Multibeam Reflectarrays in Ka-Band for Efficient Antenna Farms Onboard Broadband Communication Satellites

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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 antennaantenna 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 single antenna antenna aperture. aperture. Then, Then, a dual-reflectarray a dual-reflectarray system system and and a 1.8a 1.8m mparabolic parabolic reflectarray reflectarray will will be beshown shown in inSections Sections 4 4and and 5,5, respectively, respectively, to generategenerate halfhalf the the required required spots (two(two colors)colors) simultaneously simultaneously in in transmission transmission (Tx) (Tx) and and reception reception (Rx). (Rx).The The main main characteristics characteristics of the of four the four reflectarrays reflectarrays will bewill compared be compared in Section in Section6, proving 6, prov- the great potential of reflectarrays for multibeam satellite applications and their capability to ing the great potential of reflectarrays for multibeam satellite applications and their capa- halve the number of antennas and feed-chains required onboard the satellite to generate bility to halve the number of antennas and feed-chains required onboard the satellite to a four-color coverage. generate a four-color coverage. 2. Mission Scenario and Requirements of the Antenna System 2. Mission Scenario and Requirements of the Antenna System The main specifications of the cellular coverage, shown in Table1, have been estab- lished from those of current multibeam satellites in Ka-band [2,3,11,12] and the Rec. ITU-R S.672-4 [13]. The coverage must be formed by around 100 spots in a triangular lattice, Sensors 2021, 21, x FOR PEER REVIEW 3 of 18

The main specifications of the cellular coverage, shown in Table 1, have been estab- lished from those of current multibeam satellites in Ka-band [2,3,11,12] and the Rec. ITU- Sensors 2021, 21, 207 3 of 17 R S.672-4 [13]. The coverage must be formed by around 100 spots in a triangular lattice, generated with a four-color reuse scheme based on two different frequencies and two or- generatedthogonal with aCP. four-color The four-color reuse coverage scheme based must be on simultaneously two different frequencies generated at and Tx and two Rx user orthogonalfrequencies CP. The in four-color Ka-band, coverage which comprise must be from simultaneously 19.2 to 20.2 generated GHz for Tx at Txand and from Rx 29.0 to user frequencies30.0 GHz infor Ka-band, Rx. Thus, which both compriseTx and Rx from freq 19.2uency to bands 20.2 GHz must for be Tx divided and from into 29.0 two sub- to 30.0bands GHz forto provide Rx. Thus, the both four-color Tx and coverage Rx frequency with two bands orthogonal must be CP divided simultaneously into two in Tx sub-bandsand to Rx. provide Figure the 2 shows four-color a schematic coverage representa with two orthogonaltion of a multispot CP simultaneously coverage inwith Tx a four- and Rx.color Figure reuse2 shows scheme a schematic of two representationfrequencies (F1, of aF2) multispot and two coverage polarizations with a four-color(P1, P2) together reuse schemewith the of operating two frequencies scheme (F1, of F2)the andcurrent two mult polarizationsi-fed reflectors (P1, P2) used together onboard with the the satellite, operatingwhere scheme each of reflector the current generates multi-fed a lattice reflectors of non-contiguous used onboard spots the satellite, in a single where color, each simulta- reflectorneously generates in Tx a latticeand Rx of (the non-contiguous interlaced beams spots produced in a single by the color, four simultaneously reflectors form in the final Tx andfour-color Rx (the interlaced coverage beams of contiguous produced spots). by the The four di reflectorsameter of formthe spots the final is set four-color to 0.65° to cover coveragea circular of contiguous area of spots).around The 300 diameter km on the of theEarth. spots The is angular set to 0.65 separation◦ to cover between a circular adjacent area ofspots around from 300 center km on to the center Earth. to form The angular the approp separationriate triangular between lattice adjacent of spots spots must from be 0.56° center to(computed center to formas sin(60°)·0.65°). the appropriate The triangular minimum lattice end ofof spotscoverage must (EOC) be 0.56 gain◦ (computed of the beams is as sin(60set◦) ·to0.65 45◦ dBi,). The the minimum minimum end single-entry of coverage carrier (EOC) over gain interference of the beams ratio is set (C/I) to 45at dBi,20 dB, and the minimumthe minimum single-entry co-polar carrier over over cross-polar interference discrimination ratio (C/I) at(XPD 20 dB,) is andalso thefixed minimum to 20 dB. co-polar over cross-polar discrimination (XPD) is also fixed to 20 dB. Table 1. Antenna system requirements. Table 1. Antenna system requirements. Parameter Requirement ParameterNumber of spots Requirement 100 Number ofReuse spots scheme 100 4 colors Reuse schemeSpot lattice 4 colors Triangular Spot latticeSpot diameter Triangular 0.65° Spot diameter 0.65◦ Spot separation 0.56° Spot separation 0.56◦ EOC gainEOC gain 45 dBi 45 dBi Single-entrySingle-entry C/I C/I 20 dB 20 dB XPDXPD 20 dB 20 dB Tx frequencyTx frequency band band 19.2–20.2 19.2–20.2 GHz GHz Rx frequency band 29.0–30.0 GHz Rx frequency band 29.0–30.0 GHz

(a) (b)

Figure 2. (a) Four-colorFigure multispot 2. (a) Four-color coverage multispotrequired simultaneously coverage required in Tx simultaneously and Rx. (b) Operating in Tx and principle Rx. (b) of Operating current reflectors used onboardprinciple the satellite, of current operating reﬂectors simultaneously used onboard in theTx and satellite, Rx. operating simultaneously in Tx and Rx.

The proposedThe proposed antenna antenna configurations configurations operate operate by an improvedby an improved SFPB configuration, SFPB configuration, taking advantagetaking advantage of the ability of the ofability reflectarrays of reflectarrays to generate to generate independent independent beams atbeams different at different frequenciesfrequencies [14] or polarizations[14] or polarizations [15] with [15] a single with feed.a single The feed. proposed The proposed reflectarrays reflectarrays will be will illuminated by a cluster of 27 feeds defined with a triangular lattice, in order to provide the required multibeam coverage with a triangular grid of spots. The feeds have been modelled considering the 54 mm Ka-band feed-chain reported in [4], thus the separation between adjacent feeds has been set to 55 mm. The radiation pattern of the feeds has been Sensors 2021, 21, x FOR PEER REVIEW 4 of 18

be illuminated by a cluster of 27 feeds defined with a triangular lattice, in order to provide the required multibeam coverage with a triangular grid of spots. The feeds have been modelled considering the 54 mm Ka-band feed-chain reported in [4], thus the separation Sensors 2021, 21, 207 between adjacent feeds has been set to 55 mm. The radiation pattern of the feeds has4 been of 17 modelled in the simulations by an ideal cosq(θ) distribution. Figure 3 shows the proposed cluster of 27 feeds using the local coordinate system (xf, yf), the origin of which matches modelledthe center in of the the simulations aperture of by the an central ideal cos feedq(θ (C3) distribution. in Figure 3). Figure The3 symmetry shows the of proposed each an- clustertenna configuration of 27 feeds using will the be localused coordinateto reduce th systeme number (xf, y off), feeds the origin considered of which in matchessimulations the center(the lines ofthe of feeds aperture A, B, of C, the plotted central with feed thicke (C3 inr lines, Figure or3 ).the The array symmetry of 3x5 feeds of each plotted antenna with conﬁgurationsolid lines in Figure will be 3). used to reduce the number of feeds considered in simulations (the lines of feeds A, B, C, plotted with thicker lines, or the array of 3x5 feeds plotted with solid lines in Figure3).

FigureFigure 3.3. ClusterCluster ofof 2727 feedsfeeds deﬁneddefined toto illuminateilluminate thethe differentdifferent antennaantenna farms.farms.

