Design Approach of a Wideband Frequency Tunable Triangular Patch Array with Concurrent Polarization Alteration

Design Approach of a Wideband Frequency Tunable Triangular Patch Array with Concurrent Polarization Alteration Biswajit Dwivedy 1 and Santanu Kumar Behera 2 Department of Electronics and Communication Engg., National Institute of Technology Rourkela, India Email: 1 biswajit.dwivedy.in@ieee.org 2 skbehera@ieee.org Abstract A wideband frequency tunable triangular patch array with simultaneous polarization reconfiguring ability is designed and its performance is analysed. The antenna constitutes of three probe fed equilateral triangular patches on which narrow annular channels are created to place six varactors for continuous tuning purpose. All the triangular patches operate in dominant TM 01 mode and provide circularly polarized radiation when fed with equal amplitude and 120 sequential phase difference. The resonant frequency of the antenna can be varied from 1.93 GHz to 2.52 GHz by varing the capacitance of six varactors from 4.27 pf to 1.15 pf, operating in reverse bias mode. The antenna shows overall tuning range of 27%. Along with frequency agility the antenna array can also be operated in both circularly polarized mode (RHCP and LHCP) at all operating frequencies. To achieve polarization reconfigurability at different frequencies a wideband tri phase shift feed network (TFN) providing equal amplitude and 0, 120, 240 phase difference is also designed. The antenna possesses stable radiation pattern and wide beam-width axial ratio at all operational bands. Index Terms Frequency reconfigurable, polarization, triangular patch, varactor, wideband, axial ratio. I. INTRODUCTION In current trend multifunctional antennas have become main point of attraction for researchers among which frequency tunable antennas have special importance due to their ability to overcome electromagnetic spectrum congestion. The techniques of achieving frequency reconfigurability are shown in various communications where PIN diodes, MEMS, RF switches and varactors are mainly used to control the current distribution of the radiator [1 5]. The most common approach is to use PIN diodes as switch to change the effective length of the radiator due to their low cost and abundance availability in market in various packages [1], [2]. The main issue of poor Q-factor and high power loss in PIN diodes, sometimes limit their applications. MEMS provide better flexibility over PIN diodes in terms of lower insertion loss, high quality factor, high linearity etc. but suffers from high control voltage and large physical size problems [3]. In so many cases varactor diodes are widely employed to alter frequency bands by continuous tuning of capacitance and simplify the circuit complexity [4], [5]. Polarization agile antennas play vital role in communication as they can withstand fading caused by multipath propagation in urban areas, increase information content in SAR systems and many more. In [6] a reconfigurable circularly polarized antenna with conical beam radiation was reported where two CPs were occupying different and very narrow operational bandwidth (1.6% for LHCP, 2% for RHCP). An E-shaped microstrip antenna was demonstrated where alteration between two types of circular polarization (RHCP and LHCP) was successfully accomplished by using RF switching elements [7]. In this context, design of antennas having multiple reconfiguring capabilities (both frequency and polarization) to efficiently use the spectrum with enhanced signal quality is drawing attention and becoming the prime objective for researchers. In [8], a circularly polarized slot antenna with frequency agility is reported where the resonant frequency of the antenna can be tuned between 1.58 GHz and 2.59 GHz by varying the capacitance of four capacitors placed along the circumference of the ring slot. Similarly a wideband frequency tunable concentric circular microstrip patch antenna with simultaneous polarization agility is described in [9]. In this communication an approach to realized wideband frequency reconfigurability as well as circular polarization agility at different frequencies using a three element triangular patch array is presented. Section II of this paper presents design of the triangular patch and necessary modifications carried out to have wideband frequency reconfigurability along with circular polarization agility. In Section III design and performance analysis of the tri-feed phase shifting network for sequential rotation is presented. CP performance at different operating frequencies of the antenna using the feed network are also given in this section. Finally Section IV concludes the paper. II. DESIGN OF THE TRIANGULAR PATCH ARRAY A. Design of the Triangular Patch Design of wideband frequency and polarization tunable patch array starts with design of a simple equilateral triangular patch using a 62 mil thick FR-4 (ɛ r =4.4) substrate for resonant frequency of 2.4 GHz. The triangular patch is designed to radiate at its fundamental TM 10 mode which is linearly polarized. It is previously demonstrated that circular polarization can be achieved in the broadside direction of any array composed of linearly polarized elements placed with proper angle and fed with same phase difference [10]. Therefore the triangle is allowed to resonate at linearly polarized mode so that three patches arranged in 120 angle with each other can 978-1-5090-4442-9/17/$31.00 c 2017 IEEE 1033

TABLE I DIMENSIONAL DETAILS OF THE TRIANGULAR PATCH ANTENNA. Parameters Values (mm) Parameters Values (mm) R 68 Offset 5 L 43 d 36.1 Fig. 1. Layout of the triangular patch antenna. Surface current distribution of the triangular patch at 2.05 GHz. radiate CP wave when sequentially fed with 0, 120 and 240 phase difference. f mn = 2c 3aε 0.5 r (m 2 + n 2 + mn) 0.5 (1) a e = a + h(ε r ) 1/2 (2) ε r = (ε r +1) + (ε ( r 1) 1+ 12h ) 1/2. (3) 2 2 a Using (1) (3) side length of the equilateral triangle resonating at 2.4 GHz is found to be 43 mm where m, n are the numbers of half cycle variations in different directions, h is height of substrate, ε r is dielectric constant of the substrate and a is the length of the equilateral triangle [11]. Generally the input impedance of the triangular patch is maximum at its vertex, minimum at the centre and again increases while moving towards the side facing the vertex. For better matching a point at a distance of d from the upper vertex of the patch is chosen as feed point where the input impedance is 50 Ω. Due to this shifting of the feed point for impedance match the resonant frequency of the antenna is also shifted to 2.05 GHz. The complete layout of the single element antenna designed using commercial EM-Simulator ANSYS HFSS-15 is shown in Fig. 1 and the dimensional details are presented in Table I. The reflection coefficient plot of the patch is presented in Fig. 2 from which it is clear that the antenna resonates at 2.05 GHz. In Fig. 1, surface current distribution of the antenna at 2.05 GHz clearly indicates TM 10 mode of radiation. The antenna radiation pattern at 2.05 GHz is shown in Fig. 3. From the plot it is clear that the copolarization component is about 20 db higher than crosspolarization component within a wide beam width of ±30 and 15 db higher within ±50. It has a monopole type radiation pattern with a realized gain of 0.55 db in the bore sight direction. B. Modifications Towards Frequency Reconfigurability The prime technique used behind achieving frequency reconfigurability is, insertion of some tunable components on antenna structure to change the operating frequencies without Fig. 2. Reflection coefficient of the triangular patch antenna. Fig. 3. Radiation pattern of the triangular patch antenna at 2.05 GHz 3D pattern 2D pattern showing Co and X-pol. components. disturbing the fundamental mode of radiation. In this scenario an annular slot is etched out from triangular antenna to place a pair of varactor diodes, which can be operated in reverse biased mode for capacitance variation. The position of the slot from the upper vertex is carefully chosen to be 34 mm by parametric analysis to keep antenna mode of radiation unchanged. SMV 2025-079LF varactor diode from Skywork solutions is selected for this purpose as it has a very low series resistance of 0.8 Ω and wide capacitance variation of 1.15 pf to 8.81 pf in reverse biased mode. The varactor diode in reverse biased mode is modelled as a lumped element RLC circuit by following the manufacturer s data-sheet and used for simulation purpose. The anode of the varactor diode is connected to lower side of the triangular patch while cathode is connected to the upper side and two 10 nh inductors are used to isolate the DC from RF signals. The complete antenna structure after insertion of lumped components and the circuit equivalent of the varactor model are shown in Fig. 4. Fig. 5 shows the reflection coefficients of the antenna for variation of capacitance of varactors. From the plot, it is clear that the antenna resonant frequency reduces from 2.52 GHz to 1.93 GHz as we increase the values of capacitance from 1034

