This paper presents two slotted array antennas working in the Ka-band with switchable circular polarization capability. The first prototype is a series-fed slotted-waveguide linear array composed of ten T-shaped slots. The second antenna is a two-dimensional array with 2x2 T-shaped slots fed by a corporate distribution network. In both cases, a reflection coefficient below -10 dB has been experimentally observed within the targeted frequency band between 29.5 and 30.5 GHz. Good polarization purity is achieved for both polarization senses and in both prototypes. The fundamental contribution of the paper is to propose a simple mechanism to switch the circular polarization sense in a low-cost, low-profile and high-efficient antenna. The design and experimental results confirm that the solution is suitable for both one- and two-dimensional arrays in the millimeter-wave band

Ferrando-Rocher, Miguel

English

Repositorio Institucional de la Universidad de Alicante

1Mecanismo para conmutar el sentido de lapolarización circular en antenas ranuradas en labanda KaMiguel Ferrando-RocherAbstract—This paper presents two slotted array antennasworking in the Ka-band with switchable circular polarizationcapability. The first prototype is a series-fed slotted-waveguidelinear array composed of ten T-shaped slots. The second antennais a two-dimensional array with 2×2 T-shaped slots fed bya corporate distribution network. In both cases, a reflectioncoefficient below −10 dB has been experimentally observedwithin the targeted frequency band between 29.5 and 30.5 GHz.Good polarization purity is achieved for both polarization sensesand in both prototypes. The fundamental contribution of thepaper is to propose a simple mechanism to switch the circularpolarization sense in a low-cost, low-profile and high-efficientantenna. The design and experimental results confirm that thesolution is suitable for both one- and two-dimensional arrays inthe millimeter-wave band.Index Terms—Array, Dual-Circular Polarization, Gap Waveg-uide, Ka-Band, SATCOM.I. INTRODUCTIONNEW Ka-band high throughput satellite (HTS) systemsmake use of multibeam solutions to cover continentalregions. For example, around 80 beams are planned for theEuropean continent alone [1] with orthogonal circular polar-ization and frequency diversity. In mobile end-user systems,such as trains or airplanes, wideband devices capable ofswitching polarization sense are strongly required [2], [3].In this context, today exists a significant demand for higher-gain, lower-profile, simpler, cheaper solutions and easy tomanufacture up to millimeter-wave frequencies. Due to suchneed, it is not difficult to find recent contributions address-ing the problem from different perspectives. In general, twomethodologies are commonly followed. On the one hand,reconfigurable radiators as in [4], [5], [6] and, on the otherhand, complex reconfigurable feeding networks as in [7], [8],[9]. In general terms, these patch-based contributions sufferfrom low antenna efficiency.Some recent contributions comply with various of the abovementioned requirements. For example, [10], [11] propose all-metal structures to play the role of polarization converters,with an evident benefit in terms of antenna efficiency andreliability. However, these kind of solutions lead to bulky struc-tures, being impractical for the lowest band of the millimeter-wave spectrum. Very recently, in [12] an all-metal CP radiatingelement, able to convert the LP field radiated by a rectangularslot into a CP field in a very low-profile structure is presented.However, while all these designs achieve circular polarization(*) This text is a draft version.in high efficiency antennas, they do not provide switchingcapability. Finally, it should be noted that certainly there existsome interesting approaches that achieve circular polarizationswitching either mechanically [13], [14] or electronically [15],[16]. Yet, architectures providing circular polarization switch-ing is still a