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IEEE Access Differential Probe Fed Liquid CrystalBased Frequency Tunable Circular Ring Patch Antenna
Differential Probe Fed Liquid CrystalBased Frequency Tunable Circular Ring Patch Antenna
Xu, Guanghui, Peng, HongLi, Sun, Changli, Lu, JianGang, Zhang, Yaoping, Yin, WenYanHow much do you like this book?
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Volume:
6
Year:
2018
Language:
english
Journal:
IEEE Access
DOI:
10.1109/access.2017.2786870
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SPECIAL SECTION ON MULTIFUNCTION RF COMPONENTS FOR CURRENT AND FUTURE 5G WIRELESS COMMUNICATIONS Received October 25, 2017, accepted December 8, 2017, date of publication December 27, 2017, date of current version February 14, 2018. Digital Object Identifier 10.1109/ACCESS.2017.2786870 Differential Probe Fed Liquid CrystalBased Frequency Tunable Circular Ring Patch Antenna GUANGHUI XU 1 , HONGLI PENG1 , (Member, IEEE), CHANGLI SUN2 , JIANGANG LU2 , YAOPING ZHANG1 , (Fellow, IEEE), AND WENYAN YIN1,3 , (Fellow, IEEE) 1 Key Laboratory of Ministry of Education for Design and Electromagnetic Compatibility of High Speed Electronics Systems, Shanghai Jiao Tong University, Shanghai 200240, China 2 Center for Optoelectronic Materials and Devices, Shanghai Jiao Tong University, Shanghai 200240, China 3 College of Information and Electronic Engineering, Zhejiang University, Hangzhou 310058, China Corresponding author: HongLi Peng (hl.peng@sjtu.edu.cn) This work was supported by the Major National S&T Program of China under Grant 2016ZX03001015005. ABSTRACT A differential probefed liquid crystal (LC)based frequency tunable circularring patch antenna is presented. Besides, cavity model is extended to analyze the LCbased antenna for the differential operation. According to the cavity model, the permittivity and loss tangent of the LC are extracted from the measured differential reflection coefficients. Acceptable agreements among the measurement, simulation, and calculation for differential impedances, reflection coefficients, and radiation patterns are obtained and also validate the extraction method and model. More importantly, the extended cavity model can provide a deep physical insight into the LCbased antenna. INDEX TERMS Circular ring patch antennas, cavity model, nematic liquid crystal, differential probe fed. I. INTRODUCTION In modern wireless communication systems, the frequency reconfigurable antennas have caused considerable attention. There have been some techniques developed in anten; nas to realize frequency tunability, such as the tunable lumped elements [1], [2], micromechanical structures [3] and tunable materials. The lumped and micromechanical devices need to design complicated structures which generate the parasitic effect and lead to radiation pattern degradation. Recently, some voltagecontrolled tunable materials such as (Ba,Sr)TiO3 (BST) [4] and LC [5]–[7] have been considered as antenna substrates to realize the frequency tunability and improve the radiation performance. When the permittivity of tunable material is varied by different bias voltages, the effective antenna size with resonant frequency will be changed. The frequency tunable principle should greatly reduce parasitic effects. Among these materials, LC will be an important potential candidate for the frequency tunable antenna due to no DC power consumption and simple fabrication process [6]. To date, the reported LCbased patch antennas adopted the singleend microstrip feed structure which will lead to a larger planar size and asymmetrical Eplane radiation pattern [5]–[7]. The radiation pattern and gain as key VOLUME 6, 2018 parameters for LCbased frequency tunable antennas in Sband have not been measured [5]–[7]. In addition, permittivity properties of many LC materials are still unclear, especially in the low frequency band and are frequency dependent [8]–[11]. Therefore, it is necessary to extract the LC permittivity for study and analysis of the LCbased antenna at the early stage. For example, Polycarpou et al. [6] showed that the simulated frequency