sensors Article Multibeam Reflectarrays in Ka-Band for Efficient Antenna Farms Onboard Broadband Communication Satellites † Daniel Martinez-de-Rioja 1,* , Eduardo Martinez-de-Rioja 2 , Yolanda Rodriguez-Vaqueiro 3, Jose A. Encinar 1 and Antonio Pino 3 1 Information Processing and Telecommunications Center, Universidad Politécnica de Madrid, 28040 Madrid, Spain; [email protected] 2 Area of Signal Theory and Communications, Universidad Rey Juan Carlos, 28942 Madrid, Spain; [email protected] 3 AtlanTTic Research Center, Universidade de Vigo, 36310 Vigo, Spain; [email protected] (Y.R.-V.); [email protected] (A.P.) * Correspondence: [email protected] † This paper is an extended version of the paper entitled “Advanced Reflectarray Antennas for Multispot Coverages in Ka Band” presented at the 2020 International Workshop on Antenna Technology (iWAT 2020). Abstract: Broadband communication satellites in Ka-band commonly use four reflector antennas to generate a multispot coverage. In this paper, four different multibeam antenna farms are proposed to generate the complete multispot coverage using only two multibeam reflectarrays, making it possible to halve the number of required antennas onboard the satellite. The proposed solutions include flat and curved reflectarrays with single or dual band operation, the operating principles of which have been experimentally validated. The designed multibeam reflectarrays for each antenna farm have been analyzed to evaluate their agreement with the antenna requirements for real satellite scenarios in Ka-band. The results show that the proposed configurations have the potential to reduce the number of antennas and feed-chains onboard the satellite, from four reflectors to two reflectarrays, enabling a significant reduction in cost, mass, and volume of the payload, which provides a considerable benefit Citation: Martinez-de-Rioja, D.; for satellite operators. Martinez-de-Rioja, E.; Rodriguez-Vaqueiro, Y.; Encinar, J.A.; Keywords: reflectarray antennas; multibeam antennas; dual band reflectarrays; communication Pino, A. Multibeam Reflectarrays in satellites; Ka-band Ka-Band for Efficient Antenna Farms Onboard Broadband Communication Satellites . Sensors 2021, 21, 207. https://doi.org/10.3390/s21010207 1. Introduction In the past years, the continental contoured beams traditionally used for broadcast Received: 25 November 2020 Accepted: 28 December 2020 satellite applications in Ku-band are being replaced by cellular coverages to provide broad- Published: 31 December 2020 band services typically in Ka-band [1]. The cellular coverages are formed by between 50 and 100 slightly overlapping spot beams, generated with a frequency and polarization Publisher’s Note: MDPI stays neu- reuse scheme of four colors, where each color is associated to a unique combination of tral with regard to jurisdictional clai- frequency and polarization [2]. Thus, the four-color scheme requires splitting the available ms in published maps and institutio- user spectrum into two different frequency sub-bands and two orthogonal circular polariza- nal affiliations. tions (CP) [2,3]. The generation of four-color coverages makes it possible to spatially isolate the spots generated with same frequency and polarization, so the interference between spots is reduced and the throughput of the users can be increased without modifying the operational bandwidth of the system. The high-gain beams generated by the satellite Copyright: © 2020 by the authors. Li- antennas produce circular spots on the Earth’s surface. The directions of the beams are censee MDPI, Basel, Switzerland. adjusted to produce a triangular lattice of circular spots. Therefore, the service area can This article is an open access article be split into hexagonal cells, each of which is covered by a circular beam [2]. Figure1 distributed under the terms and con- shows an example of a four-color coverage based on hexagonal cells. The diameter of the ditions of the Creative Commons At- spots is defined depending on the capacity need of the system. Typically, the spots cover tribution (CC BY) license (https:// creativecommons.org/licenses/by/ a circular area of around 250–300 km in diameter, which corresponds to a beamwidth of ◦ 4.0/). about 0.5–0.65 for the beams produced by the satellite antennas [3]. Sensors 2021, 21, 207. https://doi.org/10.3390/s21010207 https://www.mdpi.com/journal/sensors Sensors 2021, 21, x FOR PEER REVIEW 2 of 18 spots cover a circular area of around 250–300 km in diameter, which corresponds to a Sensors 2021, 21, 207 beamwidth of about 0.5–0.65° for the beams produced by the satellite antennas [3]. 2 of 17 (a) (b) (c) FigureFigure 1. 