major challenge in Ka-band[17], [18], [19].Here, the concept described in [12] is extended to me-chanically switch between both rotation senses of circularpolarization in a very simple way. While [12] is the startingpoint of the approach, two key issues are provided in thiscontribution: firstly, the experimental validation of a circularpolarization switching mechanism in a linear array. Secondly,and more importantly, the idea is described and proved tobe extensible to bi-dimensional arrays. This fact is not minorsince the ultimate goal of this mechanism would be to useit in high-efficiency bi-dimensional flat panel antennas (FPA)for satellite communication (SATCOM) applications, whichnormally require a switchable circular polarization. Suchswitching need not be instantaneous and can be mechanical,such as the solution described here.The paper is structured as follows. Section II describeseach antenna in detail in two separate subsections: one for aseries-fed antenna and the other for a corporate-fed one. Theexperimental validation of the proposed concept is reported forboth manufactured prototypes in Section III. Finally, SectionIV draws the main conclusions.II. SWITCHABLE T-SHAPE SLOTThis section will describe two slotted waveguide antennas, a1×10 array and a 2×2 array. In both cases the same switchingmechanism is used.A. 1-D Array designBasically, the 1-D array consists of 10 series-fed T-shapedslots. A very preliminary description of this antenna was firstlyused in [20] but without any experimental validation. There,some simulated results can be consulted. A complete descrip-tion of the antenna is given here, providing the dimensionsof all relevant parameters. In addition, the antenna is nowfabricated and experimentally measured.Fig. 1a shows the radiating layer of the antenna. This layeris composed of two pieces: one fixed and the other movable.The rear side of the fixed piece is a conventional shunt slotarray, which feeds the T-shaped slots drilled in the upper face.The movable piece then plays a key role. Depending on the2(a)(b)Fig. 1: 3D sketch of the 1×10 array antenna. (a) Left figure shows both lower and upper pieces of the antenna in an explodedview. The upper piece is composed of a fixed and a movable part, with different color for an easier identification. Top andbottom view are included in the right figure. (b) Complete assembled antenna (top and side views) with two zoomed figuresshowing the switching positions.position to which it is shifted, the wider coupled arm of theT-shape will be off-center, on the left or on the right side,with respect to the feeding rectangular slot. In addition, the lidis fed through a groove gap waveguide (see Fig. 1b), whichconfines the field within the waveguide even though it hasno contact with the top cover, as demonstrated in [21]-[22].Observe how, in the side view of the antenna (Fig. 1b), thereexists an air gap (ag=556µm) between the nails and the lid.The nails of the bed are 3 mm in height, their side is 1.25 mmwide and they are periodically spaced by 2.4 mm. The entirecontour of the GGW is surrounded by 3 rows of these nails.The excitation of the waveguide is done through a port at thebottom of the antenna that corresponds to a standard WR−28flange. The distance from the center of this port to the shortof the waveguide is approximately λg/2. The cover with thetwo metallic pieces, the fixed and the movable ones, is placedon top of this GGW.A zoomed detail of the mechanism is shown in the right sideof Fig. 1b. It can be seen how the comb can mechanicallyTABLE I: Dimensions of the 1×10 slot array.Parameter Value (mm) Parameter Value (mm)dg 97.3 lg 18.1w1 3.0 l1 12.0w2 3.6 l2 10.9ds 5.5 lp 5.9dt 14.0 Lt 5.1da 21.8 Lu 6.1switch from one position to the other. The thickness of thefixed piece is 3.5 mm. One side of the fixed piece is emptiedto leave space to accommodate the movable piece. Then, thethickness of the moving piece is 1.75 mm and it has a widthof 5.9 mm. This metal tongue holds the comb