tunable range of 7% disagrees with the measurement range of 4% due to uncertain permittivity of the E7 type LC material. Considering the above problems, a differential probe fed LCbased frequency tunable circular ring patch antenna is proposed in this paper. The differential probe feed exhibits more advantages such as easy 3Dintegration with other differential RF components, compact planar size, symmetrical Eplane radiation pattern and lower crosspolarized field [12]–[16]. Here, the circular ring patch is used due to its smaller size compared with the rectangle and circular patches for a given frequency while operating in the fundamental mode [17]. In measurement, the differential reflection coefficients, radiation patterns and gain for the LCbased antenna are given. In theory, we extend cavity model and derive the 21693536 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 3051 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna mutual, differential impedance and radiation pattern expressions to analyze the LCbased circular ring patch antenna for the differential operation. The expressions for the resonant frequency, permittivity, losses and Qfactor are also shown to extract permittivity and loss tangent of the LC for further investigating the antenna. According to the extracted LC parameters, the measured, simulated and calculated impedances, reflection coefficients and radiation patterns are compared and acceptable agreements also validates the extraction method and model. In this paper, the LCbased antenna is described and the extended cavity model theory is presented in Section II, respectively. Results will be discussed in Section III. The conclusions are drawn in Section IV. network, the DC voltage is input into an SMA connector by using bias tee, and then the electrostatic field will be generated between the patch and GND2 . A. LCFILLED CAVITY LC as the tunable anisotropic material (εLC = [εx , εy , εz ]) is filled into the cavity [11]. For the electricallythin patch antenna, we can neglect the transversal electrostatic field components (Et = 0) and only consider the zdirection one (Ez 6 = 0) [18]. Therefore, the permittivity tensor of LC is just considered the component in the zdirection. In other words, the permittivity component in the zdirection (εz ) is used for the following analysis. When no DC bias voltage is applied between the ground and the patch, the LC directors are almost perpendicular to the zdirection due to the effect of the surface anchoring and the corresponding permittivity in the zdirection is εLC⊥ . If the DC (or 1kHz low frequency AC) bias voltage increases from zero to the saturation case for creating an electrostatic field, the LC directors will turn from perpendicular to parallel direction to the zdirection (the DC bias electric field direction), and the corresponding permittivity component in the zdirection εz will vary from perpendicular permittivity εLC⊥ to parallel permittivity εLC [8]. FIGURE 1. The exploded view and feed network of the differential probe fed LCbased frequency tunable circular ring patch antenna. II. THE LCBASED ANTENNA AND CAVITY MODEL THEORY Fig. 1 shows the exploded view and feed network of the differential probe fed LCbased frequency tunable circular ring patch antenna. The antenna has three layers of RT/duriod 5880 (εr = 2.2, tan δ = 0.0009, thickness t = 0.787 mm) material. Doubleside metallized bottom layer has two holes of diameter 0.9mm for two probes and openings of diameter 2 mm for insulation. Each hole is rounded by eight metalized solid vias of diameter 0.6 mm for interconnection of the doubleside grounds. The middle layer is etched to form a circular cavity extended 2 mm beyond the area of the patch and filled with the E7 type LC from injection hole. The ring patch is printed on the top layer which is inverted and placed on top of the cavity. Two bonding pads on the top layer are connected with the radiated patch by using two metallized vias so that the patch can be fed by the probe. The screw holes of the three layers are used for the fixation. In the feed 3052 FIGURE 2. The simplified