency-adjustable patchantenna, In: Proceedings of the 34th European Microwave Confer-ence, Amsterdam, The Netherlands, October 2004, pp. 825–828.3. S. Bulja, D. Mirshekar-Syahkal, M. Yazdanpanahi, R. James, F.A.Fernandez, and S.E. Day, 60 GHz reflection type phase shifterbased on liquid crystal, In: Proceedings of IEEE RWS 2010, NewOrleans, USA, 10–14 January 2010, pp. 697–699.4. S. Bulja, D. Mirshekar-Syahkal, R. James, F.A. Fernandez, andS.E. Day, Measurement of dielectric properties of nematic liquidcrystals at millimetre wavelength, IEEE Trans Microwave TheoryTech 58 (2010), 3493–3501.5. A. Penirschke, C. Damm, P. Scheele, M. Wittek, C. Weil, and R.Jakoby, Broad-band microwave characterization of liquid crystalsusing a temperature-controlled coaxial transmission line, IEEETrans Microwave Theory Tech 53 (2005), 1937–1944.6. S. Bulja and D. Mirshekar-Syahkal, Novel wide-band transitionbetween coplanar waveguide and microstrip line, IEEE TransMicrowave Theory Tech 58 (2010), 1851–1857.7. Available at: http://www.rogerscorporation.com, 2010.VC 2013 Wiley Periodicals, Inc.AN ANNULAR-RING SLOT ANTENNA FORCP OPERATIONAjay Kumar,1 A. K. Gautam,1 and Binod Kr. Kanaujia21 Department of Electronics & Communication Engineering, G.B.Pant Engineering College, Pauri Garhwal, Uttarakhand 246 194,India; Corresponding author: drakgautam@ieee.org2Department of Electronics & Communication Engineering,Ambedkar Institute of Advanced Communication Technologies &Research, Delhi 110031, IndiaReceived 27 September 2012ABSTRACT: An annular-ring slot antenna with pair of slits andtruncated corner for CP operation is proposed. A pair of slits andtruncated corners are used to enhance axial ratio bandwidth. Theproposed antenna is fabricated and simulated to analyze its radiationcharacteristics. Simulated results show a good agreement withexperimental results. The antenna also shows much larger CPbandwidths than earlier reported results. The measured impedancebandwidth and 3-dB axial ratio bandwidth (ARBW) of the proposedantenna are 94.2 and 73.8%, respectively. The proposed antenna showsconsistent return loss and radian pattern for entire frequency band.VC 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1418–1422, 2013; View this article online at wileyonlinelibrary.com. DOI10.1002/mop.27563Key words: annular-ring slot antenna; axial ratio; bandwidth; circularpolarization; microstrip antenna1. INTRODUCTIONMicrostrip antennas can be divided into two basic types bystructure, namely microstrip patch antenna [1–3] and microstripslot antenna [4, 5]. Many communication and sensor systemsTABLE 1 Measured Phase Shift per Length of LC-BasedCPW Superstrate StructureFreq. (GHz) 30 40 50 60DU (/mm) E7 1.689 2.181 2.509 2.795MDA-00–3506 1.843 2.37 2.843 3.1441418 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 6, June 2013 DOI 10.1002/mop

Effective dielectric constant of top grounded coplanar waveguide on liquid crystal superstrate

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