Fig. 4. Frequency reconfigurable triangular patch. Lumped element equivqlent of varactor diode. Fig. 6. 2D radiation pattern of the frequency tunable triangular patch antenna at 1.94 GHz 2.51 GHz. Fig. 7. H-field distribution of the array at 1.93 GHz showing LHCP. Fig. 5. Reflection coefficients of the antenna for different values of capacitance. TABLE II DIFFERENT RESONANT FREQUENCIES AND CORRESPONDING IMPEDANCE BANDWIDTHS. Frequency (GHz) Band (GHz GHz) BW (%) Frequency (GHz) Band (GHz GHz) BW (%) 1.93 1.92 1.94 1.04 2.34 2.32 2.36 1.7 1.97 1.95 1.98 1.52 2.38 2.36 2.40 1.68 1.99 1.98 2.0 1.01 2.42 2.40 2.45 2.1 2.07 2.08 2.12 2.0 2.45 2.43 2.48 2.0 2.20 2.18 2.22 1.8 2.48 2.46 2.5 1.6 2.24 2.22 2.26 1.8 2.52 2.5 2.54 1.6 2.29 2.27 2.30 1.3 1.15 pf to 4.27 pf. The list of all resonant frequencies with their corresponding impedance bandwidth are summarized in Table II. The antenna has overall tuning range of 27% covering all resonant frequencies. C. Radiation Characteristics of the Single Element Antenna at Different Frequencies Radiation patterns of the antenna at two distinct resonant frequencies (lowest and highest) are shown in Fig. 6. From the radiation patterns it is evident that the antenna has a very stable monopole type radiation patterns at the two operating frequencies of 1.94 GHz and 2.51 GHz. The difference between co-polarization and cross polarization components are more than 20 db at the two bands within a wide beam width of ±50. The antenna shows an overall directivity 5.9 dbi and 5.84 dbi, realized gain of 5.4 db and 0.7 db at the two resonant frequencies 1.93 GHz and 2.52 GHz respectively. From the simulation, it is clear that the monopole type radiation pattern and other parameters of the antenna remain unaltered at all other resonant frequencies which are not presented due to the limitation of the paper. III. SEQUENTIAL ARRANGEMENT FOR CIRCULAR POLARIZATION AGILITY One of the most efficient method of achieving circular polarization is realization of an array composed of linearly polarized elements placed with proper angle and fed with same phase difference [10]. Therefore the triangular patch array is created by symmetrically arranging three single element antennas at 120 with each other so that there is proper isolation between each element and there is no change in reflection coefficient of each port. The antenna is fed at its three ports with 0, 120, 240 phase difference for LHCP and 0, 120, 240 for RHCP radiation at each operating frequencies for simultaneous frequency and polarization reconfigurability. To clearly visualize circular polarization characteristic, H-field distribution of the antenna at 1.93 GHz and 2.52 GHz are presented in Figs. 7 and 8. At 1.93 GHz the antenna shows 1035

Fig. 10. S-parameters of the tri phase shift feed network. Fig. 8. H-field distribution of the array at 2.52 GHz showing RHCP. Fig. 11. Phase response of the tri phase shift feed network. Fig. 9. Layout of the tri phase shift feed network (TFN). LHCP where as at 2.52 GHz RHCP is observed by reversing the orientation of input phase angles. A. Design and Performance of the Tri-Feed Network for Sequential Rotation To feed the antenna array using sequential rotation at all reconfigured frequency states a wide-band tri-phase shifting network is designed. Basically the feed network is meant to provide 0, 120 and 240 phase shift with equal amplitude within 1.75 GHz to 2.75 GHz. The layout of the tri feed phase shifter is shown in Fig. 9. It basically consists of three Wilkinson s power dividers and three coupled line based 120 phase shifters. Wilkinson s power divider consists of two quarter wave transmission lines of impedance 70.70 Ω, one 100 Ω isolation resistance and has an impedance bandwidth of 1.44:1 with isolation greater than 15 db. The wideband 120 phase shifter consists of two coupled line sections of lengths λ g /3 and λ g /6. The whole tri feed network is optimized using FEM based commercial EM simulator ANSYS HFSS and lengths of two coupled lines are found equal to 29 mm, 18 mm having same width of 2.7 mm. The gap g and width of the high impedance connection line is fixed to be 0.2 mm. The simulation response of the phase shifter are presented in Figs. 10 and 11. From the Plot it is clear that the device has a wide 10 db impedance bandwidth more than 45% with transmission coefficient value varying within 7.6 db to 8.1 db through out the whole band. The port-2 and port-3 Fig. 12. Circular polarized radiation pattern of the antenna using the feed network at 2.52 GHz LHCP RHCP. of the device provide 120 and 240 degree phase difference respectively considering port-5 as reference (0 ). The port-4 of the device acts as an extra port providing 240 phase shift and should be match terminated with 50 Ω load for this particular application. The