structure of the antenna for cavity model analysis. B. EXTENDED CAVITY MODEL OF CIRCULAR RING PATCH ANTENNA A simplified structure of the antenna for cavity model analysis is shown in Fig.2. The patch of the outer radius b and inner radius a is fed by two probes at (d1 , ϕ1 ) and (d2 , ϕ2 ), respectively. According to the existing singleend cavity model theory [17], [18], the electric field under the patch is given by X X Ez = Anm ψnm = Anm Xnm (knm ρ) cos nϕ m,n = X m,n Anm Jn (knm ρ)Yn0 (knm a) − Jn0 (knm a)Yn (knm ρ) m,n × cos nϕ (1) VOLUME 6, 2018 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna where Anm R I ψ ∗ ds jωu0 R z nm . = 2 ∗ ds 2 ψnm ψnm keff − knm (2) In (1) and (2), ψmn is the eigenmode function, Jn is the norder Bessel function, Yn is the norder Bessel function of the second kind, knm is the roots of the characteristic equation (X (knm b) = 0), j is the imaginary unit, µ0 is the vacuum permeability, ω is the angular frequency, keff is the effective wave number [18], Iz is the input current density in the zdirection. The input current ribbon for two ports can be given The radiation fields can be derived as: X Rnm Eθ = jn+1 k0 FE (θ ) k m,n nm J 0 (knm a) × Jn0 (k0 a sin θ ) − Jn0 (k0 b sin θ) n0 J (knm b) ! n 2 X × I cos(n(ϕ − ϕi ))e−jφi Eϕ = −jn+1 ϕ1 − w < ϕ < ϕ1 + w, ϕ2 − w < ϕ < ϕ 2 + w elsewhere. (3) In (3), w is the effective angular width of the zdirected source current, δ(ρ) is the impulse function. For the differential operation, the two probe position satisfies the following relation: d1 = d2 and ϕ2 − ϕ1 = π. (4) According to the impedance definition, the two port mutual (transfer) impedances Zij (ij = 1, 2) can be derived as: X sin(nw) 2 Zij = jωµ0 t (nw) m,n 2 X (k d )X (k d ) cos(n(ϕ − ϕ )) π knm nm i nm j j i · i h 02 2 Jn (knm a) n2 2 2 2ε0n keff − knm (1− k 2 b2 )−(1 − k 2n a2 ) 02 Jn (knm b) nm nm X nRnm cos θ FH (θ ) sin θ knm m,n Jn (k0 a sin θ ) Jn (k0 b sin θ) Jn0 (knm a) × a b J 0 (knm b) !n 2 X × I sin(n(ϕ − ϕi ))e−jφi Iz I δ(ρ − d1 ) + δ(ρ − d2 ) d1 d2 = 0 = Z11 − Z21 − Z12 + Z22 X sin(nw) 2 = jωµ0 t (nw) m,n 2 ε0n keff 2 X (k d )2 [1 − cos(nπ )] π knm nm 1 h 02 2 J n (knm a) 2 − knm (1 − k 2n b2 ) − (1 − 02 Jn (knm b) where k0 is the free space wave number, the input current phase φi is firstly introduced in the radiation pattern expressions for the differential operation. The current phase for the differential operation satisfies φ1 = 0 and φ2 = π. FE (θ ) and FH (θ ) have the same approximate value of 2 when considering the radiated field effects of substrate and the ground plane [17], and Rnm = 2 rnε0n (keff 2 sin(nw)X (k d ) e−jk0 r 2tωµ0 knm nm 1 h 02 Jn (knm a) n2 2 (1 − k 2 b2 ) − (1 − − knm ) 0 2 Jn (knm b) n2 2 a2 knm nm nm i. ) (9) C. RESONANT FREQUENCY, PERMITTIVITY AND LOSSES The resonant frequency of these modes can be written as [18] fnm = cknm √ 2π εe (10) where c is the velocity of light, the effective permittivity [18] (11) In (11), the modified radii (ae and be ) which respectively substitute a and b for considering the fringing fields in this work can be given by i n2 2 a2 ) knm (6) which implies that if the mode order is even (n = 0, 2, 4, . . .), the corresponding mode impedance is zero. The differential operation excites the fundamental TM11 mode but will not excite higher order modes such as TM21 and TM02 mode, etc. Therefore the differential operation has much lower crosspolarization level due to canceling the spurious radiation from high modes. In addition, two symmetrical feed probes lead to a symmetrical radiation field. VOLUME 6, 2018 (8) − 1 2 10t 1 1 . εe = (εLC + 1) + (εLC − 1) 1 + 2 2 be − ae Zdd11 · i=1 (5) where ε0n is 1 for n 6 = 0 and 2 for n =0. Based on the mixedmode theory [19], the differential impedance can be derided as (7) i=1 3 ae = a − t 4 3 and be = b + t. 4 (12) The effective loss δeff which mainly comprises the LC substrate material, copper and radiation losses can be written as [18] 1 δeff = tan δLC + √ t σ π µ0 f 2ωµ0 tPr + h 02 2 Jn (knm a) η0 εr 0 2 (1 − k 2n b2 ) − (1 − Jn (knm b) nm i n2 2 a2 ) knm (13) 3053 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna where√σ is the conductivity of the patch and the ground plane, η0 = µ0 /ε0 and Z π 2 n cos2 θ Jn (k0 a sin θ) Jn (k0 b sin θ ) Jn0 (knm a) 2 Pr = − 2 2 a b Jn0 (knm b) 0 k0 sin θ Jn0 (knm a) 2 0 0 + sin θ Jn (k0 a sin θ ) − Jn (k0 b sin θ ) 0 dθ. Jn (knm b) (14) The relationship among total Qfactor, effective loss and relative bandwidth (BW) can be written as [18] 1 1 Q= = (15) δeff BW where BW = 1ω/ω0 , 1ω is the half power bandwidth of the input impedance magnitude [20]. FIGURE 3. Measured and simulated differential reflection coefficients of the airfilled cavity antenna. III. RESULTS AND DISCUSSION The extended cavity model theory is employed to analyze the LCbased antenna. By using Eqs.1015, the unknown permittivity and loss tangent of LC can be extracted from the measured differential reflection coefficients. According to the extracted LC parameters, the impedances, reflection coefficients and radiation patterns of the antenna can be respectively obtained by HFSS simulation and the cavity model calculation. The differential reference impedance is 100 [21]. The main geometrical parameters are summarized in Table 1. TABLE 1. Geometrical parameters of the antenna. FIGURE 4. Experimental setup for Sparameter (a) and radiation pattern (b). A. EXTRACTION FOR PERMITTIVITY AND LOSS TANGENT OF LC Firstly, in order to obtain the resolution of the extracted method, the measured and simulated differential reflection coefficients of the airfilled cavity antenna are given in Fig.3 [9]. The measured resonant frequency is 4.02 GHz and the corresponding simulated resonant frequency is 3.997 GHz. Their frequency deviation is just 0.023 GHz. The extracted permittivity and loss tangent of the airfilled cavity are 1.013 and 0.0018. Therefore, the extraction resolution of relative permittivity and loss tangent are limited to 0.013 and 0.0018, respectively, which is sufficient to permittivity (εLC > 2) and loss tangent (tan δLC > 0.01) of the E7 LC material [9]. Fig.4 shows the experimental setup of the LCbased antenna for Sparameter and radiation pattern measurements. In Fig.4, the bias tee is used to apply DC voltage from a feed port to the LCfilled cavity. In Fig.4 (b), the differential signal is generated by employing a balun for the radiation pattern measurement. In the Sparameter measurement, the bias tee need be calibrated. In the radiation pattern measurement, 3054 FIGURE 5. Measured differential reflection coefficients of the LCbased antenna under different bias voltages. the insertion loss of both bias tee and balun are considered for reducing their influences. Fig.5 gives the measured differential reflection coefficients of the LCbased antenna under different bias voltages. It is observed that the good impedance match can be obtained in the tunable process. When increasing the bias voltage, the differential reflection coefficients moves from high to low frequency due to the increase of the LC VOLUME 6, 2018 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna FIGURE 6. The extracted permittivity (a) and loss tangent (b) of the E7 LC. FIGURE 7. Normalized differential impedance comparisons for the measurement, simulation and calculation under bias voltages of 0 V (a) and 25 V (b). permittivity. The measured resonant frequency versus bias voltage is shown in Fig.9. The frequency tunable range from 2.532 to 2.427 GHz, the threshold voltage of 3 V and the saturation voltage of 23 V can be obtained. The measured frequency tunable range ratio of 4.2% is close to 4% of the reported work which also used the E7 LC material [6]. According to the measured reflection coefficients, the permittivity and loss tangent of LC under different bias voltages can be extracted by using Eqs.1015, and the results are shown in Fig.6. At bias voltage of 0 V, the LC directors are nearly perpendicular to the zdirection (static electric field direction), and the extracted perpendicular permittivity εLC⊥ and loss tangent tan δLC⊥ are approximate 2.696 and 0.0656, respectively. When increasing the bias voltage to the saturation case, the LC directors will turn to the direction parallel to the zdirection, and the