Highly compact size serpentine‐shaped multiple‐input–multiple‐output fractal antenna with CP diversity
2017; Institution of Engineering and Technology; Volume: 12; Issue: 4 Linguagem: Inglês
10.1049/iet-map.2017.0770
ISSN1751-8733
Autores Tópico(s)Energy Harvesting in Wireless Networks
ResumoIET Microwaves, Antennas & PropagationVolume 12, Issue 4 p. 636-640 Research ArticleFree Access Highly compact size serpentine-shaped multiple-input–multiple-output fractal antenna with CP diversity Amer T. Abed, Corresponding Author Amer T. Abed amer.t.abed@ieee.org Department of communication Engineering, Al-Mamon University College, 14th Ramadhan Street, Baghdad, IraqSearch for more papers by this author Amer T. Abed, Corresponding Author Amer T. Abed amer.t.abed@ieee.org Department of communication Engineering, Al-Mamon University College, 14th Ramadhan Street, Baghdad, IraqSearch for more papers by this author First published: 19 February 2018 https://doi.org/10.1049/iet-map.2017.0770Citations: 17AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract A novel multiple-input–multiple-output fractal antenna was proposed in this study. The proposed antenna consisted of two symmetrical radiating elements placed on opposite sides to achieve spatial diversity with a highly compact size (8 × 8 × 0.8 mm3). Each radiating element is configured by a process of three iterations, and an additional semi-crescent structure scaled down in each iteration placed in a cascade arrangement. This arrangement generated circularly polarised radiation with an axial ratio bandwidth of (2.2–3.2) GHz, which was about 24% of the entire operational range of (0.1–4.3) GHz that can be used for many wireless communications such as long-term evolution, radio-frequency identification, wireless fidelity, and worldwide interoperability for microwave access applications. The envelope correlation coefficient, mutual coupling, circular polarisation (CP), and current distribution were studied and analysed to improve the design process. Compared to other related antennas, the proposed antenna can improve many radiation characteristics such as diversity (left-hand CP) and (right-hand CP), wide axial ratio bandwidth, and acceptable gain in a small antenna size. 1 Introduction Recently, multiple-input–multiple-output (MIMO) antennas have been used as wireless communications antennas to reduce the multipath effect as well as to increase the amount of data transmitted and received. Many researchers have reported MIMO fractal [[1]–[4]], slots [[5]–[8]], or coplanar waveguide (CPW) antennas [[9], [10]]. Array of dual-patched antennas separated by modified serpentine structure for isolation enhancement reported in [[1]], the MIMO proposed antenna covered the frequency spectrum (2.43–2.49) GHz in large overall size 100 × 80 × 1.6 mm3. Four elements with octagonal shapes and an overall size of 45 × 45 × 0.8 mm3 have been used to cover the ultra-wideband with linear polarisation (LP) radiation [[2]]. Two elements with a fractal shape and a large size (121 × 61 × 0.8 mm3) have been used to generate dual operating bands [[3]]. A T-shape has been inserted between dual-fractal radiated elements to reduce the mutual coupling between them and to generate dual-frequency bands (1.6–1.9 and 2.6–6.2 GHz) with overall dimensions of 100 × 50 × 1.5 mm3 [[4]]. Moreover, a two-element integrated antenna has been reported with an operating band of 1.5–5.8 GHz, LP, and overall size of 90 × 50 × 0.76 mm3 [[5]]. The MIMO slot antenna (38.5 × 38.5 × 1.6 mm3) is preferred as it possesses linearly polarised radiation covering the frequency bands 3–5 and 6–11.8 GHz [[6]]. A two-element MIMO slot antenna measuring 35 × 33 × 0.8 mm3 has been proposed to radiate LP at the operating band of 3.1–5 GHz [[7]]. For long-term evolution (LTE) applications, a single-ring, two-port MIMO antenna has been reported in [[8]] with an impedance bandwidth of 1.85–2.6 GHz, 85% efficiency, LP, and overall size of 80 × 80 × 1.6 mm3. For wireless local area network applications, the MIMO slot antenna with linearly polarised radiation and dimensions of 56 × 20 × 1.6 mm3 has been proposed [[9]]. In [[10]], a CPW MIMO antenna operating in frequency bands 0.9/1.8/2.3/2.6 GHz, with high efficiency, LP radiation, and overall size of 80 × 60 × 0.8 mm3 has been proposed. A planar monopole MIMO antenna proposed in [[11]] has the dimensions of 15.5 × 18 × 1.6 mm3 covering the frequency range 2.2–2.51 GHz. Table 1 summarised some important specifications such as the number of elements, impedance bandwidth, efficiency, gain, axial ratio bandwidth (ARBW), envelope correlation coefficient (ECC), mutual coupling (isolation coefficient ), size, and the type of previous related antennas reported in [[1]–[11]]. Table 1. Comparison between the proposed antenna and related antennas in [[1]–[11]] Antennas Number of elements Bandwidth, GHz Efficiency, % Gain, dB ARBW, GHz ECC , dB Size, mm Type [1] 2 (2.43–2.49) — — LP 0.07 <–20 100 × 80 × 1.6 patch [2] 4 (2–5) (6–10) — –1.5 to 4 LP 0.2 <–20 45 × 45 × 0.8 fractal [3] 2 (2.4–2.5) (5–6) — — LP 0.1 <–22 121 × 61 × 0.8 fractal [4] 2 (1.6–1.9) (2.6–6.2) 88 –5 to 5 LP 0.1 <–15 100 × 50 × 1.5 fractal [5] 2 (1.5–5.5) (5.1–5.8) 60 10–14 LP 0.1 <–10 90 × 50 × 0.76 integrated [6] 2 (3–5) (6–11.8) 80 –4 to 4 LP 0.04 <–15 38.5 × 38.5 × 1.6 slot [7] 2 (3.1–5) 70 2–3 LP 0.1 <–22 35 × 33 × 0.8 slot [8] 2 (1.85–2.6) 85 — LP 0.08 <–15 80 × 80 × 1.6 slot [9] 2 (2.4–2.48) (5.1–5.8) — — LP 0.2 <–14 56 × 20 × 1.6 slot [10] 2 0.9/1.8/2.3/2.6 90 –2 to 4 LP 0.1 <–10 80 × 60 × 0.8 CPW [11] 2 (2.20–2.51) — –2.5 to 0.5 LP 0.05 <–16 15.5 × 18 × 1.6 CPW Pro 2 (0.1–4.3) 56 –1 to 3.1 2.2–3.2 0.03 <–30 8 × 8 × 0.8 fractal All previous MIMO antennas had LP radiation, so it is needed to design compact size MIMO antenna with circular polarisation (CP) and low mutual coupling. The aim of the current research is to design an MIMO fractal antenna with low mutual coupling between dual-radiating elements in a compact size of 8 × 8 × 0.8 mm3 and circularly polarised radiation with wide ARBW. 2 Antenna structure and analysis 2.1 Single element In a four-step process, the desired antenna can be configured. The 0th iteration antenna as shown in Fig. 1a consists only of a short strip line F with dimensions (1.25 × 1 mm2) printed on an Flame Retardant -4 substrate, with thickness of 0.8 mm and fully grounded by square plate (8 × 8 mm2) on the other side of the dielectric. The first iteration antenna is formed by adding a semi-crescent Sc1, which is configured by cutting elliptical slot Se [major axes 3 mm, minor axes 2 mm, and its centre at (2,3) in the X–Y-plane] in a metallic circle C, which has a radius of 2 mm. The lower corner of the semi-crescent S1 can match the right edge of the feeding strip line F. The second and the third iterations are configured by adding a semi-crescent with half the size of that used in the previous iteration with phase shifted 180°. The process of the previous iteration can be presented by the formulas Fig. 1Open in figure viewerPowerPoint Geometry of the proposed antenna (a) Four iterations, (b) Proposed antenna Or (1) (2) In the progress of antenna configuration, the dimensions of the semi-crescent are scaled down and added in order to obtain the same structure. The semi-crescents Sc1, Sc2, and Sc3 are arranged in such a way that the radiating elements take the form of serpentine shape for the purpose of generating phase shift 90° at the touch points of the structures (Sc1, Sc2, and Sc3) to achieve CP. Fig. 2 depicts the reflection coefficient values of the four iterations. It is clear that the impedance bandwidth increases at high-order iterations. The dash-dotted curve in Fig. 2 shows that a wide operating band (0.1–4 GHz) with return loss −24 dB at resonant frequency 3.2 GHz is achieved at the second iteration. At the third iteration, the operating band compresses into a frequency range of 0.1–3.4 GHz, but with low reflection coefficient (–32 dB) at a resonant frequency of 2.7 GHz Fig. 2Open in figure viewerPowerPoint Simulated reflection coefficient values for all iterations 2.2 MIMO antenna The proposed antenna shown in Fig. 1b consists of two symmetrical antennas with spatial diversity (denoted by antenna 1 and antenna 2), each having the shape of the scorpion tail. These antennas are located in close proximity to the left and right edges to reduce the mutual coupling between them to the lowest extent possible. The dual elements excited by the ports are denoted as 'port 1' and 'port 2'. Both single and dual elements have the same size (8 × 8 × 0.8 mm3). The values of the real part impedance of the MIMO antenna (the black solid curve) are closer to the port impedance (the red-dashed line) than those for the third iteration antenna (the black dashed curve) within the frequency range <2.7 GHz, as shown in Fig. 3. At a resonant frequency of 2.7 GHz, the real part impedance for both single and dual elements equal 50 Ω, which represented the best value of matching factor with input impedance. The blue-dashed curve in Fig. 3 shows low imaginary impedance values (close to zero) during the operating band for the third iteration antenna compared with those for the MIMO antenna. This occurs due to the capacitance impedance generated