3.3. Antenna FarmFarm BasedBased onon TwoTwo Single-BandSingle-Band ReflectarraysReflectarrays TheThe first first strategy strategy introduced introduced in in [10] [10 to] togenerate generate a complete a complete four-color four-color multispot multispot cov- coverageerage simultaneously simultaneously in Tx in Txand and Rx Rxis based is based on onthe the design design of ofa reflectarray a reflectarray antenna antenna to togenerate generate four four spaced spaced beams beams per per feed feed in intwo two different different operating operating frequencies frequencies and and two two or- orthogonalthogonal polarizations polarizations (i.e., (i.e., four four spaced spaced beams beams in in four four different different colors). In this way, thethe limitationlimitation of conventional conventional reflectors reflectors to to provid providee such such closely closely spaced spaced beams beams is overcome is overcome by by the generation of four adjacent beams per feed. Thus, a reflectarray illuminated by the the generation of four adjacent beams per feed. Thus, a reflectarray illuminated by the proposed cluster of 27 feeds would generate a complete four-color coverage of 108 spots proposed cluster of 27 feeds would generate a complete four-color coverage of 108 spots only for a single band (Tx or Rx). To provide multispot coverage both in Tx and Rx, two only for a single band (Tx or Rx). To provide multispot coverage both in Tx and Rx, two single-band reflectarrays designed with the same technique to produce four adjacent beams single-band reflectarrays designed with the same technique to produce four adjacent per feed would be required onboard the satellite (one for Tx and the other for Rx). The beams per feed would be required onboard the satellite (one for Tx and the other for Rx). operating scheme of the proposed solution is shown in Figure4 for a flat reflectarray to The operating scheme of the proposed solution is shown in Figure 4 for a flat reflectarray operate in the Tx band in Ka-band. to operate in the Tx band in Ka-band. This solution requires a reflectarray with independent operation at close frequencies (within the same band for Tx or Rx in Ka-band) and also independent operation in orthogonal polarizations, which increases the complexity of the reflectarray cells and the design technique. The controlled application of the beam squint effect with frequency has been used to reduce the design complexity of the reflectarray antenna, as shown in [16]. The method to generate four spaced beams in four different colors per feed has been experimentally validated in [17] for a 43 cm reflectarray antenna. The prototype operates in linear polarization (LP), but the same design technique can be applied to produce the beams in CP by an appropriate selection of the reflectarray cells. Figure5 shows the 43 cm prototype in the anechoic chamber and the measured radiation patterns of the four spaced beams in four different colors, according to the normalized angular coordinates u = sinθ·cosϕ, v = sinθ·sinϕ. Sensors 2021, 21, x FOR PEER REVIEW 5 of 18

Sensors 2021, 21, x FOR PEER REVIEW 5 of 18 Sensors 2021, 21, 207 5 of 17

Figure 4. Operating principle of the proposed single-band reflectarray.

This solution requires a reflectarray with independent operation at close frequencies (within the same band for Tx or Rx in Ka-band) and also independent operation in orthogonal polarizations, which increases the complexity of the reflectarray cells and the design technique. The controlled application of the beam squint effect with frequency has been used to reduce the design complexity of the reflectarray antenna, as shown in [16]. The method to generate four spaced beams in four different colors per feed has been experimentally validated in [17] for a 43 cm reflectarray antenna. The prototype operates in linear polarization (LP), but the same design technique can be applied to produce the beams in CP by an appropriate selection of the reflectarray cells. Figure 5 shows the 43 cm prototype in the anechoic chamber and the measured radiation patterns of the four spaced FigureFigurebeams 4. 4.in OperatingOperating four different principle principle colors, of of the the prop proposedaccordingosed single-band single-band to the normalized reflectarray. reﬂectarray. angular coordinates u = sinθ·cosφ, v = sinθ·sinφ. This solution requires a reflectarray with independent operation at close frequencies (within the same band for Tx or Rx in Ka-band) and also independent operation in orthogonal polarizations, which increases the complexity of the reflectarray cells and the design technique. The controlled application of the beam squint effect with frequency has been used to reduce the design complexity of the reflectarray antenna, as shown in [16]. The method to generate four spaced beams in four different colors per feed has been experimentally validated in [17] for a 43 cm reflectarray antenna. The prototype operates in linear polarization (LP), but the same design technique can be applied to produce the beams in CP by an appropriate selection of the reflectarray cells. Figure 5 shows the 43 cm prototype in the anechoic chamber and the measured radiation patterns of the four spaced beams in four different colors, according to the normalized angular coordinates u = sinθ·cosφ, v = sinθ·sinφ.

(a) (b)

FigureFigure 5.5. ReﬂectarrayReflectarray prototypeprototype toto generategenerate fourfour spacedspaced beamsbeams perper feedfeed [[17].17]. ((aa)) PicturePicture ofof thethe prototype,prototype, ((bb)) measuredmeasured patternpattern contourscontours atat −−1,1, −−3,3, and and −−4 dB4 dB of of the the four four beams beams generated generated by by the the same same feed. feed.

Two different approaches approaches have have been been evalua evaluatedted to toproduce produce a multispot a multispot coverage coverage fol- followinglowing the the design design technique technique developed developed in in [17]: [17]: first, first, using using a a flat flat reflectarray reflectarray as proposed in [10], [10], and and second, second, using using aa reflectarray reflectarray with with a parabolic a parabolic surface. surface. The The two two reflectarrays reflectarrays are areexpected expected to generate to generate a four-color a four-color coverage coverage for Tx for in TxKa-band in Ka-band using the using feed the cluster feed clustershown shownin Figure in 3. Figure Due to3. the Due symmetry to the symmetry of the antenna of the system, antenna the system, simulations the simulations have considered have consideredthe lines of thefeeds lines A, B, of and feeds C A,in B,Figure and C3 (plo in Figuretted with3 (plotted thicker with lines thicker in Figure lines 3), in since Figure there3), since there will be minimal differences between the beams generated by the lines of feeds B, C and D, E. The two reflectarrays have a diameter of 1.8 m and operate at 19.45 and (a) 19.95 GHz in orthogonal polarizations. The( simulationsb) have considered ideal reflectarray Figure 5. Reflectarray prototypeelements to generate to provide four spaced the required beams per phase-shift feed [17]. in(a) each Picture color of the without prototype, phase (b errors) measured or ohmic pattern contours at −1, −3, andlosses. −4 dB The of the preliminary four beams results generated will by be the used same to evaluatefeed. the strengths and weaknesses of the design method. The conclusions reached in this study can also be applied to the associated reflectarraysTwo different used forapproaches Rx, since have the design been evalua methodted would to produce be identical. a multispot coverage following the design technique developed in [17]: first, using a flat reflectarray as proposed 3.1. Flat Transmit Reflectarray to Generate Four Spaced Beams per Feed in Ka-Band in [10], and second, using a reflectarray with a parabolic surface. The two reflectarrays are expectedA flat to reflectarray generate a four-color has been proposed coverage to for generate Tx in Ka-band four spaced using beams the feed per feedcluster according shown into Figure the design 3. Due technique to the symmetry shown in of [17 the]. antenna The reflectarray system, the has simulations a diameter ofhave 1.8 considered m and it is theformed lines byof feeds 44125 A, reflectarray B, and C in cells Figure arranged 3 (plotted in a with 239 × thicker235 lattice, lines thein Figure period 3), of since which there has Sensors 2021, 21, x FOR PEER REVIEW 6 of 18

will be minimal differences between the beams generated by the lines of feeds B, C and D, E. The two reflectarrays have a diameter of 1.8 m and operate at 19.45 and 19.95 GHz in orthogonal polarizations. The simulations have considered ideal reflectarray elements to provide the required phase-shift in each color without phase errors or ohmic losses. The preliminary results will be used to evaluate the strengths and weaknesses of the design method. The conclusions reached in this study can also be applied to the associated reflectarrays used for Rx, since the design method would be identical.

