Design And Analysis Of Compact MIMO Antenna For UWB Applications
View metadata, citation and similar papers at core.ac.ukbrought to you byCOREprovided by Al-Quds University Digital RepositoryDesign and Analysis of Compact MIMO Antennafor UWB ApplicationsWatan ZaferMohammad KoualiCommunication Engineering DepartmentAl-Quds UniversityJerusalem, Palestinewatan.zafer@gmail.comCommunication Engineering DepartmentAl-Quds UniversityJerusalem, Palestinemkoali@staff.alquds.eduAbstract— The need to produce light weight and cheapcomponents and devices has been the driving force for macroelectronics. Electronics that can be stretched has recentlyattracted considerable attention. In this work a design ofprinted rectangular monopole antenna for Ultra-Wide Band(UWB) applications is presented. High-frequency structuresimulator (HFSS) is used to design the printed antenna is toachieve the best reflection coefficient. Moreover, the stretchingeffect on the antenna response is studied along x-axis and yaxis. Also, compact UWB Multiple-Input Multiple-Output(MIMO) antennas are proposed. In order to enhanceimpedance matching and improve the isolation, each UWBMIMO antenna which consists of two comparable monopoleelements is proposed with different stubs on the ground plane;the first one is a vertical slotted stub and the other is a groundslotted stub ended with a rectangular loop. The reflectioncoefficient, mutual coupling, peak gain and radiation patternshave been analysed. The obtained results show that theimpedance bandwidth of the antenna is from 3–15 GHz, lessthan -25 dB mutual coupling between the two ports. The size. As a result, it can beof the designed antenna is 32 18noticed that the proposed antenna is appropriate for UWBMIMO systems.Keywords— Monopole antenna, Ultra-Wide band MultipleInput Multiple-Output antenna, stretchable material, MicrostripAntennas.I. INTRODUCTIONIn recent years, the significant attention has focused onwearable system for monitoring human health and detectinghuman motions [1,2]. By Using the wearable wirelesscommunication, we can provide remote diagnosis andtransmit the sensory data. For transmitting and receiving, theantenna is a critical component [3].Rigid antenna fails to work properly when it is undermechanical deformation, so development of stretchable andflexible antenna leads for new device configurations. Silvernanowires (AgNWs) are a promising material for theseantennas and other components including solar cells, andsensors [4].In 2000, the Federal Communication Commission (FCC)issued a license for ultra-wideband communication systemfor bandwidth of (3.1-10.6) GHz. Since that time manymicrowave components have been implemented within theAtallah BalalemCommunication Engineering DepartmentPTUK UniversityTulkarm, Palestineatallahb@yahoo.comspecified bandwidth, these components are: Microwavefilters,couplers and antennas [5]. The UWB antenna is used inmany applications like ground penetrating radar (GPR),military applications, and medical imaging [6].Applications such as target detection, and RFID readersneed high gains and narrow beamwidths, but the existingUWB antennas have a small gains and omnidirectionalradiation patterns [7,8]. The UWB array can be good choiceto achieve good gains and directional radiation patterns.In [9], an UWB MIMO antenna with compact size waspresented. The antenna consists of 2 identical monopoleantennas and the isolation was improved with a comb-linestructure on the ground layer. In [10] the wide bandisolation has been improved by a decoupling structure whichhas been inserted between two antenna elements. Highisolation and dual reject bands can be achieved by theantenna presented in [11]. Using parasitic slots and strips onthe radiator, A dual band-notched characteristic was created.In [12], the antenna with notch-band characteristic had the, but S21 was less than 15compact size of 22 36dB for the wanted band.In this paper, a design of printed rectangular monopolepatch antenna for UWB applications using a stretchable andreversibly deformable material is presented. A stretchableand highly conductive material of AgNWs is used to designthe radiating element and the AgNWs is embedded in thesurface layer of an elastomeric substrate. It’s effectivenessand efficiency are checked under stretching to use inwearable systems like wearable textile technologies,wearable health technologies, and wearable consumerelectronics. The other geometry is a compact MIMO UWBantenna which is introduced using PDMS/AgNWs withdifferent stubs to improve and minimize the mutual couplingbetween elements and achieving compact size.II. ANTENNA DESIGNThe suggested paradigm of the monopole patch UWBantenna is shown in Fig. 1. One antenna element is printedon a 16 mm X 18 mm stretchable Polydimethylsiloxane(PDMS) substrate, with a thickness of 1mm and relativepermittivity of nearly 2.8 and loss tangent ranging from 0.01
