electronics Article
Wide-Angle Beam Scanning Leaky-Wave Antenna Using Spoof Surface Plasmon Polaritons Structure Leilei Liu 1,2, * , Jian Wang 1 , Xiaoxing Yin 2 and Zhi Ning Chen 3 1 2 3
*
College of Electronic and Optical Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210003, China;
[email protected] State Key Laboratory of Millimeter Waves, Southeast University, Nanjing 210096, China;
[email protected] Electrical and Computer Engineering Department, National University of Singapore, Singapore 117583, Singapore;
[email protected] Correspondence:
[email protected]; Tel.: +86-025-8586-6131
Received: 21 October 2018; Accepted: 19 November 2018; Published: 24 November 2018
Abstract: This paper proposes a wide-angle beam scanning leaky-wave antenna (LWA) fed by a novel spoof surface plasmon polaritons (SSPP) transmission line (TL). In the proposed LWA, circular metallic patches are periodically loaded on both sides of the SSPP TL alternately, and convert guided waves into radiating waves. The transmission characteristics of the proposed SSPP TL are analyzed, and the transmission characteristics and radiation patterns of the proposed LWA are simulated and measured. The simulated and measured results show that the proposed LWA provides approximately 12.5 dBi of radiation gain within a frequency range of 8–24 GHz, and a beam scanning range of 90◦ from forward to backward continuously by increasing the feeding frequency. The proposed LWA, based on a novel SSPP TL, has advantages of single-layer conductor, continuous wide-angle beam scanning, and high gain especially at the broadside direction, which are difficult realize using conventional LWAs. Keywords: leaky-wave antenna (LWA); spoof surface plasmon polaritons (SSPP); wide-angle beam scanning
1. Introduction Leaky-wave antennas (LWAs) have attracted significant attention in microwave engineering due to their advantages of simple feeding network, frequency beam scanning, high directivity and low cost [1]. Because a fast wave transmission line (TL) can feed a LWA to radiate, the initial design of LWAs are based on the waveguides supporting fast wave modes, such as slotted rectangular waveguides [2] and substrate integrated waveguides [3]. Many periodic structures operating on space harmonic waves are explored to feed LWAs, such as microstrip lines, groove waveguides and metamaterials structures [4–6]. With the rapid development of integrated circuits, planar antennas with single-layer conductors are explored. The spoof surface plasmon polaritons (SSPP) TL is a single-conductor line without ground plane. Since it confines the electromagnetic wave strongly around the interface between the metal and dielectric, the SSPP TL performs well in the integrated planar circuit system [7]. Surface plasmon polaritons are surface electromagnetic waves distributed at the interface of a dielectric and a conductor, which could only be excited at visible frequencies. In 2004, Pendry and his colleagues proposed a new structure that could excite surface plasmon polariton waves at lower frequencies [8]. On the basis of Pendry’s work, Garcia-Vidal proposed a scheme for realizing artificial surface plasmon with a one-dimensional periodic perforation structure [9]. The ultra-thin SSPP TL can be realized by processing the samples on flexible material films with PCB technology [10]. Early SSPP were mainly excited by probes or radiation sources, and the efficiency was not high [11–13]. Electronics 2018, 7, 348; doi:10.3390/electronics7120348
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Reference [14], MaMa used a coplanar waveguide to to excite a periodically grooved metal strip In Reference [14], used a coplanar waveguide excite a periodically grooved metal stripwith witha efficiency. Since then, varieties of TLofstructures basedbased on SSPP and widely ahigh high efficiency. Since then, varieties TL structures on have SSPPbeen haveproposed been proposed and used in used designs antennas and filtersand [7,15–20]. In Reference the SSPP was usedTL aswas a feeding widely in of designs of antennas filters [7,15–20]. In [16], Reference [16],TLthe SSPP used configuration for a dielectric antenna. As they are high confinement