Analysis of Feed Techniques on the Performance of Dual-Broadband MIMO Antenna System Gourav Banchhodiya & L.D. Malviya Deparment Of Electronics And Telecommunication Engineering, S. G. S. Institute Of Tecnology And Science, Indore, India E-mail : gouravbanchhodiya@gmail.com, ldmalviya@rediffmail.com Abstract In this paper various type of different feed technique is proposed which is applicable to a dual-band MIMO antenna element. They can be divided based on power transfer mechanism from feed line to patch. After describing various feeding techniques the paper give a better understanding of the u-shaped slots. The u-shaped slots are introduce to reduce the coupling between the two dual-broadband antenna elements. The bandwidth achieved for dual broadband antennas elements are 1.6 GHz for lower band (1.7 GHz) and 5 GHz for upper band (5.4 GHz). The measured isolation higher than 15 db in lower band and 20 db in upper band with the measured return loss higher than 10dB. The predicted result are compared with the measured data and good agreement is found.. loop, leading to a broadband performance for the lower loop. The opened loop serves as two monopole and higher mode from the outer loop result in a broadband performance for upper band. The proposed MIMO antenna design was carried out by using The simulator HFSS.v11 and the performance of proposed MIMO antenna is validated by measurement. Simulated and measured result show that the proposed MIMO antenna has high port isolation while maintain good impedance matching meanwhile the antenna is compact in size, simple in structure and easy in fabrication. I. INTRODUCTION Increasing demand for high quality and high data rate communication call for the develop-ment of multiantenna system, such as diversity and multi-input multioutput MIMO handset application,however it s big challenge to place multiple antenna within a small and compact space while maintaining good isolation between antenna element. A ring hybrid is used to improve the isolation is improved by using a network and introducing slots on ground. Nowadays, many popular communication system such as the wireless local area network (WLAN) Operate in dual-band, and thus prefer dual band components, hence it is important to Study on improving the isolation between dual-band antennas. This paper proposes a new method to achieve low dualband MIMO Antenna, elements. The low coupling is obtained by introducing two u-shaped slots. The MIMO antenna consist of two dual-broadband antenna element each of which comprises two opened loop such as an outer loop and inner loop. The opened loop act as half Wave dipole and is excited by electromagnetic coupling from inner 108
(3) STEP 4: Calculation of the length extension ( L) (4) STEP 5: Calculation of actual length of patch (L): The actual length as:... (5) Figure 1: A dual-broadband antenna element II. ANTENNA DESIGN 2.1. Physical Parameters Of Antenna STEP 1: Calculation of the width (W): The width of the micro strip patch antenna is given by:... (1) STEP 2: Calculation of the effective dielectric constant ( eff ): Equation gives the effective dielectric constant as: (2) STEP 3: Calculation of the effective length L eff : Equation gives the effective length as: STEP 6: Determine of feed point location (fx, fy, fz) a micro strip feed line is to be used in the design. As shown in figure, the feed point location is given by the co-ordinates s (fx, fy, fz) from the origin. The united feed line patch excited by the lumped port where port the input impedance is 50 for resonant frequency. Hence a hit and tried methods used to locate the feed point, the return loss (R.L.) is compared and that feed point is selected where the return loss is most negative (below-10 db). There exists a point along the length of the patch where the return loss is minimum. 