phase shifter can be directly connected to the antenna array to obtain one state of circular polarization (LHCP) while port-3 and port-2 of the feed network can be interchanged to produce RHCP radiation. B. Radiation Performance Analysis of the Antenna for Polarization Agility Using the Feed Network The antenna array and the tri phase feed network designed using ANSYS HFSS are combinedly simulated in ANSYS Designer to check circular polarization agile characteristics of the complete system at different operational frequency bands. It is verified that the feed network has negligible impact on antenna frequency reconfiguring performance as it possesses wideband phase shifting characteristics with minor mismatch less than 0.15 db between the output amplitudes. The radiation performance of the antenna for both the cases of circular polarization (LHCP and RHCP) are presented in Figs. 12 to 1036

Axial ratio beam width of the antenna at 2.52 GHz LHCP Axial ratio beam width of the antenna at 1.93 GHz LHCP Fig. 13. RHCP. Fig. 15. RHCP. Fig. 14. Circular polarized radiation pattern of the antenna using the feed network at 1.93 GHz LHCP RHCP. Fig. 16. Realized gain of the circularly polarized antenna for different capacitance values. 15. Intentionally responses only at two distinct frequencies (upper 2.52 GHz, lower 1.93 GHz) are shown due to the limitation of the paper length. From Fig. 12 it is evident that antenna co-pol. component is 15 db higher than the crosspol. component within ±30 beam-width for both LHCP and RHCP radiation at 2.52 GHz. Similarly Fig. 13 says that within ±20 of θ variation at both the planes (φ =0, 90 ) the axial ratio value is less than 3 db. Similar type of interpretation can be made for both LHCP and RHCP radiation at 1.93 GHz from plots (Figs. 14 to 15). Fig. 16 focuses on realized gain of the antenna in circularly polarized (LHCP) mode for different values of capacitances. From the plot it is clear that the realized gain of the antenna decreases from 2.55 db to 3.2 db as the reverse capacitance increases from 1.15 pf to 4.27 pf. This decrement in realized gain is mainly due to reduction of effective aperture of antenna with decrease in operating frequency. Another reason is also increase in losses in varactor diodes due to high capacitance values at low reverse bias voltage. IV. CONCLUSION A distinct approach for simultaneous realization of wideband frequency tunability and circular polarization alteration is presented. The antenna shows wide range frequency tunability from 1.93 GHz to 2.52 GHz where the radiation patterns remain unaltered at each operational frequency. The antenna can be operated in both LHCP and RHCP mode using the designed tri-phase shift feed network. Due to the sequential feeding technique, antenna shows axial ratio far less than 3 db within a wide beam width more than ±20 at all reconfigured frequencies. Overall the antenna frequency reconfigurability can help in overcoming the modern challenges and issues arising due to congestion of electromagnetic spectrum by covering various (DCS-1900, UMTS-2000, LTE-2300, LTE- 2500, WiMAX, Bluetooth) wireless standards. At the same time the device has ability to withstand the concerns regarding signal quality degradation by simultaneous alteration of its polarization states at the above mentioned frequencies. REFERENCES [1] H.A. Majid, M.K.A. Rahim, M.R. Hamid, N.A. Murad, and M.F. Ismail, Frequency-reconfigurable microstrip patch-slot antenna, IEEE Antennas Wireless Propag. Lett., vol. 12, pp. 218 220, 2013. [2] L. Ge and K.M. Luk, A band-reconfigurable antenna based on directed dipole, IEEE Trans. Antennas Propag., vol. 62, no. 1, pp. 64 71, Jan. 2014. [3] K.R. Boyle and P.G. Steeneken, A five-band reconfigurable pifa for mobile phones, IEEE Trans. Antennas Propag., vol. 55, no. 11, pp. 3300 3309, Nov. 2007. [4] J.S. Row and J.F. Tsai, Frequency-reconfigurable microstrip patch antennas with circular polarization, IEEE Trans. Antennas Propag., vol. 13, pp. 1112 1115, 2014. [5] A. Khidre, F. Yang, and A.Z. Elsherbeni, A patch antenna with a varactor-loaded slot for reconfigurable dual-band operation, IEEE Trans. Antennas Propag., vol. 63, no. 2, pp. 755 760, Feb. 2015. [6] J.S. Row and M.C. Chan, Reconfigurable circularly-polarized patch antenna with conical beam, IEEE Trans. Antennas Propag., vol. 58, no. 8, pp. 2753 2757, Aug. 2010. [7] A. Khidre, K.F. Lee, F. Yang, and A.Z. Elsherbeni, Circular polarization reconfigurable wideband e-shaped patch antenna for wireless applications, IEEE Trans. Antennas Propag., vol. 61, no. 2, pp. 960 964, Feb. 2013. 1037

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