1. ExampleExample of aa four-colorfour-color multispot multispot coverage. coverage. (a )( Hexagonala) Hexagonal service service cells cells working working in four in different four different colors. colors. (b) Hexago- (b) Hexagonalnal cells covered cells covered by circular by beamscircular generated beams generated in two orthogonal in two orthogonal polarizations polarizations (P1, P2) and two(P1,frequencies P2) and two (F1, frequencies F2). (c) Example (F1, F2).of a(c European) Example four-color of a European coverage four-color that would coverage be generated that would by abe geostationary generated by satellite. a geostationary satellite. TheThe generation generation of of the the multispot multispot coverage coverage from from the the satellite satellite is is typically typically accomplished accomplished byby using using four four multi-fed multi-fed single single offset offset reflectors reflectors operating operating by by a asingle single feed feed per per beam beam (SFPB) (SFPB) architecturearchitecture [3,4]. [3,4]. Reflectors Reflectors offer offer a areliable reliable and and simple simple operation; operation; however, however, they they cannot cannot generategenerate the the beams beams with anan angularangular separation separation between between adjacent adjacent beams beams as small as small as required as re- quiredfor this for application, this application, since since overlapping overlapping feeds feeds would would be required.be required. Thus, Thus, four four multi-fed multi- fedreflectors reflectors are are usually usually employed employed onboard onboard the th satellite,e satellite, where where each each reflector reflector produces produces all allthe the beams beams in in a specifica specific frequency frequency and and polarization, polarization, which which are are spatially spatially isolated isolated from from each eachother. other. In this In way,this way, each reflectoreach reflector generates gene therates beams the beams associated associated to one colorto one in color a lattice in a of latticenon-contiguous of non-contiguous spots (the spots spots (the are spots separated are separated by gaps thatby gaps will bethat covered will be by covered the beams by thegenerated beams generated by the other by the reflectors). other reflectors). Therefore, Therefore, the interlaced the interlaced beams produced beams produced by the four by thereflectors four reflectors form the form final the four-color final four-color cellular cellular coverage coverage of contiguous of contiguous spots. spots. DueDue to to the the severe severe constraints constraints in inweight weight and and volume volume of satellites, of satellites, the theuse useof four of fourre- flectorsreflectors can can be beseen seen as asa suboptimal a suboptimal antenna antenna farm. farm. Different Different multibeam multibeam antenna antenna solutions solutions havehave been been proposed proposed lately lately to reduce to reduce the thenumb numberer of antennas of antennas onboard onboard the satellite the satellite [5]. The [5]. useThe of use lenses of lenses [6] or [6array] or arrayantennas antennas [7] reduce [7] reducess the radiation the radiation efficiency efficiency and increases and increases the complexitythe complexity of the of antenna the antenna architecture, architecture, while while the highly the highly oversized oversized reflector reflector proposed proposed in [8]in comprises [8] comprises a prohibitive a prohibitive stowage stowage volume. volume. In this In paper, this paper, four fourdifferent different multibeam multibeam an- tennaantenna solutions solutions are proposed are proposed based based on reflectarray on reflectarray antennas antennas [9], following [9], following the study the study in- troducedintroduced in [10]. in [10 Each]. Each multibeam multibeam reflectarray reflectarray is isintended intended to to generate generate half half the the required required multispotmultispot coverage, coverage, making making it itpossible possible to to reduce reduce the the number number of of antennas antennas onboard onboard the the geostationarygeostationary satellite satellite from from four four reflectors reflectors to to two two reflectarrays. reflectarrays. The The antenna antenna specifications specifications commonlycommonly required required in in real real scenarios scenarios (described (described in in Section Section 2)2) will will be be used used to to analyze analyze the the performanceperformance of of the the proposed proposed reflectarrays. reflectarrays. In InSection Section 3, a3 ,1.8 a 1.8m flat m flat reflectarray reflectarray and and a 1.8 a m1.8 parabolic m parabolic reflectarray reflectarray will be will proposed be proposed to generate to generate a four-color a four-color coverage coverage only onlyfor the for transmissionthe transmission link linkwith with a single a