2.2 Dual-Broadband Antenna Element The geometry of the dual band antenna element is shown in figure 1. The dual-broadband antenna elements consist of two opened loop, an inner loop and outer loop. The inner loop is integrated in the front side of a FR4 substrate with thickness 0.8 mm and relative permittivity of 4.4. The total dimension of the substrate is L W = 63 mm 24 mm with rectangular round plane size of Lg Wg = 48 mm 24 mm and connect to a 50 while the outer loop is integrated in the backside of the substrate with ground plan and electromagnetically (EM) coupled to inner loop dipole at a lower frequency for the lower band while the inner loop act as a half wave dipole at higher frequency for lower band. The software used to model and simulate the micro strip patch in HFSS software. HFSS software is full-wave electromagnetic simulator based on the finite 109
element method. It analyses 3D and multilayer structures of general shapes. It has been widely used in the design of MICs, RFICs, patch antennas, wire antennas, and other RF/wireless antennas Table.1: Design parameter of dual-broadband antenna element DESIGN PARAMETER OF DUAL BROAD ANTENNA ELEMENT L g W g L o W o 48 mm 28 mm 15.8 mm 12 mm W l i l o l s W n 1.4 mm 1.4 mm 5 mm 4 mm 2 mm III. TYPE OF FEED TECHNIQUE A. COAXIAL PROB FEED A coaxial inner conductor extends through the ground plane and is connected to the patch conductor of size 63 mm X 24 mm at height 0.8 mm. The probe of radius 0.3 mm provides the impedance control in a similar manner to inserting the feed for an edge-fed patch. Probe feed mechanism is in direct contact with the antenna and most of the feed network is isolated from the patch which provides an efficient feeding and minimizes spurious radiation. However it s more complicated to manufacture. Probe-fed patches have small bandwidth and are difficult to accurately analyse. The probe used to couple power to the patch can generate somewhat high cross-polarized fields if electrically thick substrates are used. Figure 2. Co-Axial feed patch antenna Figure 3: Measured s-parameter ( S11 ) of dual-broadband antenna Table.2 Result of coaxial probe feed PARAMETER FREQUENCY 1.7 GHz S11-30.42 db GAIN 3.45 db EFFCIENCY 70% WIDTH 1.62GHz B. APERTURE COUPLING FEED In basic aperture coupled patch antenna the radiating micro strip patch element is etched on the top of the antenna substrate, and the micro strip feed line is etched on the bottom of the feed substrate. The thickness and dielectric constants of these two substrates may thus be chosen independently to optimize the distinct electrical functions of radiation and circuitry. Although the original prototype antenna used a circular coupling aperture, it was quickly realized that the use of a rectangular slot would improve the coupling, for a given aperture area, due to its increased magnetic polarization. The aperture coupled micro strip antenna involves over a dozen material and dimensional parameters, and we summarize the basic trends with variation of these parameters below: antenna substrate dielectric constant, antenna substrate thickness, micro strip patch length, micro strip patch width, feed substrate dielectric constant, feed substrate thickness, slot length, slot width, feed line width, feed line position relative to slot, position of the patch relative to the slot. 110
