Microstrip Patch Antenna Assisted Compact Dual Band Planar Crossover

electronics

Article Microstrip Patch Antenna Assisted Compact Dual Band Planar Crossover

Sreedevi K. Menon ID

Department of Electronics and Communication Engineering, Amrita University, Amritapuri 690525, India; [email protected]

Received: 13 September 2017; Accepted: 28 September 2017; Published: 30 September 2017

Abstract: In Microwave Monolithic Integrated Circuits, crossovers maintain signal purity when transmission lines overlap with each other. A simple crossover for dual band applications, particularly suitable for the development of smart antennas, is presented in this paper. Derived from conventional patch antenna, the proposed crossovers are easy to design and fabricate, thus reducing the overall complexity. Design is veriﬁed for a dual band crossover at 2.4/5.23 GHz on FR4 (Fiberglass Reinforced) epoxy and tested using Keysight E5080 A Vector Network Analyser. The results obtained by simulation and measurement are in agreement. The proposed crossovers ﬁnd application in a Butler matrix for phased array and smart antenna systems.

Keywords: butler matrix; crossover; reconﬁgurable patch; microstrip patch; dual band

1. Introduction Microstrip patch antennas continue to gather attention for communication systems, due to their affordability, compactness, light weight, and simple methods of fabrication [1]. Patch antennas provide dual-frequency operation, feed line flexibility and frequency agility. They can be modified for beam scanning omnidirectional patterning, dual and circular polarizations and broad bandwidth [1]. These characteristics are particularly suitable for the design of reconfigurable antennas. Patch antennas has been reconfigured for frequency [2], radiation pattern [3] and so on. Different methods to analyse the performance of this class of antennas are also available in the literature [1–4]. In this work a reconfigured microstrip patch antenna for crossover applications is proposed. Crossovers are used to avoid signal interference at the intersection of two transmission lines, usually in phased arrays and smart antennas. The high capacity in wireless networks employing smart antenna systems is accomplished by orienting the radiation in the desired direction. Smart antennas employ an array of antennas for radiation and can be deployed as a switched beam system or an adaptive array system [5]. In this, the switched beam antenna can be realized easily at low cost in comparison with an adaptive antenna array. A switched beam antenna requires radiating elements, a beamforming network and Radio Frequency (RF) switches [6] while adaptive array systems are much more complex, requiring advanced signal processing [7] techniques. Butler matrix is one of the common method used in switched beam system has 90◦ hybrid coupler, delay lines and crossover associated with it [8]. Generally available crossovers can be classified into three different types. In the first type, the signal path is provided as an air-bridge [9] or bond wires [10]. Air wedge is realized using microelectromechanical systems (MEMS) processing and bond wires, increasing the cost and complexity of fabrication. The second kind adopts the multilayer configuration, where the signals are transmitted between different layers by microstrip–coplanar waveguide–microstrip transitions [11,12] and microstrip–slotline–microstrip transitions. Since double side alignment is required for these crossovers they become more complex and it is also inconvenient to integrate them with other circuits and components. The third kind is the recently introduced planar crossover. Cascaded two section

Electronics 2017, 6, 74; doi:10.3390/electronics6040074 www.mdpi.com/journal/electronics Electronics 2017,, 6,, 74 22 of of 8 8 section branchline couplers were first introduced as planar crossovers providing an isolation of about branchline couplers were first introduced as planar crossovers providing an isolation of about 20 dB [13]. 20 dB [13]. There after, many planar crossovers were introduced, including multi‐section cascaded There after, many planar crossovers were introduced, including multi-section cascaded branchline branchline structure [14] and double ring structures [15]. A compact planar crossover using quarter structure [14] and double ring structures [15]. A compact planar crossover using quarter wavelength wavelength lines providing wide bandwidth is reported in [16]. A multimode cross‐shaped resonator lines providing wide bandwidth is reported in [16]. A multimode cross-shaped resonator can perform can perform as both crossover and filter [17]. By the inclusion of defected ground structures, a multi‐ as both crossover and filter [17]. By the inclusion of defected ground structures, a multi-layered layered compact crossover is realized using coupled lines in [18]. compact crossover is realized using coupled lines in [18]. The aforementioned crossovers operate at single frequency only. In the case of multi band The aforementioned crossovers operate at single frequency only. In the case of multi band operation, passive components required for transceiver systems should be cost effective and compact operation, passive components required for transceiver systems should be cost effective and compact with performance at multiple frequencies. A dual band crossover was designed with three section with performance at multiple frequencies. A dual band crossover was designed with three section branchline structures by extending the single band crossovers [19]. Stub‐loaded branchline couplers branchline structures by extending the single band crossovers [19]. Stub-loaded branchline couplers are also found to have good performance as crossovers [20]. These crossovers were designed for only are also found to have good performance as crossovers [20]. These crossovers were designed for small‐frequency ratio applications and provide less isolation. Henin etal., introduced the use of patch only small-frequency ratio applications and provide less isolation. Henin etal., introduced the use of antenna as planar crossovers [21]. Many modifications of this have been reported [22,23] which patch antenna as planar crossovers [21]. Many modifications of this have been reported [22,23] which confirms the utilization of patch antenna as planar crossover. This paper extends the concept of patch confirms the utilization of patch antenna as planar crossover. This paper extends the concept of patch as crossover to have dual band characteristics. The Diamond Patch Crossover (DPC) analysed here as crossover to have dual band characteristics. The Diamond Patch Crossover (DPC) analysed here works at 2.4 GHz and 5.23 GHz. A comparative study of other patch based crossover with the dual works at 2.4 GHz and 5.23 GHz. A comparative study of other patch based crossover with the dual band one is also presented. band one is also presented.