low profile, as a feeding configuration forresonator a dielectric resonator antenna. As they are high and confinement andSSPP low TLs areSSPP especially for suitable designing leaky-wave antenna [20–22]. the high profile, TLs aresuitable especially forthe designing the leaky-wave antennaHowever, [20–22]. However, confinements also bring difficulties in converting the SPPthe mode theinto space mode. the high confinements also bring difficulties in converting SPPwave modeinto wave theradiating space radiating To radiate waves free into space, an space, asymmetrical SSPP waveguide is periodically modulated mode. To leaky radiate leakyinto waves free an asymmetrical SSPP waveguide is periodically to achieve ato continuously beam scanning LWA [20]. Since are patches good radiators, a radiators, LWA using modulated achieve a continuously beam scanning LWApatches [20]. Since are good a patchusing array fed byarray an SSPP proposedwas [21]. However, theHowever, scanning angle and radiation LWA patch fedwaveguide by an SSPPwas waveguide proposed [21]. the scanning angle gainradiation is not as good traditional LWAs. In Reference [22] a patch loaded SSPPloaded LWA with artificial and gain isasnot as good as traditional LWAs. In Reference [22] a patch SSPPan LWA with magnetic conductor plane was plane introduced to improvetoperformances, and it turned a turned singlean artificial magnetic ground conductor ground was introduced improve performances, and it conductor structurestructure into a complex 3D structure, which lost the advantage of SSPP.of SSPP. alayer single-layer conductor into a complex 3D structure, which lost the advantage In this article, we propose a wide-angle beam scanning scanning LWA fed by a novel SSPP TL which is grooved by rectangular slots in a metallic strip. Since the SSPP structure has first been presented to LWA, the the characteristics characteristics of this TL are discussed. Due to the advantages feed LWA, advantages of of the the proposed proposed SSPP, SSPP, the antenna is fabricated on a single layer conductor which is easy to be integrated and conformed. conformed. proposedLWA LWA loaded circular patches in a wide-angle ofcontinuously 90 degrees The proposed loaded withwith circular patches scans inscans a wide-angle range of 90range degrees continuously forwardwithin to backward within a frequency range 8–24 GHz, and achieves from forward from to backward a frequency range of 8–24 GHz, andof achieves a measured averagea measuredgain average radiation gain of 12.5 dBi with a high radiation efficiency. radiation of 12.5 dBi with a high radiation efficiency.
2. SSPP Transmission Transmission Line Line Design Design The proposed SSPP TL, shown in Figure 1, is composed of a metal strip with periodic periodic drilled drilled square grooves and fabricated on on aa single single layer layer FR4 FR4 substrate substrate (with (with εεrr == 4.3, 4.3, tanσ tanσ = = 0.025 and 0.5 mm thickness). In order to integrate such an SSPP TL with traditional traditional microwave circuits, the matching transitions between the SSPP TL and co-planar waveguide (CPW) structures are adopted. Since the SSPP TL supports SPP mode and the CPW works on a quasi-TEM mode, a gradual transition part is designed into CPW to make smooth conversions, and the two side ground planes of CPW gradually flare to achieve a wave-vector matching. Rectangular grooves are also gradually smaller at two ports of the SSPP TL to created a better impedance matching. The CPW is designed of 50 Ω Ω to connect the conventional Sub-miniature A (SMA) connectors. The width of the SSPP TL is a = 70 (SMA) connectors. is a = mm, in order to have a good matching, matching, and and the the length length of of the the SSPP SSPP TL TL is is b = 404 mm which is related to the main beam width of the associated LWA. The width of grooves is LWA. The width of grooves is gg == 1.6 mm, the depth of grooves is ds = 2 mm, the length of a unit cell is is pp = 4 mm, and width of strip is ws = 4 mm. These parameters determine the shape of dispersion curve of the SSPP TL, and, in turn, determine the ranges of the scanning angles and frequencies.
Figure 1. The geometry configuration of the SSPP transmission line.