to 0.05. According we modelled the substrate material withloss tangent of 0.02. Conductivity of the AgNW/PDMSstretchable conductor is nearly 8130 Sbefore stretchingas in [13]. AgNWs is used for the patch and the groundplates.the ground plane underneath each feeding line. Finally,ground stubs were added on the ground plane of theantenna.Diverse methods have been checked to merge MIMOtechniques with UWB technology trying to slash the mutualcoupling between elements and getting a compact size. Theproposed structures can efficiently improve isolation andenhance the bandwidth of the antenna. The optimizeddimensions for the substrate of the UWB MIMO antenna are32 X 18and all other dimensions are the same like theprevious antenna.The UWB antenna consists of a monopole antenna. Theradiating element has a rectangular shape, and there is asymmetrical staircase structure on the two bottom corners ofthe radiating element.To get the desired frequency, the rectangular patch wasdesigned to be 7 mm X 11 mm, backed with an 18 mm X 5.9mm ground plane. A staircase structure was employed Formatching the input impedance of a 2 mm X 6 mm 50 Ωmicrostrip feed line,. The dimensions of the antenna wereoptimized in ANSYS HFSS v.13 to get the best reflectioncoefficient and improve the coupling between the radiatingelement and the feed line. The optimized antenna’sdimensions are listed in Table 1.III.RESULTS AND DISCUSSIONA. Antenna ElementsThe designed UWB antenna consists of a rectangularpatch element. The monopole patch antenna was designedusing a stretchable material, PDMS for the substrate andAgNWs for the top and ground plates. The optimaldimensions for the structure of the UWB patch antenna wereshown in the previous section.The UWB rectangular monopole patch antenna hasapproximately a lower resonance frequency at [14] GHz (1) Where, ,aare the length of the ground, thelength of the radiating element and the gap between themis about 7 GHz. Fig. 3respectively. The calculatedshows the simulated ׀ ׀ which is less than -10 dB for theUWB operation.Fig. 1 Monopole UWB antennaTABLE 1 Dimensions of the proposed UWB antenna (mm).1618w7L110.519121.15.9The other geometry is a compact UWB MIMO antennawhich shown in Fig. 2. This antenna was designed with thesame material parameters. Two identical patches withdimensions 7 mm X 11 mm were arranged in parallel. Setup more than one radiating elements on the close space, themutual coupling between them can be very large. In thisdesign, the edge-to-edge spacing between the patches is9mm which is nearly equal to the half wavelength for thehighest frequency ( 15 GHz) which is equal to 20mm. Each antenna is feed by a 50 Ω microstrip line. Thedimensions of the ground plane areXandfor improving the impedance matching at high frequencies,a rectangular slot with dimensions ofXis cut onh2Fig. 2 The top view of the UWB MIMO antenna.For input impedance matching improvement andenhancement the bandwidth of the antenna, a staircasestructure was done for each patch and a rectangular slot wascut on the ground plane underneath each feeding line. Bychanging and, the lower cut off frequency could bechanged to reach 6 GHz and ׀ ׀ will be less than -10 dB