2.2. Crossover Crossover IntersectionIntersection of of transmission transmission lines lines in in a acircuit circuit leads leads to toreduced reduced signal signal-to-noise‐to‐noise ratio ratio (SNR). (SNR). A ‘crossover’A ‘crossover’ at atthe the intersection intersection addresses addresses this this effectively. effectively. As As presented presented in in Figure Figure 1,1, the crossover hashas decoupleddecoupled adjacent ports with power maintained between opposite ports.

Figure 1. Crossover. Figure 1. Crossover.

AssumingAssuming reciprocal, reciprocal, lossless lossless and and matched matched conditions conditions and andneglecting neglecting phase phase variations; variations; with perfectwith perfect isolation isolation between between adjacent adjacent ports ports the ideal the idealS matrixS matrix for the for crossover the crossover in Figure in Figure 1 will1 willbe, be,   000 0 1 00  0 0 0 1  [S] = 0 0 0 1  S  1 0 0 0  1 0 0 0  0 1 0 0  0 1 0 0 Due to the overlap in a practical circuit, the transmission coefﬁcient will not be 1 due to attenuationDue to andthe overlap isolation in will a practical not be zero. circuit, So the for designingtransmission any coefficient crossover, will the not isolation be 1 due should to attenuationbe maximized and and isolation insertion will loss not (attenuation) be zero. So shouldfor designing be minimum any crossover, maintaining the compactness.isolation should In this be maximizedpaper, the intersection and insertion is realized loss (attenuation) as patch with should feed be location minimum exciting maintaining perpendicular compactness. polarization In this for paper,adjacent the ports intersection and parallel is realized polarizations as patch for with opposite feed location ports. This exciting ensures perpendicular the signal ﬂow polarization is maintained for adjacent ports and parallel polarizations for opposite ports. This ensures the signal flow is maintained

ElectronicsElectronics 2017 2017, 6, 746, 74 3 of 3 of8 8 betweenbetween the the opposite opposite ports ports while while the the adjacent adjacent ports ports are are decoupled. decoupled. The The design design and and detailed detailed study study between the opposite ports while the adjacent ports are decoupled. The design and detailed study are areare presented presented in in the the next next section. section. presented in the next section. 3. 3.Microstrip Microstrip Patch Patch Reconfigured Reconfigured as as Planar Planar Crossover Crossover 3. Microstrip Patch Reconﬁgured as Planar Crossover ForFor initial initial analysis, analysis, a circulara circular patch patch is ismodified modified as as crossover crossover as as shown shown in in Figure Figure 2. 2.The The patch patch is is For initial analysis, a circular patch is modiﬁed as crossover as shown in Figure2. The patch energizedenergized at at four four locations locations using using 50 50 Ω Ω microstripmicrostrip lines, lines, making making adjacent adjacent ports ports to to have have orthogonal orthogonal is energized at four locations using 50 Ω microstrip lines, making adjacent ports to have polarization.polarization. orthogonal polarization.

Figure 2. Circular Patch Crossover(CPC). (a) Schematic of CPC; (b) Photo of the fabricated prototype. FigureFigure 2. 2.Circular Circular Patch Patch Crossover(CPC). Crossover(CPC). (a) (a) Schematic Schematic of ofCPC; CPC; (b ()b Photo) Photo of ofthe the fabricated fabricated prototype. prototype.