between the dual-radiating elements in the MIMO antenna. Fig. 3Open in figure viewerPowerPoint Impedance values of the third iteration and MIMO antenna The solid curve in Fig. 4 shows that the impedance bandwidth compresses into a frequency range of 0.1–3 GHz compared with that for the third iteration antenna (the dotted curve). A good isolation coefficient value (<−30 dB) is observed in Fig. 4 (the dashed curve) during the entire operating band. Fig. 4Open in figure viewerPowerPoint Simulated S-parameter values for single and dual elements 2.3 Circular polarisation The left-hand CP (LHCP) (black curve) and the right-hand CP (RHCP) (red curve) radiation patterns of both antennas at the frequencies of 1.5, 2.7, 3, and 5 GHz are depicted in Fig. 5. As can be seen, the phase difference between radiation patterns at 1.5 and 5 GHz are and , respectively, for both antennas, whereas the LHCP and RHCP patterns at 2.7 and 3 GHz are shifted by (almost perpendicular to each other) for both radiating elements. This means that there are CP characteristics generated by the antenna around the frequencies of 2.7 and 3 GHz. Fig. 5Open in figure viewerPowerPoint Simulated left (black curves) and right (red curves) polarisation at frequencies of 1.5, 2.7, 3, and 5 GHz The previous investigation about CP matches the simulated values of the axial ratio during the progress of iteration presented in Fig. 6. The blue curve shows that there is no CP at the first iteration, at the second iteration an ARBW observed at frequency range 2.4–3 GHz (the red curve), whereas at the third iteration (the black curve) an ARBW <3 dB during the frequency range of 2.1–3.6 GHz obtained. Fig. 6Open in figure viewerPowerPoint Simulated AR during the progress of iteration Fig. 7 represents the surface current distribution at phase references , , , and when antenna 1 is excited. It is clear that the current flows toward +Y-direction along structure and flows toward +X along structure at phase references and , while it flows toward −Y-direction along structure and flows toward X along structure at phase references and . Fig. 7Open in figure viewerPowerPoint Simulated surface current at 2.7 GHz for the sequential phases 0°, 90°, 180°, and 270° So, the direction of the surface current changes sequentially during changing the phase. For all phase references in Fig. 7 at the touch points B and C between the structures , , and , two current components perpendicular to each other observed which generate two normal electric fields that are required for CP generation. When exciting antenna 1, the maximum current concentrates along the edges of the structure 1 (F, Sc1, Sc2, and Sc3) for antenna 1, and there are no surface currents observed along antenna 2, as shown in Fig. 8a. Vice versa when antenna 2 is excited (Fig. 8b). This means that the mutual coupling between antennas is very low during the operating band. This analysis matches the values of the isolation coefficient (<−30 dB) during the operating band, as shown in Fig. 4 (the dashed curve). Fig. 8Open in figure viewerPowerPoint Surface current distribution at 2.7 GHz (a) Antenna 1 excited, (b) Antenna 2 excited 2.4 Diversity performance The diversity performance of the proposed MIMO antenna can be investigated by using the ECC, which can be calculated by either using three-dimensional (3D) radiation pattern values [[12]] or by using simulated or measured scattering parameter values as in (3) [[13]]. The acceptable value of ECC is <0.5 (3) Fig. 9a represents the ECC values for different types of diversities. The red curve in the mentioned figure shows the lowest values of ECC when the dual ports are located at the same side of the MIMO antenna with pattern diversity. The blue curve represents the ECC values when the dual antennas are excited by dual ports are located on the same side without pattern diversity. The worst indicator of the diversity (the black curve) for the proposed MIMO antenna takes place when the ports are located on the opposite side. This type of diversity is called the spatial diversity and is shown in Fig. 1b. Fig. 9Open in figure viewerPowerPoint Simulated values of AR and ECC for different types of diversities (a) ECC values, (b) AR The black curve in Fig. 9b shows that CP radiation with ARBW of 2.1–3.6 GHz can be achieved only by the spatial diversity of the dual-radiating elements in the proposed MIMO antenna. The values of AR for other types of diversities are poor (the red and blue curves). Thus, the spatial diversity as shown in Fig. 1b is the best due to the acceptable values of ECC and the CP characteristics. 