The ofof thethe CPW TLTL close to the ports werewere chosen as w as = 30 The strip strip width, width,gap gapsize, size,and andlength length CPW close to the ports chosen w mm, = 30 gap = 0.5 mm, and l = 16 mm, respectively. The length of the transition region was set to be l = 66 1 2 mm, gap = 0.5 mm, and l1 = 16 mm, respectively. The length of the transition region was set to bemm l2 = 66 mm based on a tradeoff between a good matching and a small size of the proposed LWA. The
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origin of the coordinate was set at the center of the CPW port. To convert the quasi-TEM to a SSPP origin on of athe coordinate wasaset at the center and of the CPWsize port. theLWA. quasi-TEM to a of SSPP based tradeoff between good matching a small ofTo theconvert proposed The origin the wave with a high efficiency in broadband, the flaring ground was designed as an exponential curve wave with awas high in broadband, the flaring ground the wasquasi-TEM designed astoan exponential curve coordinate setefficiency at the center of the CPW port. To convert a SSPP wave with a as follows [14]: as follows [14]: high efficiency in broadband, the flaring ground was designed as an exponential curve as follows [14]: 1) + 𝜃 (1) 𝑦 = 𝑓(𝑥) = 𝑤 · 𝑒 𝛼(𝑥−𝑙 1) (1) 𝑦 = 𝑓(𝑥) = 𝑤 · 𝑒 𝛼(𝑥−𝑙 α ( x − l1 ) + 𝜃 y = f ( x ) = w·e +θ (1) where where where 𝛼 = 𝑙𝑛2/𝑙2 , 𝜃 = −𝑤 + 𝑔𝑎𝑝 + 𝑤𝑠 /2 (2) 𝛼 = 𝑙𝑛2/𝑙2 𝜃 = −𝑤 + 𝑔𝑎𝑝 + 𝑤𝑠 /2 (2) α = ln2/l2 ,, θ = −w + gap + ws /2 (2) θ is a coefficient to move the beginning of the curve to the coordinate origin. The design can give θ is a coefficient to move the beginning of the curve to the coordinate origin. The design can give to moveproperties the beginning thefrequency curve to the coordinate origin. The design canofgive θ is a coefficient the structure good matching in a of wide band by matching the momentum the the structure good matching properties in a wide frequency band by matching the momentum of the the structure good matching properties in a wide frequency band by matching the momentum of the conventional CPW mode into a SSPP waveguide mode. conventional CPW mode into a SSPP waveguide mode. conventional modeTL into a SSPP waveguide mode. AlthoughCPW an SSPP does not have a ground plane, it can also bound electromagnetic waves, Although an SSPP TL does not have a ground plane, it can also bound electromagnetic waves, an SSPP TL does not have a ground plane,of it the can proposed also bound electromagnetic waves, as as weAlthough know. Figure 2 shows the dispersion characteristic SSPP TL which is simulated as we know. Figure 2 shows the dispersion characteristic of the proposed SSPP TL which is simulated we know. Figure 2 shows the dispersion characteristic of the proposed SSPP TL which is simulated with the Eigen-mode solver of CST Microwave Studio. The dispersion curve of the SSPP TL is always with the Eigen-mode solver of CST Microwave Studio. The dispersion curve of the SSPP TL is always with solverline of in CST Microwave Studio. dispersion of the SSPP is belowthe theEigen-mode light’s dispersion its working band becauseThe an SSPP wave iscurve a slow-wave (𝑣 TL < below the light’s dispersion line in its working band because an SSPP wave is a slow-wave (𝑣𝑝−𝑆𝑆𝑃𝑃 < 𝑝−𝑆𝑆𝑃𝑃 always the light’s dispersion line under in its working becauseFigure an SSPP wave isthe a slow-wave 𝑣𝑝−𝑙𝑖𝑔ℎ𝑡 ),below and there is no radiation mode the cutoffband frequency. 3 displays simulated 𝑣𝑝−𝑙𝑖𝑔ℎ𝑡 ), and there is no radiation mode under the cutoff frequency. Figure 3 displays the simulated (v v p−lightdistribution ), and there isonnothe radiation mode frequency. Figure displays p−SSPP < E-field y-direction substrate at 10under GHz,the andcutoff it appears binding TM 3waves on the the y-direction E-field distribution on the substrate at 10 GHz, and it appears binding TM waves the simulated y-direction E-field distribution onalso the substrate at 10 GHz, and itbetween appears guided bindingwaves TM on waves surface metal strip. From Figure 3 we can see smooth conversions and surface metal strip. From 3 we can also smooth conversions between guided waves and on the surface metal strip. Figure From Figure 3 we cansee also seeare smooth conversions between guided waves SSPPs. In the CPW sections, conventional guided waves supported. Along the matching transition SSPPs. In the CPW sections, conventional guided waves are supported. Along the matching transition and SSPPs. the CPW sections, conventional guidedtowaves supported. thethe matching sections, the In guided waves are gradually transformed SSPP are waves. In the TLAlong section, E-field sections, the guided waves are gradually transformed to SSPP waves. In the TL section, the E-field transition sections, the guided gradually transformed to SSPP waves. the TL section, distribution behaviors like a waves surfaceare plasmon polaritons, which means that In SSPP waves are distribution behaviors like a surface plasmon polaritons, which means that SSPP waves are the E-field distribution behaviors like a surface plasmon polaritons, which means that SSPP waves supported. supported. are supported.
Figure 2. The the unit cell. cell. Figure The dispersion dispersion curve curve of of the SSPP SSPP Figure 2. 2. The dispersion curve of the SSPP unit unit cell.