ThisThis crossover crossover provides provides single single band band operation operation with with ~15 ~15 dB dB isolation isolation between between the the adjacent adjacent ports ports andand the the performance performance is isplotted plotted in in Figure Figure 33.. 3. EventhoughEventhough Eventhough returnreturn return lossloss loss ofof of 2020 20 dBdB dB isis observedisobserved observed atat at 5.25.2 5.2 GHzGHz GHz thethe transmission transmission and and isolation isolation are are very very poor, poor, thusthus thus notnot not performingperforming performing asas as crossovercrossover crossover atat at thatthat that frequency.frequency. frequency.

(a)( a) (b()b)

FigureFigure 3. 3.(a )( aVariation) Variation of ofS parametersS parameters with with frequency frequency for for a CPC;a CPC; (b ()b Current) Current distribution distribution of ofCPC CPC at at Figure 3. (a) Variation of S parameters with frequency for a CPC; (b) Current distribution of CPC at 2.45 GHz. 2.452.45 GHz. GHz.

ToTo enable enable the the microstrip microstrip patch patch assisted assisted crossover crossover to to have have dual dual band band characteristics, characteristics, the the intersectionintersectionTo enable of of thetransmission transmission microstrip lines patch lines is assisted ismodified modified crossover as as square square to have and and dualdiamond diamond band patches. characteristics, patches. The The observations theobservations intersection are are presentedofpresented transmission in in detail detail lines in in isthe modiﬁedthe next next section. section. as square and diamond patches. The observations are presented in detail in the next section.

ElectronicsElectronics2017 2017,,6 6,, 74 74 44 ofof 88

4. Microstrip Patch Reconfigured as Dual Band Planar Crossover 4. Microstrip Patch Reconﬁgured as Dual Band Planar Crossover

4.1.4.1. SquareSquare PatchPatch CrossoverCrossover (SPC)(SPC) AA squaresquare patchpatch crossovercrossover (SPC)(SPC) isis realizedrealized byby replacingreplacing thethe transmissiontransmission lineline intersectionintersection withwith half-wavelengthhalf‐wavelength square patchpatch excitedexcited withwith twotwo setssets ofof perpendicularperpendicular feedfeed lineslines asas demonstrateddemonstrated inin FigureFigure4 4..

Figure 4. Square Patch Crossover (SPC). (a) Schematic of SPC; (b) Photo of the fabricated prototype. Figure 4. Square Patch Crossover (SPC). (a) Schematic of SPC; (b) Photo of the fabricated prototype.

ToTo ensure ensure isolation isolation between between adjacent adjacent ports, ports, the the orthogonal orthogonal feed feed lines lines are are provided provided at theat the corners corners of theof patchthe patch which which energizes energizes the Transverse the Transverse Magnetic Magnetic (TM) modes (TM) TM 010modesand TMTM100010 modes.and TM Considering100 modes. cavityConsidering model cavity [1] for model the patch, [1] for the the magnetic patch, ﬁeldthe magnetic distribution field for distribution TM100 mode for is TM100 mode is

 xπ x HH y = AA1 SinSin , H, xH 0 =01 (1) y 1 L L x andand forfor TMTM010010 modemode field field is: is: π y Hx = A2 Sin y , Hy = 0 (2) H  A Sin L, H  0  2 x 2 L y where A1 and A2 are the fields amplitude and L is the patch length. The operating frequency fr is givenwhere as: A1 and A2 are the fields amplitude and L is the patch length. The operating frequency fr is given as: C fr = √ (3) 2L εr C f r  3 Since the mode is TM, Ez is present and the two fundamental modes have perpendicular magnetic 2L  r fields. The Poyting Vector (S) is perpendicular to electric and magnetic fields. Since the mode is TM, Ez is present and the two fundamental modes have perpendicular magnetic fields. The Poyting Vector (S) is perpendicularS = E × H to electric and magnetic fields. (4)

S  E  H  4 For TM100 mode, For TM100 mode, Sx = Ez × Hy (5)