3.1. Flat Transmit Reflectarray to Generate Four Spaced Beams per Feed in Ka-Band

Sensors 2021, 21, 207 A flat reflectarray has been proposed to generate four spaced beams per6 offeed 17 according to the design technique shown in [17]. The reflectarray has a diameter of 1.8 m and it is formed by 44125 reflectarray cells arranged in a 239 × 235 lattice, the period of which been sethas to 7.5 been mm set to to avoid 7.5 mm grating to avoid lobes. grating As a result lobes. of As the a designresult of method, the design four method, different four dif- phase distributionsferent phase are distributions implemented are in implemented the reflectarray in the surface reflectarray (one for surface each combination (one for each com- of frequencybination and of polarization). frequency and Figure polarization).6 shows the Figure required 6 shows phase the distributions required phase at 19.45 distributions and 19.95at GHz 19.45 for and one 19.95 CP (there GHz arefor noone appreciable CP (there differencesare no appreciable with the phasedifferences distributions with the phase for the orthogonaldistributions CP). for The the largeorthogonal size of CP). the reflectarrayThe large size and of its the flat reflectarray configuration and areits flat the configu- reason whyration the are phase the distributionsreason why the depicted phase in distribu Figure6tions show depicted a large numberin Figure of 6 360 show◦ cycles. a large num- The abruptber variationsof 360° cycles. in the The phase abrupt distributions variations limit in the the phase antenna distributions performance limit and the reduce antenna per- the operationalformance bandwidth. and reduce the operational bandwidth.

(a) (b) Figure 6. RequiredFigure phase 6.distributionsRequired phase of the distributions 1.8 m flat reflectarray of the 1.8 mat ﬂatthe reﬂectarray(a) lower and at ( theb) upper (a) lower operating and (b )frequencies upper for the same CP. operating frequencies for the same CP.

The resultsThe of results the conducted of the conducted simulations simulations when the 1.8when m reﬂectarraythe 1.8 m reflectarray is illuminated is illuminated by the 16 feedsby the placed 16 feeds in the placed lines A, in B, the and lines C in A, Figure B, and3 are C shownin Figure in Figure 3 are 7shown, which in presents Figure 7, which the patternpresents contours the ofpattern the beams contours for a 46of the dBi beams gain. The for simulateda 46 dBi gain. pattern The contours simulated show pattern a con- coveragetours of 64 show beams a coverage generated of in 64 a beams four-color generated reuse scheme.in a four-color The distribution reuse scheme. of beams The distribu- Sensors 2021, 21, x FOR PEER REVIEW 7 of 18 meets thetion required of beams triangular meets the lattice required of spots, triangular as well aslatti thece separationof spots, as and well diameter as the separation of the and ◦ ◦ circular spotsdiameter (0.56 of andthe circular 0.65 , respectively). spots (0.56° and 0.65°, respectively).

Figure 7. SimulatedFigure 7. 46 Simulated dBi pattern 46 contoursdBi pattern of thecontours 64 beams of the generated 64 beams by generated 16 feeds by in 16 four feeds different in four colors different with a 1.8 m ﬂat reﬂectarray.colors with a 1.8 m flat reflectarray.

The main cutsThe of the main radiation cuts of thepatterns radiation have patterns been evaluated have been to estimate evaluated the to interfer- estimate the interfer- ences betweenences beams between generated beams in generated the same inco thelor. sameFigure color. 8 shows Figure the8 shows horizontal the horizontal cut of cut of the the radiation radiationpatterns in patterns the plane in thev = plane0 for the v = beams 0 for the generated beams generatedat both frequencies. at both frequencies. Due Due to to the monofocalthe monofocal antenna design, antenna the design, extreme the beams extreme of beamsthe coverage of the coverageshow some show aberra- some aberrations tions that increasethat increase the C/I. the Since C/I. the Since side-l theobe side-lobe levels levelsof the ofbeam thes beams can be can reduced be reduced by by slightly slightly increasingincreasing the antenna the antenna aperture aperture and andreducing reducing the theedge edge illumination, illumination, this this draw- drawback can be back can be easily overcome (space reflectors in Ka-band usually have a diameter of 2.3 m). Thus, the diameter of 1.8 m has been maintained for the rest of the reflectarray antennas proposed in this paper, expecting similar C/I levels. The use of ideal reflectarray cells prevents an accurate characterization of the cross-polar radiation, which is mainly produced by the reflectarray elements and the offset antenna configuration. Previous reflectarray prototypes with offset configurations have provided XPD above 25 dB [17,18]. The main constraint of this strategy is the operational bandwidth of the reflectarray, limited by the abrupt phase variations shown in Figure 6 and the independent operation at two close frequencies within the same link (Tx or Rx) in Ka-band.

(a) (b) Figure 8. Cut of the simulated radiation patterns in v = 0 for the beams generated at the (a) lower and (b) upper frequency in the same circular polarization (CP).

3.2. Parabolic Transmit Reflectarray to Generate Four Spaced Beams per Feed in Ka-Band Sensors 2021, 21, x FOR PEER REVIEW 7 of 18

Figure 7. Simulated 46 dBi pattern contours of the 64 beams generated by 16 feeds in four different colors with a 1.8 m flat reflectarray.

The main cuts of the radiation patterns have been evaluated to estimate the interfer- ences between beams generated in the same color. Figure 8 shows the horizontal cut of the radiation patterns in the plane v = 0 for the beams generated at both frequencies. Due to the monofocal antenna design, the extreme beams of the coverage show some aberrations that increase the C/I. Since the side-lobe levels of the beams can be reduced by slightly increasing the antenna aperture and reducing the edge illumination, this draw- Sensors 2021, 21, 207 back can be easily overcome (space reflectors in Ka-band usually have a diameter of7 of2.3 17 m). Thus, the diameter of 1.8 m has been maintained for the rest of the reflectarray antennas proposed in this paper, expecting similar C/I levels. The use of ideal reflectarray cells preventseasily overcome an accurate (space characterization reflectors in Ka-band of the cross-polar usually have radiation, a diameter which of 2.3 is m). mainly Thus, pro- the duceddiameter by ofthe 1.8 reflectarray m has been elements maintained and the for theoffset rest antenna of the reflectarrayconfiguration. antennas Previous proposed reflec- tarrayin this prototypes paper, expecting with offset similar configurations C/I levels. The have use provided of ideal XPD reflectarray above 25 cells dB prevents[17,18]. an accurateThe main characterization constraint of of this the strategy cross-polar is the radiation, operational which bandwidth is mainly of produced the reflectarray, by the reflectarray elements and the offset antenna configuration. Previous reflectarray prototypes limited by the abrupt phase variations shown in Figure 6 and the independent operation with offset configurations have provided XPD above 25 dB [17,18]. at two close frequencies within the same link (Tx or Rx) in Ka-band.

(a) (b)

Figure 8. CutCut of of the the simulated simulated radiation radiation patterns in v = 0 0 for for the the beams beams generated generated at at the the ( (aa)) lower lower and and ( (bb)) upper upper frequency frequency inin the the same same circular polarization (CP).

3.2. ParabolicThe main Transmit constraint Reflectarray of this strategy to Generate is the Four operational Spaced Beams bandwidth per Feed ofin theKa-Band reﬂectarray, limited by the abrupt phase variations shown in Figure6 and the independent operation at two close frequencies within the same link (Tx or Rx) in Ka-band.