Figure 3. Simulated E-field magnitude distribution at 10 GHz. Figure 3. Simulated E-field magnitude distribution at 10 GHz.
Figure the simulated results of of reflection reflection and transmission coefficients coefficients of Figure 44 shows shows the simulated results and transmission of the the proposed proposed Figure 4 shows the simulated results ofpropagation reflection and transmission of coefficients of the SSPP TL. The simulated results certify the characteristics SSPP waves, asproposed Figure SSPP TL. The simulated results certify the propagation characteristics of SSPP waves, as Figure 22 SSPP TL. The simulated results certify the propagation characteristics of SSPP waves, as Figure 2 indicates. Sincethe theloss lossofofFR4 FR4isispretty prettylarge largeand andthe theSSPP SSPPTLTLis is long, loss indeed affects indicates. Since long, thethe loss indeed affects thethe Sindicates. Since the loss of FR4 is pretty large and the SSPP TL is long, the loss indeed affects the SS-parameters. However, transmission loss is still to the intrinsic property of SSPP an SSPP 21 still parameters. However, thethe transmission loss S21Sis lowlow duedue to the intrinsic property of an TL parameters. However, the transmission loss S21 is stilloflow due to theTL, intrinsic property of an SSPP TL TL [23]. To study the transmission loss performance such a SSPP numerical simulations of the [23]. To study the transmission loss performance of such a SSPP TL, numerical simulations of the [23]. To study the transmission5.loss performance of such that a SSPP TL, numerical simulations of the SSPP wewe observe thethe depth of grooves ds determines the SSPP TL TL are arepresented presentedininFigure Figure 5.InInFigure Figure5a,5a, observe that depth of grooves ds determines SSPP TL are presented in Figure 5. In Figure 5a, we observe that the depth of grooves ds determines the cut-off frequency of the SSPP TL. As the cut-off frequency decreases, the transmission loss the cut-off frequency of the SSPP TL. As the cut-off frequency decreases, the transmission loss
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cut-off frequency of the SSPP TL. As the cut-off frequency decreases, the transmission loss decreases, decreases, which leads from aEM stronger EM field confinements. At the samethe time, the width of grooves which from a stronger field At theAt same time, of grooves g has decreases,leads which leads from a stronger EMconfinements. field confinements. the same time,width the width of grooves g has little effect as shown in Figure 5b. Therefore, the deeper the groove is, the lower the cut-off littlelittle effect as shown in Figure 5b. Therefore, the deeper the groove is, the lower cut-off g has effect as shown in Figure 5b. Therefore, the deeper the groove is, thethe lower thefrequency cut-off frequency and theitsstronger its confinements. To make better energy radiation, wetochoose to loosen and the and stronger confinements. To make radiation, we choose loosen the field frequency the stronger its confinements. Tobetter makeenergy better energy radiation, we choose to loosen the field confinements, and the geometrical parameters are chosen as Figure 1. confinements, and the geometrical parameters are chosen as Figure 1. the field confinements, and the geometrical parameters are chosen as Figure 1.
Figure 4. The simulated S-parameter of SSPP TL. Figure 4. The simulated S-parameter of SSPP Figure 4. The simulated S-parameter of SSPP TL.TL.
(a)
(a)
(b) (b) Figure 5. The simulated S21 of SSPP SSPP TL TL with with different different size of slots. (a) (a) Different ds when p = 21 of = 1.6 mm; Figure 5. The simulated S21 of SSPP TL with different size of slots. (a) Different ds when p = 1.6 mm; (b) different different gg when when ds ds == 2 mm. (b) different g when ds = 2 mm.