from 3.37 GHz to 15.2 GHz which means that the staircaseand the slot in the ground plate have a huge outcome byimproving the input impedance matching as shown in Fig. 4.After these improvements, the stretching was applied onthe antenna along x-axis and y-axis, along its width andlength respectively, the applied stretching was between (015) %. The value of S gives an indication about how thewaves are reflected under stretching and how this affectedunderantenna bandwidth. Fig. 5 represents the value ofstretching along x-axis and Fig. 6 forwith stretchingalong y-axis. In UWB application, it is acceptable when itsvalue is less than -10 dB continuously.Fig. 3 Simulated ׀ SFig. 4 Simulated ׀ SFig. 5 Simulated ׀ S ׀ with stretching along x-axisFig. 6 Simulated ׀ S ׀ with stretching along y-axis ׀ Due to the applied strain, we have a shift at the resonancefrequencies and this change is accounted by changing thedimensions of the antenna as a function of the applied strain.PDMS is a typical hyper-elastic material and underconsideration of constant total volume during deformation[15]. The length and thickness of the antenna shrinkproportionally, when it is stretched along its width and thelower resonance frequencyis determined using equation1. When a strain of s is applied along x-axis, the newdimensions of the antenna, width w, length L, thickness h asthe function of s are [9]: ׀ after doing staircase and ground slotW L h 1 (2)(3)(4)When a strain is applied along y-axis, the width andthickness shrink proportionally. As we can see in Fig. 5 and
Fig. 6, with stretching, the antenna bandwidth was nearlyconstant for each value of s.As shown in Fig. 2, when placing the two antennasclose to each other, the UWB frequency band is covered.Fig. 7 shows the S-parameters S , and S , in dB for theMIMO UWB antenna.S is less than -10 dB and S is less than -15 dB forthe desired frequencies, S is good and S is wanted to beless than -25 dB for UWB applications. So, we need toimprove isolation between antenna elements.Fig. 8 (a) Geometries of MIMO antenna with vertical stub.Fig. 8 (b) Geometries of the stub.Fig. 7 S-Parameters for MIMO antenna.The other structure was a stub which made in the groundlayer as shown in Fig. 10. The mutual coupling for thedesired band was improved, and S was less than -25 dB forthe band of 3.5-10.6 GHz. Shown in Fig. 11.B. Improving of IsolationTo maintain high efficiency, the mutual couplingbetween radiating elements should be minimized [9]. Toreduce it in the UWB band, several structures are used in thepresented MIMO antenna. These structures achieve highisolation because of different mechanisms. One of them,changing the current distribution in the ground plane usingstubs, which will reduce the mutual coupling by capturingthe current towards them. Also, an improvement in theimpedance matching will happen.One of these structures is an AgNWs stub which wasintroduced vertically between the radiating elements,between the substrate of each antenna. Using this stub, themutual coupling was improved for the band of UWB and theisolation was less than -25 dB for the frequency band of 3.9 –11.9 GHz, shown in Fig. 9. Due to the symmetry of theantenna and as S and S are identical to S and S ,respectively, we have only presented S and S .Fig. 9 S-Parameters for MIMO antenna with vertical stub.Initially, we introduced an AgNWs sheet between thesubstrates and then two slots were created to improve themutual coupling, each slot has 7 mm length and 0.5 mm ofthickness as shown in Figure 8 a and b.Fig. 10 Geometries of the ground stub.
Finally, a rectangular loop was added at the end of theground sub, shown in Fig. 12. The proposed loop improvesthe mutual coupling and the isolation for the f band of 3-10.6GHz as shown in Fig. 13.C. Other antenna parameterThe paradigm of the UWB and UWB MIMO antennasdescribed in previous sections have been simulated. The sparameters of the final design are presented. These resultsprove that this antenna specify the specifications of theUWB MIMO operation across the whole band. Radiation PatternsThe simulated radiation patterns of the UWBantenna at the frequencies of 4.5 GHz and 9 GHzin 3D polar plot are shown in Fig. 14 (a) and (b).The radiation pattern for the antenna is nearlyomnidirectional for these frequencies. At 4.5 GHzfrequency, we have less deviation in the radiationpattern but when the frequency is increased thesimilarity in the radiation pattern decreases.Fig. 11 S-Parameters for MIMO antenna with and without a groundstub.Fig. 14 3D radiation pattern for the UWB antenna at (a) 4.5 GHz.Fig. 12 Geometries of the ground stub with a loop.(b) 3D radiation pattern for the UWB antenna at 9 GHz.Fig. 13 S-Parameters for MIMO antenna with and without a rectangularloop.