3.2. Parabolic Transmit Reflectarray to Generate Four Spaced Beams per Feed in Ka-Band The design of large and flat reflectarrays results in inappropriate phase distributions (with fast phase variations) that limit the antenna performance. The previous design of a multibeam reflectarray to generate four spaced beams per feed has been improved to fit a 1.8 m parabolic reflectarray. In this configuration, the parabolic surface shapes the beams as a conventional reflector, while the reflectarray elements only introduce a small phase correction to slightly deviate the beams produced in orthogonal polarizations and/or at different operating frequencies. The four required phase distributions that must be implemented in the parabolic reflectarray have been computed and Figure9 shows the required phase distributions at 19.45 and 19.95 GHz for one CP (the phase distributions for the orthogonal CP have a similar behavior). The phase distributions in Figure9 show a great improvement with respect to the equivalent distributions for a flat reflectarray (see Figure6), presenting smooth phase variations and a reduced number of 360◦ cycles. The 1.8 m parabolic reflectarray antenna has been simulated considering the same cluster of feeds as in the previous design, producing a four-color coverage of 64 beams similar to that shown in Figure7. There are no appreciable differences between the simulated radiation patterns of both flat and parabolic reflectarrays (a comparison between the antenna performances of flat and parabolic reflectarrays to produce four beams per feed can be seen in detail in [19]). The results obtained for this (flat or parabolic) antenna farm show that a single reflectarray with 27 feeds would produce a four-color coverage of 108 beams only in the Tx or Rx link of the Ka-band, so two reflectarrays would be needed to generate a complete multispot coverage both in Tx and Rx. As a result, the number of antennas would be halved Sensors 2021, 21, x FOR PEER REVIEW 8 of 18

The design of large and flat reflectarrays results in inappropriate phase distributions (with fast phase variations) that limit the antenna performance. The previous design of a multibeam reflectarray to generate four spaced beams per feed has been improved to fit a 1.8 m parabolic reflectarray. In this configuration, the parabolic surface shapes the beams as a conventional reflector, while the reflectarray elements only introduce a small phase correction to slightly deviate the beams produced in orthogonal polarizations and/or at different operating frequencies. The four required phase distributions that must be implemented in the parabolic reflectarray have been computed and Figure 9 shows the required phase distributions at 19.45 and 19.95 GHz for one CP (the phase distributions for the orthogonal CP have a similar behavior). The phase distributions in Figure 9 show a great improvement with respect to the equivalent distributions for a flat reflectarray (see Figure 6), presenting smooth phase variations and a reduced number of 360° cycles. The 1.8 m parabolic reflec- Sensors 2021, 21, 207 tarray antenna has been simulated considering the same cluster of feeds as8 in of 17the previous design, producing a four-color coverage of 64 beams similar to that shown in Figure 7. There are no appreciable differences between the simulated radiation patterns of both flat with respectand to theparabolic conventional reflectarrays four-reﬂector (a comparison conﬁguration, between and the the antenna same wouldperformances happen of flat and with the numberparabolic of feed-chains reflectarrays (since to produce each feed four produces beams per four feed beams can inbe Txseen or in Rx). detail in [19]).

(a) (b)

Figure 9. RequiredFigure phase 9. Required distributions phase of distributions the 1.8 m parabolic of the 1.8 reflectarray m parabolic at reflectarraythe (a) lower at and the ((ab)) lower upper and operating (b) upper frequencies for the sameoperating CP. frequencies for the same CP.

Moreover, thisThe solutionresults obtained makes it for possible this (flat to reduce or parabolic) the complexity antenna of farm the feed-chains,show that a single re- since each reflectarrayflectarray with would 27 feeds be illuminated would produce by single-band a four-color feeds, coverage instead of 108 of the beams current only in the Tx dual-bandor feed-chains Rx link of [the4]. TheKa-band, use of so independent two reflectarrays reflectarrays would be to needed operate to in generate Tx and a complete Rx can bemultispot exploited tocoverage reduce both the sizein Tx of and the RxRx. antenna,As a result, due the to itsnumber higher of operating antennas would be frequency.halved However, with the respect main to limitation the conventional is related four to the-reflector independent configuration, operation and at the two same would close frequencies.happen Althoughwith the number the parabolic of feed-chains surface has(since overcome each feed the produces drawback four associated beams in Tx or Rx). with the requiredMoreover, phase distributions, this solution furthermakes it research possible is to needed reduce onthe the complexity reflectarray of the cell feed-chains, that shouldsince be used each in reflectarray a real scenario would to achievebe illuminated dual-band by single-band operation at feeds, near frequencies. instead of the current The cell proposeddual-band in [ 20feed-chains] could be [4]. scaled The to use operate of independent at two close reflectarrays frequencies withinto operate the Txin Tx and Rx band in Ka-bandcan be inexploited dual CP. to Once reduce the the reflectarray size of the cell Rx is defined,antenna, intensedue to optimizationsits higher operating fre- should be appliedquency. toHowever, provide the an independentmain limitation operation is related in to both the frequencies independent with operation a stable at two close performancefrequencies. in band. Although the parabolic surface has overcome the drawback associated with the required phase distributions, further research is needed on the reflectarray cell that 4. Antenna Farm Based on Two Dual-Band Dual Reflectarray Configurations should be used in a real scenario to achieve dual-band operation at near frequencies. The The constraintscell proposed arising in from[20] could the design be scaled of reflectarrays to operate with at two independent close frequencies operation within at the Tx two close frequenciesband in Ka-band (within in the dual same CP. band Once for Txthe or reflectarray Rx) can be overcomecell is defined, by implementing intense optimizations the dual-frequencyshould be operation applied to in provide two separate an independent frequency op bands.eration Then,in both the frequencies reflectarray with a stable must generateperformance two spaced in band. beams per feed in orthogonal CP simultaneously at Tx and Rx frequencies in Ka-band. In this way, the reflectarray is expected to generate all the beams in two different colors simultaneously in Tx and Rx. A second dual-band reflectarray with slightly different operating frequencies would generate the second half of the coverage, and the interlaced coverages of the two reflectarrays would form the complete four-color coverage in Tx and Rx. However, the design of a reflectarray with independent operation in dual CP simultaneously at two frequency bands is a complex task. In this strategy, the reflectarray has been defined with a dual-antenna configuration based on a flat subreflector reflectarray and a main parabolic reflectarray. The use of a dual configuration makes it possible to simplify the implementation of the dual-band operation in dual CP thanks to the larger number of degrees of freedom provided by the use of two reflectarrays. In this way, the feeds will operate in dual-LP, which implies a lesser complexity of the feed-chain than in the case of dual-CP operation (there is no need for a polarizer in the feed-chain). Then, the subreflectarray will be designed to deviate (without focusing) the dual-LP beams radiated by the feeds by ±0.28◦ (in opposite directions for each LP) simultaneously in Tx and Rx. The main reflectarray will be designed to convert the incident dual-LP field into dual-CP, at the same time as its parabolic surface focuses high-gain beams. Figure 10a shows the operating principle of the dual reflectarray antenna to generate two spaced beams in left-handed and right-handed CP (LHCP and RHCP, respectively) from a single feed operating in horizontal and vertical LP (H and V, Sensors 2021, 21, x FOR PEER REVIEW 9 of 18