spikes, whichare spaced 7 mm apart. Finally, all relevant geometrical valuesof this antenna are shown in Table I.B. 2-D Array designNow, it is presented how to adapt the mechanism for notonly one-dimensional arrays but also for two-dimensional3(a)Fig. 2: 3D sketch of the 2×2 array antenna. From left to right, the first figure shows the complete assembled antenna. There, themovable piece is located in position 1. Second figure also shows the assembled antenna but with movable piece in position 2.In the third figure the movable piece has been removed for a better identification and visualization of the lid. The two rightmostfigures correspond to the coaxial-fed cavity-backed slots and the input port through WR-28 standard flange, respectively.(a) (b) (c) (d)Fig. 3: Manufactured prototypes: (a) complete 1×10 manufactured antenna. The movable piece of the lid has been placedaside for better visualization of the fixed lid; (b) fully assembled 1×10 slot array; (c) complete 2×2 manufactured antennawith the cover slipped out and (d) top view of the 2×2 antenna.TABLE II: Dimensions of the 2×2 slot array.Parameter Value (mm) Parameter Value (mm)Dt 9.0 Lc 2.0Dc 1.0 Lu 7.1Ds 5.7 Lz 9.0Dg 26.0 Lg 26.0ht 2.0 Lb 2.929.6 29.8 30 30.2 30.40123Frequency (GHz)AxialRatio(dB)Sim. RHCPSim. LHCPMeas. RHCPMeas. LHCP(a)29.6 29.8 30 30.2 30.4−40−30−20−100Frequency (GHz)ReflectionCoeff.(dB)Sim. Pos.1Sim. Pos.2Meas. Pos.1Meas. Pos.2(b)Fig. 4: Results of the 1×10 series-fed array for each pieceposition: (a) Measured and simulated axial ratio and (b)measured and simulated reflection coefficient.ones. Note that the lid described for the series-fed array isfully valid to transform the linear polarization into a circularone and, at the same time, to switch between both circularpolarization senses, RHCP and LHCP. Usefully, just a smallmodification is enough to adapt the idea for a two-dimensionalarray. Fig. 2 shows from left to right the complete cover, thefixed and the movable part, respectively. Obviously, being a29 30 310123Frequency (GHz)AxialRatio(dB)Sim. RHCPSim. LHCPMeas. RHCPMeas. LHCP(a)29 30 31 32−40−30−20−100Frequency (GHz)ReflectionCoeff.(dB)Sim. Pos.1Sim. Pos.2Meas. Pos.1Meas. Pos.2(b)Fig. 5: Results of the 2×2 corporate-fed array antenna for eachpiece position: (a) Measured and simulated axial ratio and (b)measured and simulated reflection coefficient.two-dimensional array in this case, two combs are needed. Ametal arm connects both combs making the moving part U-shaped. All relevant dimensions of this new part are indicatedagain, and are detailed in the Table II.The fourth sketch of Fig. 2 shows the cavities that arebacking the slots. It consists of 4 coaxial cavities excited bya central groove which in turn is fed by a slot coupled fromthe input port. This type of coaxial cavities in gap waveguidetechnology have been used and fully described in the past in[23], [24].III. EXPERIMENTAL VALIDATIONBoth fabricated prototypes are shown in several picturesin Fig. 3. Likewise, both prototypes have been fabricated in-41802252703153604590135-40-30-20-100XZ-planeRHCPφ (deg)Position 129.5 GHz 30.0 GHz 30.5 GHz(a)1802252703153604590135-40-30-20-100YZ-planeRHCPφ (deg)Position 129.5 GHz 30.0 GHz 30.5 GHz(b)1802252703153604590135-40-30-20-100XZ-planeLHCPφ (deg)Position 229.5 GHz 30.0 GHz 30.5 GHz(c)1802252703153604590135-40-30-20-100YZ-planeLHCPφ (deg)Position 229.5 GHz 30.0 GHz 30.5 GHz(d)Fig. 6: Measured radiation patterns of the 1×10 array in apolar representation. Upper plots correspond to position 1(RHCP), lower plots to position 2 (LHCP): (a) XZ plane and(b) YZ plane, (c) XZ plane and (d) YZ plane.house in the Antennas and Propagation Laboratory facilitieswith a milling machine. The unique material used in both casesis AL7079 aluminum. Being the two prototypes fully metallic,high radiation efficiencies, above 80% in both cases, have beenmeasured. Table III shows the measured gain for both antennasand for both switching positions at five frequency points withinthe