corresponding parallel permittivity εLC of 2.95 and loss tangent tan δLC of 0.0295 can be extracted. It is noted that when the bias voltage increases, the permittivity increases and loss tangent decreases. From above results, we can observe that the permittivity of the E7 type LC has a small tunability range with the large loss tangent. As a result, the frequency tunable range is small. In fact, some LC materials (e.g. GT323001) with lager tunablity range and low loss tangent have been produced by Merck for microwave and millimeterwave applications [22]. However, we can only use the type E7 LC material to model VOLUME 6, 2018 and analyze properties of the LCbased antenna under our laboratory conditions. B. COMPARISONS OF THE MEASURED, SIMULATED AND CALCULATED DIFFERENTIAL IMPEDANCES AND REFLECTION COEFFICIENTS The above extracted permittivity and loss tangent of LC under different bias voltages are introduced into the HFSS simulation and the cavity model calculation, respectively. Fig.7 compares the measured, simulated and calculated normalized differential impedances under bias voltages of 0 V and 25 V. In order to uniformly compare the results, the normalized operation is adopted. The measured and calculated results are very close. The simulated results show smaller frequency deviation and slightly larger reactance value. Similarly, the measured, simulated and calculated differential reflection coefficients under the bias voltages of 0 V, 10 V and 25 V are compared in Fig.8. The measured and calculated results are also very close. The simulated results have smaller frequency deviation. To further analyze the frequency deviation, the resonant frequency variations for the measurement, simulation and calculation are shown in Fig.9. These resonant frequencies versus bias voltage has the same variation trend. 3055 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna FIGURE 8. Measured, simulated and calculated differential reflection coefficients under bias voltages of 0 V, 10 V and 25 V. FIGURE 9. Resonant frequency comparisons for the measurement, simulation and calculation. The maximum resonant frequency difference between the measurement and simulation is only 0.019 GHz. C. RADIATION PERFORMANCE Fig.10 compares the measured, simulated and calculated copolar radiation patterns under the bias voltages of 0 V and 25 V. Pattern differences among calculation, simulation and measurement are acceptable. The calculated Eplane patterns have a slightly wider upper half plane due to an infinite ground size considered in the cavity model [14]. However, the simulation and measurement use a finite ground size. A good performance can be found that the patterns basically maintain the same shape under bias voltages of 0 V and 25 V. This is because the permittivity and loss tangent of the LCfilled cavity uniformly vary under the electrostatic field. Due to a larger inherent loss tangent of the E7 LC in the Sband, the simulated realized gain at resonant frequency under bias voltage of 25 V can achieve the maximum value of 1.2 dBi and the corresponding measured value is 0.1 dBi. Their difference is acceptable when considering tolerance in measurement. Therefore, the LCbased antenna can be used in many lowpower wireless communications. From Eqs.78, when operating in the fundamental TM11 mode, higher order modes such as TM21 and TM02 mode 3056 FIGURE 10. Measured, simulated and calculated radiation patterns: in the Hplane (a) and Eplane (b) at resonant frequency under bias voltage of 0V; in the Hplane (c) and Eplane (d) at resonant frequency under bias voltage of 25V. are not excited and the spurious radiation from high modes is reduced. In addition, the differential operation cancels cross polar components from feed probes [14]. Therefore, lower VOLUME 6, 2018 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna crosspolarization levels can be obtained. The simulated and measured crosspolarization levels are at least 25 dB lower than their peak copolarization levels. IV. CONCLUSION The extended cavity model is presented to analyze the differential probe fed LCbased frequency tunable