4. Antenna Farm Based on Two Dual-Band Dual Reflectarray Configurations The constraints arising from the design of reflectarrays with independent operation at two close frequencies (within the same band for Tx or Rx) can be overcome by implementing the dual-frequency operation in two separate frequency bands. Then, the reflectarray must generate two spaced beams per feed in orthogonal CP simultaneously at Tx and Rx frequencies in Ka-band. In this way, the reflectarray is expected to generate all the beams in two different colors simultaneously in Tx and Rx. A second dual-band reflectarray with slightly different operating frequencies would generate the second half of the coverage, and the interlaced coverages of the two reflectarrays would form the complete four-color coverage in Tx and Rx. However, the design of a reflectarray with independent operation in dual CP simultaneously at two frequency bands is a complex task. In this strategy, the reflectarray has been defined with a dual-antenna configuration based on a flat subreflector reflectarray and a main parabolic reflectarray. The use of a dual configuration makes it possible to simplify the implementation of the dual-band operation in dual CP thanks to the larger number of degrees of freedom provided by the use of two reflectarrays. In this way, the feeds will operate in dual-LP, which implies a lesser complexity of the feed-chain than in the case of dual-CP operation (there is no need for a polarizer in the feed-chain). Then, the subreflectarray will be designed to deviate (without focusing) the dual-LP beams radiated by the feeds by ±0.28° (in opposite directions for each LP) simultaneously in Tx and Rx. The main reflectarray will be designed to convert the incident dual-LP field into dual-CP, at the same time as its parabolic surface focuses high-gain beams. Figure 10a shows the operating principle of the dual reflectarray antenna to generate two spaced beams in left-handed and right-handed CP (LHCP and RHCP, respectively) from a single feed operating in horizontal and vertical LP (H and V, respectively). Figure 10b shows the operating scheme of the proposed antenna to generate half the required multispot coverage (two colors) simultaneously in Tx and Rx. The operating principles behind this solution have been experimentally validated Sensors 2021, 21, 207 separately. The design of a dual-band reflectarray with independent operation in dual9 of LP 17 at two separated frequencies can be reached in a direct way, as proposed in [21], by means of reflectarray cells based on orthogonal sets of parallel dipoles for each band. A polariz- ingrespectively). reflectarray Figure to convert 10b shows dual theLP into operating dual CP scheme with ofbroadband the proposed operation antenna from to generate 20 to 30 GHzhalf the (covering required both multispot Tx and coverageRx bands) (two has colors)been demonstrated simultaneously in [22]. in Tx and Rx.

(a) (b)

Figure 10. DualDual reflectarray reﬂectarray system system proposed proposed to toop operateerate simultaneously simultaneously in Tx in and Tx and Rx. Rx.(a) Operating (a) Operating principle principle of the of dual- the dual-conﬁguration,configuration, (b) functional (b) functional scheme scheme of the of prop theosed proposed antenna antenna on the on satellite. the satellite.

The dual operating reflectarray principles designed behind to thisproduce solution multiple have spot been beams experimentally in Ka-band validated has been definedseparately. on the The basis design of a of Cassegrain a dual-band system. reflectarray The flat with subreflectarray independent has operation a diameter in dual of 0.65 LP mat twoand separatedit is formed frequencies by 11,497 cancells be disposed reached in in a a 117 direct × 125 way, grid, as proposedconsidering in [the21], same by means type of reflectarray cells based on orthogonal sets of parallel dipoles for each band. A polarizing reflectarray to convert dual LP into dual CP with broadband operation from 20 to 30 GHz (covering both Tx and Rx bands) has been demonstrated in [22]. The dual reflectarray designed to produce multiple spot beams in Ka-band has been defined on the basis of a Cassegrain system. The flat subreflectarray has a diameter of 0.65 m and it is formed by 11,497 cells disposed in a 117 × 125 grid, considering the same type of reflectarray cells used in [23] (based on two orthogonal set of parallel dipoles) with a period of 5.3 mm. The main parabolic reflectarray has a diameter of 1.8 m, formed by 62,654 cells disposed in a 286 × 279 grid. It has been designed using the polarizing cells shown in [22] (based on three coplanar parallel dipoles) with a period of 5 mm. In contrast to the complex phase distributions computed for the previous flat 1.8 m reflectarray (see Figure6), the flat subreflectarray only deviates the orthogonal LP beams radiated by the feeds in opposite directions, so the required phase distributions present a smooth phase variation at both frequencies. Figure 11 shows the phase distributions implemented in the subreflectarray at 19.7 GHz and 29.5 GHz in one LP; note that the phase distributions in the orthogonal LP show the opposite phase variation at each frequency to deviate the orthogonal LP beam in the opposite direction. Then, the main parabolic reflectarray only converts the dual-LP incident field into dual-CP, so their cells have been designed to introduce a phase difference of 90◦ between the two orthogonal components of each incident LP [22]. The design of the subreflectarray and the main parabolic reflectarray has been carried out separately. The radiation patterns of the designed subreflectarray (without the main reflectarray) have been simulated and the results have been compared with those obtained considering ideal reflectarray cells (providing the required phase distributions without any phase errors and zero dielectric losses). As an example, Figure 12a shows the comparison between the ideal and realized patterns in the azimuth plane (ϕ = 90◦) for the subreflectarray in H polarization at 19.7 and 29.5 GHz. Note that the patterns in Figure 12a show a wide main lobe since the subreflectarray does not focus the beam radiated by the feed (the parabolic surface of the main reflectarray will focus the beam). In the same way, the simulated radiation patterns of the designed polarizing main reflectarray (without the Sensors 2021, 21, x FOR PEER REVIEW 10 of 18

of reflectarray cells used in [23] (based on two orthogonal set of parallel dipoles) with a period of 5.3 mm. The main parabolic reflectarray has a diameter of 1.8 m, formed by 62,654 cells disposed in a 286 × 279 grid. It has been designed using the polarizing cells shown in [22] (based on three coplanar parallel dipoles) with a period of 5 mm. In contrast to the complex phase distributions computed for the previous flat 1.8 m reflectarray (see Figure 6), the flat subreflectarray only deviates the orthogonal LP beams radiated by the Sensors 2021, 21, 207 feeds in opposite directions, so the required phase distributions present a smooth 10phase of 17 variation at both frequencies. Figure 11 shows the phase distributions implemented in the subreflectarray at 19.7 GHz and 29.5 GHz in one LP; note that the phase distributions in thesubreflectarray, orthogonal LP so show that the themain opposite reflectarray phase variation is illuminated at each from frequency the virtual to deviate focus the of theor- thogonaldual system) LP beam have beenin the computed opposite direction. and compared Then, with the main the simulations parabolic reflectarray performed withonly convertsideal cells. the Figure dual-LP 12b incident shows the field comparison into dual-C betweenP, so their the ideal cells andhave realized been designed patterns into thein- troduceazimuth a plane phase at difference 19.7 and 29.5of 90° GHz between for the the polarizing two orthogonal main reflectarray components inRHCP of each (obtained incident LPfrom [22]. the H polarization radiated by the feed).

(a) (b)

Figure 11. Required phase distributionsdistributions ofof the the 0.65 0.65 m m ﬂat flat subreﬂectarray subreflectarray at at the the (a )(a lower) lower and and (b )(b upper) upper operating operating frequencies frequen- Sensors 2021, 21, x FOR PEER REVIEW 11 of 18 forcies the for samethe same linear linear polarization polarization (LP). (LP).