IV. CONCLUSIONFig. 15 3D radiation pattern for the MIMO UWB antenna with a rectangularloop at (a) 4.5 GHzWe have presented a class of monopole patch UWBantennas which are stretchable. An UWB monopole antennawas simulated and the main antenna parameters weremeasured and analyzed such that S parameters, and radiationproperties of the antenna were characterized under stretchingalong x-axis and y-axis. Also a tight UWB MIMO antennawas proposed and an isolation improvement was made for aUWB application. This improvement was presented by usingdifferent stubs to improve mutual coupling and impedancematching. It is noticed from results that the antenna cancover the UWB band of 3.1–10.6 GHz with good mutualcoupling and isolation in the whole UWB band. Based onthe simulated performance, the MIMO antenna is goodchoice for UWB applications.REFERENCES(b) 3D radiation pattern for the MIMO UWB antenna with a rectangularloop at 9 GHz.3D radiation pattern results of the proposed MIMO UWBantenna with a ground stub ended with a rectangular loop atthe frequencies of 4.5 GHz and 9 GHz in 3D polar plot areshown in Fig. 15 (a) and (b).As we can see, when the MIMO UWB antenna wasintroduced, the radiation pattern became more directiveespecially when the stubs were added to the ground plane.D. Performance ComparisonIn this section, a quick comparison will be done between theproposed antenna and recently designed antennas in[9,10,11,12] on the size, isolation and bandwidth. Table 2shows this comparison. Our antenna achieves good isolationperformance with compact size. Therefore, we can concludethat it is a good choice for UWB MIMO systemapplications.TABLE 2. Performance comparison of the proposed antenna and somereferance antennas.ReferenceSize ()Bandwidth(GHz)Isolation (dB)926 31 0.782.8–12-251035 40 0.83 – 11.6-161127 30 0.83 – 11-201222 36 1.63.1 – 11-15Our Work32 18 13 – 15-25[1] Kim, D. H., Lu, N., Ma, R., Kim, Y. S., Kim, R. H., Wang, S., . & Yu,K. J. (2011). Epidermal electronics. science, 333(6044), 838-843.[2] Lipomi, D. J., Vosgueritchian, M., Tee, B. C., Hellstrom, S. L., Lee, J.A., Fox, C. H., & Bao, Z. (2011). Skin-like pressure and strainsensors based on transparent elastic films of carbon nanotubes. Naturenanotechnology, 6(12), 788.[3] Song, L., Myers, A. C., Adams, J. J., & Zhu, Y. (2014). Stretchable andreversibly deformable radio frequency antennas based on silvernanowires. ACS applied materials & interfaces, 6(6), 4248-4253.[4] Rai, T., Dantes, P., Bahreyni, B., & Kim, W. S. (2013). A stretchableRF antenna with silver nanowires. IEEE Electron DeviceLetters, 34(4), 544-546.[5] US Federal Communications Commission. (2002). Revision of part 15of the commissions rules regarding ultra-wideband transmissionsystems. First Report and Order, ET Docket, 98-153.[6] Oppermann, I., Hämäläinen, M., & Iinatti, J. (Eds.). (2005). UWB:theory and applications. John Wiley & Sons.[7] Ojaroudi, M., Yazdanifard, S., Ojaroudi, N., & Naser-Moghaddasi, M.(2010). Small square monopole antenna with enhanced bandwidth byusing inverted T-shaped slot and conductor-backed plane. IEEETransactions on Antennas and Propagation, 59(2), 670-674.[8] Shambavi, K., Alex, Z. C., & Krishna, T. N. P. (2009). Design andanalysis of high gain millimeter wave microstrip antenna array forwireless applications. Journal of Theoretical and Applied InformationTechnology, 2005-2009.[9] Malekpour, N., & Honarvar, M. A. (2016). Design of high-isolationcompact MIMO antenna for UWB application. Progress InElectromagneticsResearch, 62,119-12.[10] Zhang, S., Z. Ying, J. Xiong, and S. He, “UltrawidebandMIMO/diversity antennas with a tree-like structure to enhancewideband isolation,” IEEE Antennas Wireless Propag. Lett., Vol. 8,1279– 1282, 2009.[11] Li, J. F., Q. X. Chu, Z. H. Li, and X. Xia, “Compact dual band-notchedUWB MIMO antenna with high isolation,” IEEE Trans. AntennasPropag., Vol. 61, 4759–4766, 2013.[12] Liu, L., S. W. Cheung, and T. I. Yuk, “Compact MIMO antenna forportable UWB applications with band-notched characteristic,” IEEETrans. Antennas Propag., Vol. 63, 1917–1924, 2015.[13] Xu, F., & Zhu, Y. (2012). Highly conductive and stretchable silvernanowire conductors. Advanced materials, 24(37), 5117-5122.[14] Thomas, K. G., & Sreenivasan, M. (2009). A simple ultrawidebandplanar rectangular printed antenna with band dispensation. IEEETransactions on Antennas and Propagation, 58(1), 27-34.[15] Xu, F., Durham III, J. W., Wiley, B. J., & Zhu, Y. (2011). Strainrelease assembly of nanowires on stretchable substrates. ACSnano, 5(2),1556-1563.
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