circular ring patch antenna. The impedance and radiation pattern expressions for the differential operation are derived. Expressions for resonant frequency, permittivity, losses and Qfactor are shown for extracting the permittivity and loss tangent of the LC. According to the extracted LC parameters, simulation and calculation of the antenna can be performed for analyzing properties of the differential impedances, reflection coefficients and radiation patterns under the different bias voltages. Comparisons among measurement, simulation and calculation validates the extraction method and model. Therefore, the cavity model can provide a deep physical insight into the LCbased antenna. Similarly, the cavity model can be also applied to analyze the LCbased rectangular, circular patch antennas and their arrays. It is believed that the LC is an important candidate material for realizing frequency tunability in antennas. REFERENCES [1] A.F. Sheta and S. F. Mahmoud, ‘‘A widely tunable compact patch antenna,’’ IEEE Antennas Wireless Propag. Lett., vol. 7, pp. 40–42, 2008. [2] Y. Cai, Y. J. Guo, and T. S. Bird, ‘‘A frequency reconfigurable printed Yagi–Uda dipole antenna for cognitive radio applications,’’ IEEE Trans. Antennas Propag., vol. 60, no. 6, pp. 2905–2912, Jun. 2012. [3] E. Erdil, K. Topalli, M. Unlu, O. A. Civi, and T. Akin, ‘‘Frequency tunable microstrip patch antenna using RF MEMS technology,’’ IEEE Trans. Antennas Propag., vol. 55, no. 4, pp. 1193–1196, Apr. 2007. [4] Y. Wang et al., ‘‘A frequency reconfigurable microstrip antenna based on (Ba,Sr)TiO3 substrate,’’ IEEE Trans. Antennas Propag., vol. 63, no. 2, pp. 770–775, Feb. 2015. [5] L. Liu and R. J. Langley, ‘‘Liquid crystal tunable microstrip patch antenna,’’ Electron. Lett., vol. 44, no. 20, pp. 1179–1180, Sep. 2008. [6] A. C. Polycarpou, M. A. Christou, and N. C. Papanicolaou, ‘‘Tunable patch antenna printed on a biased nematic liquid crystal cell,’’ IEEE Trans. Antennas Propag., vol. 62, no. 10, pp. 4981–4987, Oct. 2014. [7] N. C. Papanicolaou, M. A. Christou, and A. C. Polycarpou, ‘‘Frequencyagile microstrip patch antenna on a biased liquid crystal substrate,’’ Electron. Lett., vol. 51, no. 3, pp. 202–204, 2015. [8] M. Yazdanpanahi, S. Bulja, D. MirshekarSyahkal, R. James, S. E. Day, and F. A. Fernández, ‘‘Measurement of dielectric constants of nematic liquid crystals at mmwave frequencies using patch resonator,’’ IEEE Trans. Instrum. Meas., vol. 59, no. 12, pp. 3079–3085, Dec. 2010. [9] S. Bulja, D. MirshekarSyahkal, R. James, S. E. Day, and F. A. Fernández, ‘‘Measurement of dielectric properties of nematic liquid crystals at millimeter wavelength,’’ IEEE Trans. Microw. Theory Techn., vol. 58, no. 12, pp. 3493–3501, Dec. 2010. [10] D. E. Schaub and D. R. Oliver, ‘‘A circular patch resonator for the measurement of microwave permittivity of nematic liquid crystal,’’ IEEE Trans. Microw. Theory Techn., vol. 59, no. 7, pp. 1855–1862, Jul. 2011. [11] P. Deo, D. MirshekarSyahkal, L. Seddon, S. E. Day, and F. A. Fernández, ‘‘Microstrip device for broadband (15–65 GHz) measurement of dielectric properties of nematic liquid crystals,’’ IEEE Trans. Microw. Theory Techn., vol. 63, no. 4, pp. 1388–1398, Apr. 2015. [12] Y. P. Zhang, J. J. Wang, Q. Li, and X. J. Li, ‘‘Antennainpackage and transmit–receive switch for singlechip radio transceivers of differential architecture,’’ IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 55, no. 11, pp. 3564–3570, Dec. 2008. [13] Q. Xue, X. Y. Zhang, and C.H. K. Chin, ‘‘A novel differentialfed patch antenna,’’ IEEE Antennas Wireless Propag. Lett., vol. 5, pp. 471–474, Dec. 2006. VOLUME 6, 2018 [14] Y. P. Zhang and J. J. Wang, ‘‘Theory and analysis of differentiallydriven microstrip antennas,’’ IEEE Trans. Antennas Propag., vol. 54, no. 4, pp. 1092–1099, Apr. 2006. [15] H. Li Peng, Z. Tang, Y. P. Zhang, and J. F. Mao, ‘‘Cavity model analysis of a dualprobefeed circular microstrip patch antenna,’’ IEEE Antennas Wireless Propag. Lett., vol. 15, pp. 44–47, Feb. 2016. [16] N. Liu, L. Zhu, X. Zhang, and W.W. Choi, ‘‘A wideband differentialfed dualpolarized microstrip antenna under