band. Anyhow, the main objective of this study is to test thefeasibility of circular polarization switching, which is verifiedin Figs. 4 to 7. Fig. 4 shows the measured and simulated axialratio (AR) in the band of interest. Good polarization purityis obtained for both positions, with a maximum measuredAR less than 2.5 dB in either case. It can be observed howthe purest circular polarization occurs around 29.75 GHz,when a minimum AR centered at 30 GHz was expected. Thisdisplacement of the AR is mainly due to the effect of thescrews used to fix the bottom and top pieces. These screwswould presumably have a minor effect on a larger array.Fig. 4b shows the measured reflection coefficient below the−10 dB threshold.Next, Fig. 5 shows the measurements for the 2×2 antenna.The conclusions that can be drawn from these experimentalresults are similar to those for the series-fed antenna. The ra-diation patterns measured in the main cuts, XZ and YZ planes,are presented in Figs. 6 and 7. Note that thick lines correspondto co-polar component while thin lines to crosspolar ones.The directivity, beam-width and stability of the patterns at thesampled frequencies are as good as expected, thus validating1802252703153604590135-40-30-20-100XZ-planeRHCPφ (deg)Position 129.5 GHz 30.0 GHz 30.5 GHz(a)1802252703153604590135-40-30-20-100YZ-planeRHCPφ (deg)Position 129.5 GHz 30.0 GHz 30.5 GHz(b)1802252703153604590135-40-30-20-100XZ-planeLHCPφ (deg)Position 229.5 GHz 30.0 GHz 30.5 GHz(c)1802252703153604590135-40-30-20-100YZ-planeLHCPφ (deg)Position 229.5 GHz 30.0 GHz 30.5 GHz(d)Fig. 7: Measured radiation patterns of the 2×2 array in a polarrepresentation. Upper plots correspond to position 1 (RHCP),lower plots to position 2 (LHCP): (a) XZ plane and (b) YZplane, (c) XZ plane and (d) YZ plane.TABLE III: Measured gain in dBi of both antennas for bothswitching positions.Frequency (GHz) 29.5 29.75 30.0 30.25 30.51×10 array (Pos.1) 16.12 16.69 16.85 16.97 16.831×10 array (Pos.2) 16.11 16.45 15.76 16.85 16.692×2 array (Pos.1) 12.96 12.85 12.72 12.52 12.352×2 array (Pos.2) 12.75 12.61 12.51 12.23 12.11the performance of both antennas.IV. CONCLUSIONSThis paper presents two slotted array antennas operating inKa-band with switchable circular polarization capability. Theproposed mechanism allows easy switching between RHCPand LCHP and vice versa. Experimental results showed anaxial ratio consistently below 3 dB from 29.5 GHz to 30.5 GHzfor both switching positions in the two aluminum prototypes.Future lines of work should integrate the concept into a largerarray as well as incorporate a motor to handle the mechanicalswitching.V. ACKNOWLEDGEMENTSThis work has been funded by the Spanish Gover-ment, Agencia Estatal de Investigacion (AEI), under projectPID2019-107688RB-C22.5REFERENCES[1] H. Fenech, A. Tomatis, S. Amos, V. Soumpholphakdy, and J. Ser-rano Merino, “Eutelsat hts systems,” International Journal of SatelliteCommunications and Networking, vol. 34, no. 4, pp. 503–521, 2016.[2] S. S. Gao, Q. Luo, and F. Zhu, Introduction to circularly polarizedantennas. Wiley-IEEE Press, 2014.[3] P. Naseri, S. A. Matos, J. R. Costa, C. A. Fernandes, and N. J.Fonseca, “Dual-band dual-linear-to-circular polarization converter intransmission mode application to k/ka-band satellite communications,”IEEE Transactions on Antennas and Propagation, vol. 66, no. 12, pp.7128–7137, 2018.[4] M. Li, Y. Wu, W. Wang, and A. A. Kishk, “Wideband polarizationreconfigurable differential circularly polarized antenna,” IEEE Access,vol. 7, pp. 64 697–64 703, 2019.[5] W. Lin, S.-L. Chen, R. W. Ziolkowski, and Y. J. 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Mecanismo para conmutar el sentido de la polarización circular en antenas ranuradas en la banda Ka

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Mecanismo para conmutar el sentido de la polarización circular en antenas ranuradas en la banda Ka

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Repositorio Institucional de la Universidad de Alicante