The design of the subreflectarray and the main parabolic reflectarray has been carried out separately.19.7 GHz The ideal radiation patterns of the designed subreflectarray (without the main 20 CP reflectarray)19.7 have GHz been real. simulated and the results have been compared with those obtained XP considering29.5 ideal GHz reflectarray ideal cells (providing the required phase distributions without any phase errors29.5 GHz and real. zero dielectric losses). As an example, Figure 12a shows the compar- 10 ison between the ideal and realized patterns in the azimuth plane (φ = 90°) for the subreflectarray in H polarization at 19.7 and 29.5 GHz. Note that the patterns in Figure 12a show a 0 wide main lobe since the subreflectarray does not focus the beam radiated by the feed (the parabolic surface of the main reflectarray will focus the beam). In the same way, the simulated Gain [dBi] Gain radiation patterns of the designed polarizing main reflectarray (without the subreflectarray, − 10 so that the main reflectarray is illuminated from the virtual focus of the dual system) have been computed and compared with the simulations performed with ideal cells. Figure 12b − 20 shows the comparison between the ideal and realized patterns in the azimuth plane at 19.7 and 29.5 GHz for the polarizing main reflectarray in RHCP (obtained from the H polarization − 40 − 20 0radiated 20by the feed). 40 The patterns in Figure 12 show a satisfactory agreement between the designed and [deg] ideal(a) performances of both reflectarrays. The dual system configuration(b) results in relatively large incidence angles from the feed on the subreflectarray, which complicates the FigureFigure 12. 12. SimulatedSimulated radiation radiationdesign patterns patterns of the at at19.7 elements 19.7 and and 29.5 29.5of GHz the GHz insubreflectarray. th ine theazimuth azimuth plane As plane of a theresult, of ideal the the idealand simulated realized and realized antenna patterns antenna com- of the ponents.components. (a) Subreflectarray (a) Subreflectarray designedfor H for polarization, H subreflectarray polarization, (b) main (b) mainshow reflectarray reflectarray a slightly for right-handed forlower right-handed gain circularand circularhigher polarizations polarizationscross-polar (RHCP). levels (RHCP). than in the ideal case (see Figure 12a). On the other hand, the simulated results of the designed Finally,The patterns the dual in antenna Figure 12 configuration show a satisfactory is illuminated agreement by the betweencluster of the27 feeds designed shown and in main reflectarray show an excellent performance, which is practically identical to the ideal Figureideal performances 3 rotated 90° ofin boththe xf reflectarrays.yf-plane. The simulations The dual system of the configuration complete dual results reflectarray in relatively have behavior (see Figure 12b). consideredlarge incidence 15 of the angles 27 feeds from (an the array feed of on 3 the× 5 subreflectarray,feeds, plotted with which solid complicates lines in Figure the 3), design since theof thebeams elements produced of the by subreflectarray.the remaining feeds As aare result, expected the simulated to have similar patterns behavior. of the The designed simu- latedsubreflectarray pattern contours show of a slightlythe 30 beams lower generate gain andd by higher the designed cross-polar dual levels reflectarray than in simultane- the ideal ouslycase (seeat 19.7 Figure GHz 12anda). 29.5 On theGHz other are depicted hand, the in Figure simulated 13 at results the EOC of thelevels designed of the beams main (betweenreflectarray 45 and show 46 andBi). excellent The simulated performance, beams whichmatch iswith practically the required identical spot distribution, to the ideal wherebehavior there (see is room Figure for 12 theb). beams that would be generated by a second dual reflectarray op- eratingFinally, at slightly the dualdifferent antenna frequencies configuration to form is the illuminated complete byfour-color the cluster multispot of 27 feeds coverage shown in ◦ Txin Figureand Rx.3 rotated 90 in the xfyf-plane. The simulations of the complete dual reflectarray haveThe considered main cuts 15 of of the the radiation 27 feeds patterns (an array have of 3 revealed× 5 feeds, similar plotted C/I with levels solid as those lines of in the previous 1.8 m reflectarray. Figure 14 shows the cut of the radiation patterns in the plane u = 0 for the beams produced by the central row of feeds at both frequencies. The simulated cross-polar radiation provides an XPD lower than 20 dB for some beams. The analysis of the results has shown that the increase of the cross-polar component is mainly produced in the subreflectarray, so an optimization procedure should be applied to the dual-band dual-LP cells to reduce the cross-polarization. Moreover, the use of a dual configuration could be used to implement a bifocal technique to improve the shaping of the beams generated in the edge of the coverage [24]. Sensors 2021, 21, 207 11 of 17

Figure3), since the beams produced by the remaining feeds are expected to have similar behavior. The simulated pattern contours of the 30 beams generated by the designed dual reﬂectarray simultaneously at 19.7 GHz and 29.5 GHz are depicted in Figure 13 at the EOC levels of the beams (between 45 and 46 dBi). The simulated beams match with the required spot distribution, where there is room for the beams that would be generated by a second Sensors 2021, 21, x FOR PEER REVIEWdual reﬂectarray operating at slightly different frequencies to form the complete four-color12 of 18 multispot coverage in Tx and Rx.

FigureFigure 13.13. Simulated pattern pattern contours contours for for the the 30 30 beams beams generated generated by by15 15feed feedss illuminating illuminating the the proposedproposed dual dual reﬂectarray reflectarray at at 19.7 19.7 GHz GHz and and 29.5 29.5 GHz. GHz.

The main cuts of the radiation patterns have revealed similar C/I levels as those of the previous 1.8 m reflectarray. Figure 14 shows the cut of the radiation patterns in the plane u = 0 for the beams produced by the central row of feeds at both frequencies. The simulated cross-polar radiation provides an XPD lower than 20 dB for some beams. The analysis of the results has shown that the increase of the cross-polar component is mainly produced in the subreflectarray, so an optimization procedure should be applied to the dual-band dual-LP cells to reduce the cross-polarization. Moreover, the use of a dual configuration could be used to implement a bifocal technique to improve the shaping of the beams generated in the edge of the coverage [24]. In conclusion, the simulated results of the dual reflectarray configuration prove the capability of the proposed multibeam antenna to generate half the required four-color coverage simultaneously in Tx and Rx. The dual-band operation involving separate frequencies simplifies the design of the reflectarray elements, while the dual configuration simplifies the process of generating two spaced beams in orthogonal CP per feed, split-

ting it into two straightforward stages (deviation of the LP beams in opposite directions (a) (b) and dual-LP to dual-CP conversion), the principles of which have been experimentally Figure 14. Cut of the simulatedvalidated radiation previously. patterns Thein u simulated= 0 for the beams results generated show that at ( aa) further 19.7 GHz optimization and (b) 29.5 isGHz required in same CP. for the subreflectarray to improve the XPD, although the flexibility provided by the dual configuration could be also used to improve the overall performance of the antenna. In conclusion, the simulated results of the dual reflectarray configuration prove the capability of the proposed multibeam antenna to generate half the required four-color coverage simultaneously in Tx and Rx. The dual-band operation involving separate frequencies simplifies the design of the reflectarray elements, while the dual configuration simplifies the process of generating two spaced beams in orthogonal CP per feed, splitting it into two straightforward stages (deviation of the LP beams in opposite directions and dual-LP to dual-CP conversion), the principles of which have been experimentally validated previously. The simulated results show that a further optimization is required for the subreflectarray to improve the XPD, although the flexibility provided by the dual configuration could be also used to improve the overall performance of the antenna. Sensors 2021, 21, x FOR PEER REVIEW 12 of 18

Sensors 2021, 21, 207 12 of 17 Figure 13. Simulated pattern contours for the 30 beams generated by 15 feeds illuminating the proposed dual reflectarray at 19.7 GHz and 29.5 GHz.

Sensors 2021, 21, x FOR PEER REVIEW 13 of 18

(a) (b)