radiation of dual improved oddorder resonant modes,’’ IEEE Access, vol. 5, pp. 23672–23680, Nov. 2017. [17] J. R. James and P. S. Hall, Handbook of Microstrip Antennas. Stevenage, U.K.: Peregrinus, 1989, pp. 179–181. [18] K. F. Lee and K. M. Luk, Microstrip Patch Antennas. London, U.K: Imperial College Press, 2011. [19] Z. H. Tu and Y. P. Zhang, ‘‘Comparison of three methods for the measurement of the differential impedance of a balanced device [measurements corner],’’ IEEE Antennas Propag. Mag., vol. 55, no. 5, pp. 142–151, Oct. 2013. [20] D. M. Pozar, Microwave Engineering. New York, NY, USA: Wiley, 2005, pp. 275–277. [21] W. R. Eisenstadt, B. Stengel, and B. M. Thompson, Microwave Differential Circuit Design Using Mixed Mode SParameters. Boston, MA, USA: Artech House, 2006, p. 52. [22] S. Bildik, S. Dieter, C. Fritzsch, W. Menzel, and R. Jakoby, ‘‘Reconfigurable folded reflectarray antenna based upon liquid crystal technology,’’ IEEE Trans. Antennas Propag., vol. 63, no. 1, pp. 122–132, Jan. 2015. GUANGHUI XU is currently pursuing the Ph.D. degree with the Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, China. His research interests include liquid crystalbased reconfigurable patch antennas and arrays. HONGLI PENG (M’10) was born in Shangluo, China, in 1966. He received the B.S., M.S., and Ph.D. degrees in electromagnetic field and microwave technique from Xidian University, Xi’an, China, in 1988, 1991, and 2005, respectively. From 1994 to 1999, he was a Senior Researcher and the Project Leader for conformal antenna and its system designing with the National Telemeter Center, Xi’an. In 1999, he joined the ZTE Corporation, Shanghai, China, as a Scientific Researcher and the Project Leader, where he contributed 11 essential patents. Since 2008, he has been an Associate Professor in electromagnetic fields and microwave techniques with the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University. He has authored or coauthored over 65 technical papers and a book on MIMO indoor channel modelling. He holds over 25 patents. His current research interests mainly include the tunable RF and microwave passive circuits research, reconfigurable compact antennas/array analysis and design, and spatial wireless channel modeling. CHANGLI SUN is currently pursuing the Ph.D. degree with the Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, China. Her research interests include liquid crystal technologies and liquid crystal display modes. 3057 G. Xu et al.: Differential Probe Fed LCBased Frequency Tunable Circular Ring Patch Antenna JIANGANG LU received the Ph.D. degree from the College of Information Science and Engineering, Zhejiang University, Hangzhou, China, in 2003. He focused on LC displays with the NextGeneration LCD Research Center, LCD business, Samsung Electronics, from 2003 to 2009. Since 2009, he has been an Associate Professor with the National Engineering Laboratory for TFTLCD Materials and Technologies, Shanghai Jiao Tong University, Shanghai, China. His research interests include liquid crystal materials, polymer materials, liquid crystal display modes, and 3D displays. YAOPING ZHANG (M’03–SM’07–F’10) received the B.E. and M.E. degrees in electronic engineering from the Taiyuan Polytechnic Institute and the Shanxi Mining Institute, Taiyuan University of Technology, China, in 1982 and 1987, respectively, and the Ph.D. degree in electronic engineering from The Chinese University of Hong Kong, Hong Kong, in 1995. He was with Shanxi Electronic Industry Bureau, China, from 1982 to 1984; with the Shanxi Mining Institute from 1987 to 1990; with the University of Liverpool, Liverpool, U.K., from 1990 to 1992; and also with the City University of Hong Kong, Hong Kong, from 1996 to 1997. He was promoted to a Full Professor with the Taiyuan University of Technology in 1996. He was with The University of Hong Kong from 1997 to 1998. He is currently a Professor with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore. He has delivered scores of invited papers/keynote addresses at international scientific conferences. He has organized or chaired dozens of technical