For TM010 mode, Sx Ez Hy 5 Sy = Ez × Hx (6) For TM010 mode, So the power ﬂow for the TM100 mode and for the TM010 mode is in perpendicular directions (x-direction and y-direction respectively).Sy SoE thez H oppositex 6 ports (port 1 and 3 and port 2 and 4) get properly aligned to couple energy from the same modes. The adjacent ports become decoupled by So the power flow for the TM100 mode and for the TM010 mode is in perpendicular directions (x‐ orthogonal modes. Thus proper isolation (between the adjacent ports) and low insertion loss (between direction and y‐direction respectively). So the opposite ports (port 1 and 3 and port 2 and 4) get the opposite ports) is maintained among the concerned ports. properly aligned to couple energy from the same modes. The adjacent ports become decoupled by Designed using Equation (3), on the dielectric substrate FR4 epoxy (ε = 4.4, h = 1.6 mm, orthogonal modes. Thus proper isolation (between the adjacent ports) andr low insertion loss tan δ = 0.018) a square patch is simulated using ANSYS HFSS (14, ANSYS Inc., Pittsburgh, PA, USA) (between the opposite ports) is maintained among the concerned ports.

Electronics 2017, 6, 74 5 of 8 Electronics 2017, 6, 74 5 of 8 ElectronicsDesigned2017, 6, 74 using Equation (3), on the dielectric substrate FR4 epoxy (εr = 4.4, h = 1.6 mm, tan5 of δ 8= 0.018)Designed a square using patch Equation is simulated (3), on using the dielectric ANSYS HFSSsubstrate (14, FR4ANSYS epoxy Inc., (ε rPittsburgh, = 4.4, h = 1.6 PA, mm, USA) tan δand = 0.018)fabricated a square to have patch crossover is simulated characteristics using ANSYS at 2.4 HFSS GHz. (14, The ANSYS feed lines Inc., arePittsburgh, designed PA, for USA) 50 Ω .and The fabricatedandreflection fabricated toand have transmission to have crossover crossover characteristics characteristics characteristics of at SPC 2.4 at areGHz. 2.4 plotted GHz. The feedand The shownlines feed linesare in designedFigure are designed 5. for 50 for Ω. 50 TheΩ . reflectionThe reﬂection and andtransmission transmission characteristics characteristics of SPC of SPCare plotted are plotted and andshown shown in Figure in Figure 5. 5.

Figure 5. Variation of S parameters with frequency for a Square Patch Crossover. Figure 5. Variation of S parameters with frequency for a Square Patch Crossover. Figure 5. Variation of S parameters with frequency for a Square Patch Crossover. TheThe edgeedge feed energizes energizes the the next next degenerate degenerate modes modes (TM (TM020 020andand TM200 TM), 200having), having resonance resonance at 5.2 atGHz, 5.2The thus GHz, edge providing thus feed providing energizes a dual band the a dual next response. band degenerate response. The current modes The distribution (TM current020 and distribution TMobtained200), having confirms obtained resonance the conﬁrmsuse at of 5.2 the GHz,thesquare use thus patch of providing the as square effective a patch dual crossover band as effective response. and is crossover presented The current and in Figure isdistribution presented 6. It can obtained in be Figure seen thatconfirms6. It diagonally can the be seenuse opposite of that the squarediagonallyports showpatch opposite theas effective same ports polarization,thus crossover show the and same is allowing presented polarization, signal in Figure flow thus 6.between allowing It can be them signal seen for that ﬂow both diagonally between the frequencies. opposite them for portsboth theshow frequencies. the same polarization,thus allowing signal flow between them for both the frequencies.

Figure 6. Current distribution of Square Patch Crossover: (a) at 2.43 GHz; (b) at 5.2 GHz. Figure 6. Current distribution of Square Patch Crossover: (a) at 2.43 GHz; (b) at 5.2 GHz. Figure 6. Current distribution of Square Patch Crossover: (a) at 2.43 GHz; (b) at 5.2 GHz. 4.2. Diamond Patch Crossover (DPC) 4.2. Diamond Patch Crossover (DPC) 4.2. DiamondA diamond Patch Crossoverpatch crossover (DPC) (DPC) is designed by changing the excitation of SPC as demonstratedA diamond in Figurepatch 7.crossover (DPC) is designed by changing the excitation of SPC as A diamond patch crossover (DPC) is designed by changing the excitation of SPC as demonstrated demonstrated in Figure 7. in Figure7. The scattering parameters observed for the DPC is as presented in Figure8. The current distribution at both the frequencies ascertain the performance as crossover and is as shown in Figure9. For DPC the area needed for designing the dual band crossover is 27% less than that of the SPC. The characteristics observed for the crossovers are consolidated in Table1. The simulated and experimental results obtained establish the use of microstrip patch antennas with proper feed location as a potential candidate as crossover.