3 Measurements and results Fig. 10 shows the photographs of the fabricated antenna, whereas Fig. 11 represents the simulated (solid) and measured S-parameters of the proposed MIMO antenna. The measurement of the S-parameters of the fabricated antenna is carried out with the panoramic network analyser (N5227A). Clearly, the measured impedance bandwidth expands to the frequency range of 0.1–4.3 GHz for antenna 1 (Fig. 11 – the black dashed curve), whereas for antenna 2 the impedance bandwidth expands to the frequency band 0.1–4 GHz as shown in Fig. 11 (the black dotted curve), whereas the measured mutual coupling () between the two antennas varies from −35 to −12 dB, as shown in Fig. 11 (the red-dashed and dotted curves). Of course, the resonant frequency at the measured curve does not match that at the simulated curve, which takes place due to the fact that the impedance of connectors and the cables at simulating results is neglected. Fig. 12b presents the measured and simulated radiation patterns in X–Y and X–Z planes at frequencies of 1.5, 2.7, and 4 GHz. Fig. 10Open in figure viewerPowerPoint Prototype photographs. (a) Radiating elements, (b) Ground plate, (c) Measuring S-parameters Fig. 11Open in figure viewerPowerPoint Simulated (solid) and the measured (dashed) S-parameters Fig. 12Open in figure viewerPowerPoint Radiation patterns (a) Simulated 3D gain, (b) Simulated and measured patterns in the X–Y and X–Z planes Generally, acceptable agreement observed in Fig. 12b between the measured patterns (dashed and dotted curves) and simulated patterns (the solid curves) in both planes (X–Y and X–Z), especially at the frequencies of 1.5 and 2.7 GHz. At the frequency of 1.5 GHz, the radiation pattern in the X–Z-plane looks omnidirectional, whereas that in the X–Y-plane is pear shaped. On the contrary, at a resonant frequency of 2.7 GHz, the radiation pattern in the X–Z-plane looks like Fig. 9, whereas it is omnidirectional in the X–Y-plane. At the frequency of 4 GHz, the simulated radiation patterns in X–Y and X–Z planes are almost semi-omnidirectional, while the measured radiation pattern in the X–Y-plane has dual major lobes at angles of 0° and 270°, whereas that in the X–Z-plane, the measured radiation pattern has a rectangular shape with buckling at angles of 0° and 190°. Fig. 13 depicts the measured and simulated gains (black curves), efficiency (red curves), and AR (blue curves). The measured ARBW is 2.2–3.2 GHz, which is about 24% of the operating band, as shown in Fig. 13 (the blue-dashed curve). The peak values of measured gain varied between −1 and 3.1 dB, whereas the maximum total efficiency is −2.5 dB (56%) at the frequency 2.7 GHz (the red-dashed curve). Fig. 13Open in figure viewerPowerPoint Simulated and measured values of AR, gain, and efficiency 4 Conclusion For the purpose of highlighting the contribution in this work, Table 1 illustrates a comparison between the proposed antenna and previous antennas investigated in [[1]–[11]] in terms of several characteristics such as the number of elements, impedance bandwidth, efficiency, gain, ARBW, ECC, mutual coupling (isolation coefficient ), and size. By comparing the proposed antenna with the others related in Table 1, we can clearly see that the proposed antenna in this paper has the smallest size, CP diversity, the lowest ECC value, wideband axial ratio, and acceptable gain, but of low efficiency due to the highly compact size. Thus, the proposed fractal MIMO antenna could be a suitable candidate when compact circularly polarised antennas are required. It can be used for many portable wireless communications such as radio-frequency identification, LTE, wireless fidelity, and worldwide interoperability for microwave access. Furthermore, due to low mutual coupling between the two radiating elements and CP diversity, the proposed antenna can be used in transceiver devices. 5 References [1]Henridass, A., Sarma, A.K., Kanagasabai, M., et al.: Deployment of Modified Serpentine Structure for Mutual Coupling Reduction in MIMO Antennas, IEEE Antennas Wirel. Propag. Lett., 2014, 13, pp. 277– 280 [2]Tripathi, S., Mohan, A., Yadav, S.: 'A compact Koch fractal UWB MIMO antenna with WLAN band-rejection', IEEE Antennas Wirel. Propag. Lett., 2015, 14, pp. 1565– 1568 [3]Peristeriano, A., Theopoulos, A., Koutinos, A.G., et al.: 'Dual-band fractal semi-printed element antenna arrays for MIMO applications', IEEE Antennas Wirel. 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Antennas Propag., 2012, 61, (2), pp. 964– 968 Citing Literature Volume12, Issue4March 2018Pages 636-640 FiguresReferencesRelatedInformation
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