3. SSPP Leaky-Wave Antenna Design and Study 3. SSPP Leaky-Wave Antenna Design and Study
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ElectronicsThe 2018, 7, x FOR REVIEW SSPP TL PEER also could serve in feeding leaky-wave
5 ofwave 11 antennas. In order to couple the SSPP 3. SSPP Leaky-Wave Antenna Design and Study to a far field radiation, circular patches were loaded on both sides of the SSPP TL. The SSPP TL also serve inthe feeding leaky-wave antennas. In order toFR4 couple SSPP wave The SSPP also could serve inof feeding leaky-wave In order to acouple thethe SSPP wave Figure 6TL shows thecould structure proposed SSPPantennas. LWA fabricated on substrate with εr = tofar a tan far field radiation, circular patches were loaded on both sides of the SSPP TL. to a4.3, field radiation, circular patches were loaded on both sides of the SSPP TL. σ = 0.025 and a thickness h = 0.5 mm. The SSPP TL and the substrate are the same as those Figure 6 shows the structure ofthe theproposed proposed SSPPLWA LWA fabricated on a sides FR4 with εrεr=The Figure shows structure SSPP fabricated FR4 substrate substrate with =4.3, shown in6Figure 1.the The circularofpatches are interlacing, placed at the two of the SSPP TL. tan and a thickness h = 0.5 mm. The SSPP TL and the substrate are the same as those shown in 4.3,patch-loading tanσ σ= =0.025 0.025 and a thickness h = 0.5 mm. The SSPP TL and the substrate are the same as those here is used to convert surface waves into radiating space waves. The distance between Figure 1. The circular patches are interlacing, placed at the two sides of the SSPP TL. The patch-loading shown in Figure 1. the Themetallic circularstrip patches are interlacing, placed at the twobased sides on of the TL. Thefor the patches and is related to the couple which is set the SSPP requirement here is used to convert surface waves intowaves radiating waves. The distance the patches patch-loading here isThe used to convert radiating space waves. The distance between radiation pattern. diameter of surface the circle patchinto is Dspace = 10 mm, which makes thebetween radius equal to one and the of metallic strip is related to the couple which is set based on the requirement for radiation thequarter patches and the metallic strip is related to the couple which is set based on the requirement forthe the waveguide wavelength, approximately. The period is d = 20 mm, which is just pattern. The diameter of the circle patch is D = 10 mm, which makes the radius equal to one quarter radiation pattern. diameter of the circle patch on is D = 10 mm, which makes the radius one distance of two The of the circle patches. The offset the both sides of patches is delta = 10equal mm, to it equals of the waveguide wavelength, approximately. The period is d = 20 mm, which is just the distance quarter of the waveguide wavelength, approximately. The period is d = 20 mm, which is just the of half of the period d, which will discussed in the following. Real copper was chosen with an electrical two of the circle patches. The offset on the both sides of patches is delta = 10 mm, it equals half of the 7 distance of two of the circle patches. The offset on the both sides of patches is delta = 10 mm, it equals conductivity of 5.96×10 S/m. All the patches are t = 0.5 mm away from the TL to couple the energy period d,period which discussed in the following. Real copper was chosen with an electrical half of the d,will which will discussed in the following. Real copper was chosen with anconductivity electrical from SSPP TL properly. 7 S/m. of 5.96 × 10 S/m. All the patches are t = 0.5 away from thefrom TL tothe couple energy SSPP conductivity of7 5.96×10 All the patches aremm t = 0.5 mm away TL tothe couple thefrom energy TL properly. from SSPP TL properly.
Figure 6. The structure of the SSPP leaky-wave antenna. D = 10 mm, delta = 10 mm, d = 20 mm, t = 0.5 mm, ws = 4 mm, gap = 0.5 mm. Figure 6. The structure ofSSPP the SSPP leaky-wave antenna. = 10 mm, delta = d10= mm, d =t =200.5 mm, Figure 6. The structure of the leaky-wave antenna. D = 10D mm, delta = 10 mm, 20 mm, t =ws 0.5= mm, ws = 4=mm, gap =theory, 0.5 mm.numerous spatial harmonics are ignited in the antenna, and the mm, 4 mm, 0.5 mm. According togap the Floquet
phases constants are: According to the Floquet theory, numerous spatial harmonics ignited in the antenna, According to the Floquet theory, numerous spatial harmonics areare ignited in the antenna, andand thethe 2𝑛𝜋 phases constants are: (3) 𝛽𝑛 = 𝛽0 + , 𝑛 = 0, ±1, ±2 … phases constants are: 𝑑 2nπ β n = β 0 +2𝑛𝜋 , n = 0, ±1, ±2 . . . (3) d, 𝑛 Equation where β0 is the fundamental mode (3)…shows that the space harmonics (3) are 𝛽𝑛 =phase 𝛽0 + constant. = 0, ±1, ±2 𝑑 where is the for fundamental mode phase Equation (3) shows harmonics all all slowβ0waves n ≥ 0, meanwhile, for nconstant. < 0, the space harmonics canthat be the fastspace waves. In order are to get where β 0 is the fundamental mode phase constant. Equation (3) shows that the space harmonics are slowmaximum waves forradation n ≥ 0, meanwhile, < 0, theaspace can be the fast nwaves. In order to get the the efficiency for andn obtain singleharmonics radiated beam, = −1 space harmonic is all usually slow waves forton convert ≥ efficiency 0, meanwhile, for n