FigureFigure 14. Cut14. Cut of theof the simulated simulated radiation radiation patterns patterns inin uu == 0 for the beams beams generated generated at at(a) ( a19.7) 19.7 GHz GHz and and (b) (29.5b) 29.5 GHz GHz in in same CP. same CP. 5. Antenna Farm Based on Two Dual-Band Offset Parabolic Reflectarrays Bearing in mind the better performance of the previous dual-band dual reflectarray 5. AntennaIn conclusion, Farm Based the simulated on Two Dual-Band results of the Offset dual reflectarray ParabolicReflectarrays configuration prove the withcapability respect of to the the proposed single-band multibeam reflectarray, antenna the tofinal generate proposed half antennathe required farm four-color is also based Bearing in mind the better performance of the previous dual-band dual reflectarray oncoverage the use simultaneouslyof a dual-band inreflectarray Tx and Rx. toThe generate dual-band all operationthe beams involving associated separate to two fre- colors with respect to the single-band reflectarray, the final proposed antenna farm is also based simultaneouslyquencies simplifies in Tx the and design Rx. In of this the case, reflecta a singlerray elements, offset parabolic while the reflectarray dual configuration is proposed insteadonsimplifies theuse of the ofthe a previousprocess dual-band of dualgenera reflectarray configuration,ting two tospaced generate reducing beams all in the theorthogonal beamsnumberassociated CP of per components feed, to splitting two colorsin the antennasimultaneouslyit into twoconfiguration. straightforward in Tx and Rx. stages In this (deviation case, a single of the offsetLP beams parabolic in opposite reflectarray directions is proposed and insteaddual-LPThe of generation to the dual-CP previous ofconversion), two dual spaced configuration, the beams principles in reducingor ofthogonal which the have CP number beenper feedexperimentally of componentsis now completely vali- in the managedantennadated previously. configuration. by the reflectarray The simulated elements results placedshow that on athe further parabolic optimizat reflectarrayion is required surface. for The operatingtheThe subreflectarray generationprinciple ofto of improvethe two antenna spaced the XPD, is beams based althou inongh orthogonal the the concept flexibility CP proposed provided per feed in by is [25], the now dualwhere completely con- a single-bandmanagedfiguration parabolic by could the reflectarraybe reflectarray also used to elements improvdeviatese placed thethe overall orthogonal on theperformance parabolic CP beams of reflectarray the focused antenna. by surface. the para- The bolicoperating surface principle in opposite of the directions antenna is by based means on of the the concept variable proposed rotation in technique [25], where (VRT) a single- ap- band parabolic reflectarray deviates the orthogonal CP beams focused by the parabolic plied to the reflectarray elements. In our solution, a dual-band offset parabolic reflectarray surface in opposite directions by means of the variable rotation technique (VRT) applied to is proposed to focus the beams by its parabolic surface and deviate the orthogonal CP by the reflectarray elements. In our solution, a dual-band offset parabolic reflectarray is proposed the application of VRT simultaneously at Tx and Rx frequencies. The dual-band operation to focus the beams by its parabolic surface and deviate the orthogonal CP by the application by VRT is achieved using the reflectarray cells proposed in [20]. As a result, a single offset of VRT simultaneously at Tx and Rx frequencies. The dual-band operation by VRT is parabolic reflectarray illuminated by dual-CP feeds generates two spaced beams in or- achieved using the reflectarray cells proposed in [20]. As a result, a single offset parabolic thogonal CP per feed at Tx and Rx frequencies in Ka-band. The operating scheme of the reflectarray illuminated by dual-CP feeds generates two spaced beams in orthogonal CP per proposed reflectarray to generate half the required four-color coverage is shown in Figure feed at Tx and Rx frequencies in Ka-band. The operating scheme of the proposed reflectarray 15. to generate half the required four-color coverage is shown in Figure 15.

Figure 15. Figure 15. OperatingOperating principle of the proposed single-band parabolic reﬂectarray.reflectarray.

Recently, the proposed operating principle has been experimentally validated in [26] by a 0.9 m parabolic reflectarray prototype that generates two spaced beams per feed at Tx and Rx frequencies using the reflectarray cell described in [20]. Figure 16 shows the prototype in the anechoic chamber and the 40.6 dBi pattern contours of the two measured beams generated simultaneously at 19.7 GHz and 30 GHz with a single dual-CP feed.

Sensors 2021, 21, 207 13 of 17

Recently, the proposed operating principle has been experimentally validated in [26] by a 0.9 m parabolic reﬂectarray prototype that generates two spaced beams per feed at Tx and Rx frequencies using the reﬂectarray cell described in [20]. Figure 16 shows the Sensors 2021, 21, x FOR PEER REVIEWprototype in the anechoic chamber and the 40.6 dBi pattern contours of the two measured14 of 18

beams generated simultaneously at 19.7 GHz and 30 GHz with a single dual-CP feed.

(a) (b) Figure 16. Dual band parabolicparabolic reflectarray reflectarray prototype prototype to to generate generate two two spaced spaced beams beams per per feed feed [26]. [26]. (a) Picture (a) Picture of the of prototype, the prototype,(b) measured (b) measured 40.6 dBi 40.6 pattern dBi contourspattern contours of the two of beamsthe two generated beams generated at 19.7 and at 3019.7 GHz and by 30 the GHz same by feed.the same feed.

AA 1.8 1.8 m m parabolic parabolic reflectarray reflectarray formed formed by by 62,654 62,654 reflectarray reflectarray cells cells with with a period a period of 6.5 of ◦ mm6.5 mm has hasbeen been designed designed to deviate to deviate the the beams beams in orthogonal in orthogonal CP CP ± 0.28°± 0.28 simultaneouslysimultaneously at 19.7at 19.7 and and 29.5 29.5 GHz. GHz. Since Since the the parabolic parabolic surface surface of of the the antenna antenna focuses focuses the the beams beams radiated radiated byby the feeds, thethe requiredrequired phase phase distributions distributions on on the the reflectarray reflectarray to to split split the the orthogonal orthogonal CP CPbeams beams present present a similar a similar aspect aspect as those as those of the of subreflectarraythe subreflectarray shown shown in Section in Section4. Figure 4. Fig- 17 ureshows 17 shows the required the required phase distributions phase distributions on the parabolic on the parabolic surface at surface 19.7 GHz at 19.7 and GHz 29.5 GHzand 29.5in one GHz CP in (the one phase CP (the distributions phase distributions in the orthogonal in the orthogonal CP show CP the show opposite the phaseopposite variation phase variationto deviate to the deviate orthogonal the orthogonal CP beams CP in beams the opposite in the direction).opposite direction).

(a) (b)

Figure 17. Required phasephase distributions distributions of of the the 1.8 1.8 m parabolicm parabolic reflectarray reflectarray at the at ( athe) lower (a) lower and ( band) upper (b) upper operating operating frequencies frequencies for the same CP. quenciesfor the same for the CP. same CP.

AsAs in in the the first first antenna antenna solution solution shown shown in in Section Section 33,, the proposed dual-band parabolicparabolic reflectarrayreflectarray has has been been illuminated illuminated by by the the clus clusterter of of 27 27 feeds feeds shown shown in in Figure Figure 33,, andand thethe simulationssimulations have have considered considered three three lines lines of of feeds (16 feeds). Thus, the designed parabolic reflectarrayreflectarray is is expected expected to to generate generate 32 32 beams beams in in orthogonal orthogonal CP CP simultaneously simultaneously in in Tx Tx and Rx.Rx. The The simulated simulated pattern pattern contours contours of ofthe the 32 32beams beams generated generated by bythethe 1.8 1.8m parabolic m parabolic reflectarrayreflectarray at 19.7 at 19.7 and and 29.5 29.5 GHz GHz are depicted are depicted in Figure in Figure 18, where 18, where the contours the contours of the beams of the atbeams 19.7 and at 19.7 29.5 and GHz 29.5 correspond GHz correspond to a gain to level a gain of level46 and of 45 46 dBi, and respectively. 45 dBi, respectively. In a similar In a waysimilar to the way previous to the previous dual reflectarray dual reflectarray configuration, configuration, a second a second parabolic parabolic reflectarray reflectarray operating at slightly different frequencies would produce the second half of the required four- color coverage. Sensors 2021, 21, x FOR PEER REVIEW 15 of 18

0.04 LHCP 19.7 GHz Sensors 2021, 21, 207 RHCP 19.7 GHz 14 of 17 0.03 LHCP 29.5 GHz RHCP 29.5 GHz Sensors 2021, 21, x FOR PEER REVIEW 15 of 18 0.02 operating at slightly different frequencies would produce the second half of the required four-color coverage. 0.01

0.040 LHCP 19.7 GHz RHCP 19.7 GHz − 0.010.03 LHCP 29.5 GHz − 0.06 − 0.04 − 0.02 0 0.02 0.04 0.06RHCP 0.0829.5 GHz 0.02 u Figure 18. Contours of 32 beams generated simultaneously at 19.7 GHz and 29.5 GHz by the 1.8-m dual-band parabolic reflectarray0.01 illuminated by 16 feeds.