C. MICROSTRIP LINE FEED Micro strip line feed is one of the easier methods to fabricate as it is a just conducting strip connecting to the patch and therefore can be consider as extension of patch. It is simple to model and easy to match by controlling the inset position. The disadvantage of this method is that as substrate thickness increases, surface wave and spurious feed radiation increases which limit the bandwidth. Figure 4. Co-Axial feed patch antenna Table 3. Result of aperture coupled feed PARAMETER FREQUENCY 1.7 GHz S11-30.05 db GAIN 3.44 db EFFCIENCY 70% WIDTH 1.56GHz In such technique, the ground plane separates the radiating patch and the micro strip feed line. Coupling between the patch and the feed line is made through a slot or an aperture (usually centred under the patch) in the ground plane hence spurious radiation is minimized. The geometry of aperture coupled antenna is shown in Figure 4 with S11 in Figure 5 and radiation pattern in Fig 6. Here in this slot dimensions are 0.11mm X 0.9 mm and stub length is 1.7 mm. Figure 6. Micro-strip line feed Micro strip line feed is one of the easier methods to fabricate as it is a just conducting strip connecting to the patch and therefore can be consider as extension of patch. It is simple to model and easy to match by controlling the inset position. The disadvantage of this method is that as substrate thickness increases, surface wave and spurious feed radiation increases Table 4.Result of micro-strip line feed PARAMETER LOWER UPPER FREQUENCY 1,7 GHz 5.4 GHz S11-30.12 db -27.43 db GAIN 3.40 db 4 db EFFCIENCY 70% 76% WIDTH 1.6 GHz 4.9 GHz Figure 5: Measured s-parameter ( S11 ) of dual-broadband antenna 111
Figure 7: Measured s-parameter ( S11 ) of dualbroadband antenna V. COMPARISON OF FEEDING TECHNIQUES Table 5. Comparison of feeding techniques PERFORMANCE COMPARISON OF FEEDING TECHNIQUES Specifications Microstrip coupled Aperture Coaxial feed feed Frequency(GHz) 1.7 1.7 1.7 Gain(dB) 3.48 3.45 3.44 Directivity (db) 6.418 6.712 6.2399 Return loss(db) -30.05-30.42-30.12 Efficiency(%) 70% 70% 70% Radiated power (Watts) 0.0538 0.0054 0.0084 VI. DUAL BROAD MIMO ANTENNA SYSTEM The dual-broadband antenna element is used for development of two element MIMO antenna system. In MIMO antenna system the coupling in the lower band need to be reduced. In past few year, a many decoupling techniques to be used to reduce the coupling between in the antenna element, Table 6: Geometric parameter for a dual-broadband antenna system DESIGN PARAMETER OF A DUAL BROAD- MIMO ANTENNA SYSTEM L L g W g dd g l s l g w g c 68 mm 52 mm 52 mm 24 mm 21 mm 5 mm 2 mm 0.4 mm Such as decoupling slots, decoupling line and decoupling circuits. In this paper, we introduce two u-shaped slots. This slots are help to reduce the coupling between two broadband antenna elements for the lower band. The inner slot works at a higher frequency of lower band, and the outer slot operates at a lower frequency. The mechanism for decoupling is that two u-shaped slots block the current flowing form the excited port 2 to the coupled port1. The current flowing into the port 1 is reduced as using of u-shaped slots. The depth of u-shaped slots is determine for the lowest coupling. The optimal length is found to be about a quarterwavelength in the lower band. The inner loop is patch on the front side of a thin substrate, while the outer loop is printed on the backside of the substrate with the ground plane and electromagnetically (EM) coupled to the inner loop. The EM coupling help in the bandwidth enhancement for the lower band. VII. ADUAL-BROAD MIMO ANTENNA SYSTEM WITHOUT U-SLOTS The measured S-parameter are plotted in figure 5, it is observed that the S11 is 17 db in the lower band and upper band. 112
db(s(waveport1,waveport1)) db(s(waveport1,waveport1)) ITSI Transactions on Electrical and Electronics Engineering (ITSI-TEEE) Figure 9: A dual-broadband MIMO antenna system without u-shaped slots The peak gain is about 3 dbi for lower band and 2-6 dbi for upper band. The antenna efficiency is about 70% for lower band and 70-80% for upper band Ansoft Corporation 0.00 XY Plot 1 HFSSDesign1 Figure 10.Geometry for A dual-broadband MIMO antenna system with u-shaped slots Ansoft Corporation 0.00-5.00 XY Plot 2 HFSSDesign1-5.00-10.00-10.00-15.00-20.00-25.00-15.00-20.00-25.00 Curve Info db(s(waveport1,waveport1)) Setup1 : Sw eep1-30.00-35.00 Curve Info db(s(waveport1,waveport1)) Setup1 : Sw eep1-40.