Electronics 2017, 6, 74 6 of 8

Electronics 2017, 6, 74 6 of 8 Electronics 2017, 6, 74 6 of 8

Figure 7. Diamond Patch Crossover (DPC). (a) Schematic of DPC; (b) Photo of the fabricated prototype.

The scattering parameters observed for the DPC is as presented in Figure 8. The current Figure 7. Diamond Patch Crossover (DPC). (a) Schematic of DPC; (b) Photo of the fabricated distributionFigure 7. atDiamond both the Patch frequencies Crossover ascertain (DPC). (thea) Schematic performance of DPC; as crossover (b) Photo ofand the is fabricated as shown prototype. in Figure 9. prototype.

The scattering parameters observed for the DPC is as presented in Figure 8. The current distribution at both the frequencies ascertain the performance as crossover and is as shown in Figure 9.

Figure 8. Variation ofof SS parametersparameters withwith frequencyfrequency forfor DPC.DPC.

Figure 8. Variation of S parameters with frequency for DPC.

Figure 9. Current distribution of Diamond Patch Crossover; (a) at 2.43 GHz; (b) at 5.2 GHz. Figure 9. Current distribution of Diamond Patch Crossover; (a) at 2.43 GHz; (b) at 5.2 GHz.

Figure 9. Current distribution of Diamond Patch Crossover; (a) at 2.43 GHz; (b) at 5.2 GHz.

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Table 1. Comparison of Dual band Crossover characteristics — Experimental data (εr = 4.4, h = 1.6 mm, tan δ = 0.018). SPC: Square patch crossover; DPC: Diamond patch crossover.

Frequency of Return Loss Insertion Loss Isolation Type of Crossover Area (mm2) Operation (GHz) (dB) (dB) (dB) 2.43 28 1 12 930.25 SPC 5.2 40 1.2 33 - 2.43 23 2 12 676 DPC 5.2 32 1.5 34 -

5. Discussion The performance of the proposed crossover compared to the others to which this paper has referred is presented in Table2. The reduction in isolation for the ﬁrst band is due to the high loss tangent of FR4 which could be appreciably improved on using low loss substrates.

Table 2. Comparison of Crossovers.

Substrate Characteristics Type of Crossover Performance Isolation(dB) Size(λg × λg) εr tan δ Ref [16] 10.2 0.0035 Single band <20 0.25 × 0.25 Ref [17] 3.38 0.0027 Single band <20 0.24 × 0.33 Ref [18] 2.65 0.003 Single band <20 0.45 × 0.28 Ref [19] 2.2 0.0009 Dual band >10/>10 0.58 × 0.58 Ref [20] 3.55 0.0027 Dual band >25/>25 1.00 × 0.78 Ref [21] 10.2 0.0035 Single band <17 0.63 × 0.63 SPC 4.4 0.018 Dual band >12/>33 0.50 × 0.50 DPC 4.4 0.018 Dual band >12/>34 0.42 × 0.42

6. Conclusions In this paper a new microstrip patch antenna assisted planar dual frequency crossover for maintaining signal purity at transmission line intersections is presented. From Table2, it can be inferred that the proposed crossover is compact compared to the other microstrip counter parts, provides dual frequency operation and is potentially suitable in designing phased array and smart antenna systems.

Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Constantine, A. Balanis: Antenna Theory and Design, 3rd ed.; John Wileys & Sons, Inc.: Hoboken, NJ, USA, 2005. 2. Majid, H.A.; Rahim, M.K.A.; Hamid, M.; Murad, N.A.; Ismail, M.F. Frequency-Reconﬁgurable Microstrip Patch-Slot Antenna. IEEE Antennas Wirel. Propag. Lett. 2013, 12, 218–220. [CrossRef] 3. Lu, Z.; Yang, X.; Tan, G. A Multidirectional Pattern-Reconﬁgurable Patch Antenna With CSRR on the Ground. IEEE Antennas Wirel. Propag. Lett. 2017, 16, 416–419. [CrossRef] 4. Santhosh, S.; Arunselvan, S.J.; Aazam, S.H.; Gandhiraj, R.; Soman, K.P. Performance analysis of microstrip antenna using GNU radio with USRP N210. Smart Innov. Syst. Technol. 2015, 33, 239–249. 5. Fernandes, M.; Bhandare, A.; Dessai, C.; Virani, H. A Wideband Switched Beam Patch Antenna Array for LTE and Wi-Fi. In Proceedings of the 2013 Annual IEEE India Conference (INDICON), Mumbai, India, 13–15 December 2013; pp. 1–6. 6. Anderson, S.; Millnert, M.; Viberg, M.; Wahlberg, B. An adaptive array for mobile communication systems. IEEE Trans. Veh. Technol. 1991, 40, 230–236. [CrossRef] Electronics 2017, 6, 74 8 of 8