0 Figure 19 shows the horizontal cut in the plane v = 0 of the radiation patterns for the beams generated at 19.7 and 29.5 GHz. The main cuts of the radiation pattern have shown − 0.01 − 0.06 similar− 0.04 values− 0.02 of C/I to those 0 of the 0.02 previous configurations, 0.04 0.06 with a 0.08XPD larger than 20 dB. Due to the large electrical size of the antenna at 29.5 GHz, the beams in Rx show a higher maximum gain than the beamsu in Tx; however, the shaping of the beams in Tx and Figure 18. Contours of 32 beamsRx can generated be matched simultaneously by the optimization at 19. 19.77 GHz procedure and 29.5 GHz applied by the in 1.8-m [26]. dual-band parabolic reflectarrayreﬂectarray illuminated by 16 feeds.In conclusion, the simulated results of the dual-band parabolic reflectarray have demonstrated a very satisfactory performance. The simulations show a suitable distribution ofFigureFigure the beams 19 shows with the appropriate horizontal values cut in of the gain plane and v XPD. = 0 0 of As the in radiation the previous patterns configura- for the tions,beamsbeams the generated generated C/I can beat 19.7 improved and 29.5 by GHz. a small The The incr main mainease cuts cuts of theof of the theantenna radiation radiation size pattern patternto reduce have the shown edge illuminationsimilarsimilar values levels ofof C/IC/I on to the thosethose reflectarray. ofof thethe previous previous Moreov conﬁgurations, configurations,er, the reflectarray with with a cell XPDa XPD considered larger larger than than in 20 the dB.20 designdB.Due Due to makes the to largethe it largepossible electrical electrical to size implement ofsize the of antenna theaddition antenna atal 29.5 optimization at GHz, 29.5 theGHz, beams to the shape beams in Rx the showin beams Rx ashow higherin Rx a andhighermaximum to maintainmaximum gain thanthe gain cross-polar the than beams the beams inlevels Tx; however, inin Tx;band, however, theas in shaping [26], the shapingby of a the proper beams of the adjustment inbeams Tx and in Tx Rxof andthe can dimensionsRxbe matchedcan be matched and by the rotation optimizationby the an optimizationgle of procedure the elements procedure applied [27]. applied in [26]. in [26]. In conclusion, the simulated results of the dual-band parabolic reflectarray have demonstrated a very satisfactory performance. The simulations show a suitable distribution of the beams with appropriate values of gain and XPD. As in the previous configurations, the C/I can be improved by a small increase of the antenna size to reduce the edge illumination levels on the reflectarray. Moreover, the reflectarray cell considered in the design makes it possible to implement additional optimization to shape the beams in Rx and to maintain the cross-polar levels in band, as in [26], by a proper adjustment of the dimensions and rotation angle of the elements [27].

(a) (b)

FigureFigure 19. 19. CutCut of of the the simulated simulated radiation radiation patterns patterns in in v v = = 0 0 for for the the beams beams generated generated at at ( (aa)) 19.7 19.7 GHz GHz and and ( (bb)) 29.5 29.5 GHz GHz in in samesame CP. CP.

6. ConclusionsIn conclusion, the simulated results of the dual-band parabolic reflectarray have demonstrated a very satisfactory performance. The simulations show a suitable distribution In this paper, four novel antenna farms based on reflectarrays for broadband com- of the beams with appropriate values of gain and XPD. As in the previous configurations, munication satellites in Ka-band have been proposed and evaluated. The exclusive char- the C/I can be improved by a small increase of the antenna size to reduce the edge acteristics of reflectarrays, which are able to operate independently at different frequen- illumination(a) levels on the reflectarray. Moreover, the reflectarray(b) cell considered in the cies or polarizations, have been applied in different ways to improve the SFPB operation Figure 19. Cut of the simulateddesign radiation makes patterns it possible in v to= 0 implement for the beams additional generated optimization at (a) 19.7 GHz to and shape (b) the29.5 beamsGHz in in Rx same CP. and to maintain the cross-polar levels in band, as in [26], by a proper adjustment of the dimensions and rotation angle of the elements [27]. 6. Conclusions In this paper, four novel antenna farms based on reflectarrays for broadband communication satellites in Ka-band have been proposed and evaluated. The exclusive characteristics of reflectarrays, which are able to operate independently at different frequencies or polarizations, have been applied in different ways to improve the SFPB operation Sensors 2021, 21, 207 15 of 17

6. Conclusions In this paper, four novel antenna farms based on reflectarrays for broadband communication satellites in Ka-band have been proposed and evaluated. The exclusive characteristics of reflectarrays, which are able to operate independently at different frequencies or polarizations, have been applied in different ways to improve the SFPB operation compared to conventional reflectors. The impact of using flat or curved surfaces, single or dual-band operation, and single or dual antenna configurations for large reflectarrays in Ka-band has been assessed to achieve the best antenna performance. Table2 summarizes the main characteristics of the proposed antenna solutions.

Table 2. Main characteristics of the proposed antenna solutions.

Flat Single-Band Parabolic Single-Band Dual-Band Dual Parabolic Dual-Band Parameter Reflectarray Reflectarray Reflectarray Reflectarray Antenna configuration Single aperture Single aperture Dual configuration Single aperture Parabolic (main) + flat Aperture type Flat Parabolic Parabolic (sub) Beams per feed 4 4 2 2 Frequency bands 1 (Tx or Rx) 1 (Tx or Rx) 2 (Tx and Rx) 2 (Tx and Rx) Required antennas 2 2 2 2 Complexity of the cells Very high Very high Low Low Operation in frequency Very hard to implement Hard to implement Readily achievable Readily achievable Operation in CP Hard to implement Hard to implement Readily achievable Readily achievable

The preliminary simulations of single band reflectarrays to generate four beams per feed permits operation with a single antenna for Tx and an independent antenna for Rx, which simplifies the simultaneous operation in both bands as well as the design of the feed-chains. However, this solution involves a major difficulty in providing a stable performance in band due to the independent operation at close frequencies, requiring complex reflectarray cells to which intense optimizations must be applied. On the other hand, the two proposed dual-band reflectarrays show promising results. The simulations of the dual reflectarray system would require a further optimization of the subreflectarray to slightly reduce the cross-polar radiation, although the use of a dual configuration with two reflectarrays provides greater flexibility of operation than single aperture systems, which can be used to correct the shaping of the beams in the edge of the coverage. Finally, the dual-band parabolic reflectarray achieves an excellent antenna performance with a simple antenna configuration similar to that of the currently used reflectors. Moreover, the reflectarray elements can be optimized to properly shape the beams in Tx and Rx without the need to shape the antenna surface. The core technologies of the proposed reflectarrays have been experimentally validated, which confirms the capabilities of reflectarrays to provide novel efficient multibeam antenna farms for broadband communications satellites in Ka-band. All the proposed antenna solutions make it possible to halve the number of antennas and feed-chains required onboard the satellite, from four reflectors to two reflectarray antennas, and from a total of N feed-chains (to produce a coverage of N spots) to N/2 feeds, allowing an important reduction in cost, as well as in mass and volume of the payload, which are severely limited in the satellite.

Author Contributions: Conceptualization, J.A.E. and A.P.; methodology, J.A.E. and A.P.; software, D.M.-d.-R., E.M.-d.-R. and Y.R.-V.; validation, D.M.-d.-R., E.M.-d.-R. and J.A.E.; formal analysis, D.M.-d.-R., E.M.-d.-R. and Y.R.-V.; investigation, D.M.-d.-R., E.M.-d.-R., Y.R.-V., J.A.E. and A.P.; data curation, Y.R.-V. and A.P.; writing—original draft preparation, D.M.-d.-R.; writing—review and editing, E.M.-d.-R., Y.R.-V., J.A.E. and A.P.; visualization, D.M.-d.-R., E.M.-d.-R. and Y.R.-V.; Sensors 2021, 21, 207 16 of 17

supervision, J.A.E. and A.P.; project administration, J.A.E.; funding acquisition, J.A.E. All authors have read and agreed to the published version of the manuscript. Funding: This research has been supported in part by the Spanish Ministry of Economy and Com- petitiveness under the project TEC2016-75103-C2-1-R, and under the project FJCI-2016-29943; by the European Regional Development Fund (ERDF), and by the European Space Agency (ESA) under contract 4000117113/16/NL/AF. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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