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Freq [GHz] Figure 8: Measured S-parameter ( S11 and S21) of the dualbroadband antenna system without u shaped slots VII. A DUAL-BROAD ANTENNA SYST- EM WITH U-SHAPED SLOTS The measured S-parameter are plotted in figure7. It is observed that the S11 is lower than - 10 db in lower band. -30.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 Freq [GHz] Figure11: measured S-parameter of A dual-broadband MIMO antenna system with u-shaped slots VIII. CONCUSION The simulated result shows that performance of Micro strip antenna feeding techniques. Realized antennas with compact size, relatively good radiation characteristics have a wide beam width and low return loss. Based on simulated results in Table.5, coaxial fed having good performance. Coaxial feed was chosen and fabricate for WLAN applications. A dual-broadband MIMO antenna system is developed for WLAN communication handsets. Each of which comprises two opened loops, an outer loop and inner loop. The outer loop act as half-wave dipole and is 113
excited by electromagnetic coupling from inner loop. The inner loop serves as two monopoles. The bandwidth achieved for dual-broadband antenna element is 1.5-2.8 GHz for lower band and 4.7-8.5GHz for upper band. The u shaped slots are introduced to reduce the coupling between the two dual-broadband antenna elements. The measured isolation is higher than 15dB in lower band and 20dB in upper band with the measured return loss of higher than 10dB. IX. REFERENCES [1] Xiang Zhou, Xulin Quan, and RongLin LI, A dual-broadband MIMO antenna system for GSM/UMTS/LTE and WLAN handsets: A feature overview, IEEE antenna and wireless propagation letter, vol.11, 2012 [2] Q.Li, G.Li, W.Lee, D. Mazzarese, B.Cleackx, and Z. Li MIMO techniques in WiMAX and LTE : A feature overview, IEEE Commum. Mag, vol.48, no. 5, pp,86-92 may 2010 [3] Chiu, C.-Y., C.-H. Cheng, R. D. Murch, and C. R. Rowell, Reduction of mutual coupling between closely-packed antenna elements, IEEE Trans. Antennas Propag., Vol. 55, No. 6, 1732 1738, 2007. [4] Abouda, A. A. and S. G. H aggman, Effect of mutual coupling on capacity of MIMO wireless channels in high snr scenario, ProgressIn Electro-magnetics Research, Vol. 65, 27 40, 2006. [5] Ying, Z. and D. Zhang, Study of the mutual coupling, correlation efficiency of two PIFA antennas on a small ground plane, IEEEAntennas Propag. Soc. Int. Symp., Washington, DC, Jul. 2005. [6] Thaysen, J. and K. B. Jakobsen, MIMO channel capacity versus mutual coupling in multi antenna element system, AntennaMeasurem. Techniques Assoc. (AMTA2004), Stone Mountain, GA, Oct. 2005. [7] Wong, K.-L., J.-H. Chou, S.-W. Su, and C.-M. Su, Isolation between GSM/DCS and WLAN antennas in a PDA phone, Microwave Opt. Technol. Let., Vol. 45, No. 4, 347 352, May 20,2005. [8] Su, C.-M., C.-L. Tang, S.-H. Yeh and K.-L. Wong, Optimized isolation between internal antennas for a dual-network wireless device, IEEE Antennas Propag. Soc. Int. Symp., Washington, DC, Jul. 2005. [9] Li, H.-J. and C.-H. Yu, MIMO channel capacity for various polarization combinations, Journal of Electromagnetic Waves and Applications, Vol. 18, No. 3, 301 320, Mar. 2004. [10] Chaloupka, H. J. and X. Wang, Novel approach for diversity and MIMO antennas at small mobile platforms, Personal, Indoorand Mobile Radil Communications. 2004. 15th IEEE InternationalSymposium, Vol. 1, 637 642, Sep. 2004. [11] Hwang, R. B., A broadband CPW-fed T-shaped antenna for wireless communications, IEE Proc. - Microw. Antennas Propag., Vol. 151, No. 6, Dec. 2004. [12] Laheurte, J.-M., L. P. B. Katehi, and G. M. Rebeiz, CPW-fed slot antennas on multilayer dielectric substrates, IEEE Trans.Antennas Propag., Vol. 44, No. 8, 1102 1111, 1996, [13] Tilley, K., X.-D. Wu, and K. Chang, Coplanar waveguide fed coplanar strip dipole antenna, Electron. Lett., Vol. 30, No. 3,176 177, 1994. 114