sessions of international symposia. His current research interests include radio science and technology. Dr. Zhang received the SinoBritish Technical Collaboration Award in 1990 for his contribution to the advancement of subsurface radio science and technology, the Best Paper Award from the Second International Symposium on Communication Systems, Networks and Digital Signal Processing, Bournemouth, U.K., in 2000, the Best Paper Prize from the Third IEEE International Workshop on Antenna Technology, Cambridge, U.K., in 2007, the William Mong Visiting Fellowship from The University of Hong Kong in 2005, and the IEEE APS Schelkunoff Prize 2012. He was a Guest Editor of the International Journal of RF and Microwave ComputerAided Engineering and an Associate Editor of the International Journal of Microwave Science and Technology. He serves as an Editor for ETRI Journal and an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION and the International Journal of Electromagnetic Waves and Applications. He serves on the editorial boards of a large number of journals, including the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES and the IEEE Microwave and Wireless Components Letters. 3058 WENYAN YIN (M’99–SM’01–F’13) received the M.Sc. degree in electromagnetic field and microwave technique from Xidian University, Xi’an, China, in 1989, and the Ph.D. degree in electrical engineering from Xi’an Jiao Tong University, Xi’an, in 1994. He was an Associate Professor with the Department of Electronic Engineering, Northwestern Polytechnic University, Xi’an, from 1993 to 1996. From 1996 to 1998, he was a Research Fellow with the Department of Electrical Engineering, Duisburg University, Duisburg, Germany. Since 1998, he has been a Research Fellow with the Monolithic Microwave Integrated Circuit Modeling and Package Laboratory, Department of Electrical Engineering, National University of Singapore (NUS), Singapore . In 2002, he joined Temasek Laboratories, NUS, as a Research Scientist and the Project Leader of highpower microwave and ultrawideband electromagnetic compatibility (EMC)/electromagnetic interference. Since 2005, he has been a Professor of electromagnetic fields and microwave techniques with the School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai, China, where he is currently the Director and the Adjunct Ph.D. Candidate Supervisor with the Center for Microwave and RF Technologies. In 2009, he joined the National State Key Laboratory of Modern Optical Instrumentation, Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou, China, as a Qiu Shi Chair Professor. As the Lead Author, he has authored over 230 international journal articles (more than 100 IEEE papers), including one international book and 17 book chapters. His current research interests include passive and active RF and millimeterwave device and circuit modeling, ultrawideband interconnects and signal integrity, nanoelectronics, EMC and electromagnetic protection of communication platforms, and computational multiphysics and its application. Dr. Yin received the Science and Technology Promotion Award of the First Class from the Local Shanghai Government of China in 2005, the National Technology Invention Award of the Second Class from the Chinese Government in 2008, the National Technology Progress Award of the Second Class from the Chinese Government in 2012, and the best paper awards from the Asia–Pacific Symposium on Electromagnetic Compatibility in 2008 and 2012. He was also the Technical Chair of EDAPS in 2006. He was an IEEE EMC Society Distinguished Lecturer from 2011 to 2012, and the General CoChair of the IEEE Electrical Design of Advanced Packaging and Systems Symposium (EDAPS) in 2011, technically sponsored by the IEEE Components, Packaging and Manufacturing Technology Committee. He has been an Associate Editor of the IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY and the International Journal of Numerical Modeling: Electronic Networks, Devices and Fields since 2011. He is a reviewer for many international journals, including eight IEEE Transactions and IEEE Letters. VOLUME 6, 2018