7. Ibrahim, S.Z.; Rahim, M.K.A. Switched Beam Antenna using Omnidirectional Antenna Array. In Proceedings of the Asia-Paciﬁc Conference on Applied Electromagnetics, Melaka, Malaysia, 4–6 December 2007; pp. 1–4. 8. Bhowmik, W.; Srivastava, S. Optimum Design of a 4×4 Planar Butler Matrix Array for WLAN Application. J. Telecommun. 2010, 2, 68–74. 9. Young-Min, J. Broad Band Patch Antennas Using a Wedge-Shaped Air Dielectric Substrate. IEEE Antennas Propag. Soc. Int. Symp. 1999, 2, 932–935. 10. Goossen, K.W. On the Design of Coplanar Bond Wires as Transmission Lines. IEEE Microw. Guided Wave Lett. 1999, 9, 798–799. [CrossRef] 11. Liu, W.; Zhang, Z.; Feng, Z.; Iskander, M.F. A compact wideband microstrip crossover. IEEE Microw. Wirel. Compon. Lett. 2012, 22, 254–256. [CrossRef] 12. Abbosh, A.; Ibrahim, S.; Karim, M. Ultra-wideband crossover using microstrip-to-coplanar waveguide transitions. IEEE Microw. Wirel. Compon. Lett. 2012, 22, 500–502. [CrossRef] 13. Wight, J.S.; Chudobiak, W.J.; Makios, V. A microstrip and stripline crossover structure. IEEE Trans. Microw. Theory Tech. 1976, 24, 270. [CrossRef] 14. Lin, F.; Chu, Q.X.; Wong, S.W. Dual band planar crossover with two section branch line structure. IEEE Trans. Microw. Theory Tech. 2013, 61, 2309–2316. [CrossRef] 15. Tang, C.; Chuang, W. Design of the Planar Six-Port Crossover With Double Rings. IEEE Microw. Wirel. Compon. Lett. 2015, 25, 651–653. [CrossRef] 16. Xin, N.L.; Cuiping, Y.; Yuanan, L.; Shulan, L.; Fan, W.; Ming, S. A Novel Compact Planar Crossover with Simple Design Procedure. In Proceedings of the Asia-Paciﬁc Microwave Conference, Yokohama, Japan, 7–10 December 2010; pp. 1633–1636. 17. Guo, Q.Y.; Zhang, X.Y.; Gao, L. Novel Compact Planar Crossover With Bandpass Response Based on Cross-Shaped Resonator. IEEE Trans. Compon. Packag. Manuf. Technol. 2017, PP, 1–9. [CrossRef] 18. Feng, W.; Zhang, T.; Che, W. Compact Single-band Planar Crossover Based on Coupled Lines. In Proceedings of the 46th European Microwave Conference, London, UK, 4–6 October 2016. 19. Lee, Z.-W.; Pang, Y.-H. Compact planar dual-band crossover using two-section branch-line coupler. Electron. Lett. 2012, 48, 1348–1349. [CrossRef] 20. Shao, J.; Ren, H.; Arigong, B.; Li, C.; Zhang, H. A fully symmetrical crossover and its dual-frequency application. IEEE Trans. Microw. Theory Tech. 2012, 60, 2410–2416. [CrossRef] 21. Henin, B.; Abbosh, A. Design of compact planar crossover using Sierpinski carpet microstrip patch. IET Microw. Antennas Propag. 2015, 7, 54–60. [CrossRef] 22. Jayakrishnan, V.M.; Menon, S.K. Circular Patch Antenna Based Planar Crossover. In Proceedings of the 3rd International Conference on Signal Processing and Integrated Networks, Noida, India, 11–12 February 2016; pp. 250–254. 23. Purnima, G.; Menon, S.K. Microstrip patch based high isolation planar crossover for beamforming applications. In Proceedings of the IEEE International WIE Conference on Electrical and Computer Engineering (WIECON-ECE), Pune, India, 19–21 December 2016; pp. 184–187.

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