electronics

Article Development of Enabling Technologies for Ku-Band Airborne SATCOM Phased-Arrays

Alfredo Catalani 1,2,* , Giovanni Toso 1, Piero Angeletti 1, Mario Albertini 3 and Pasquale Russo 2 1 European Space Agency (ESA-ESTEC), TEC-EFA, 2201AZ Noordwijk, The Netherlands; [email protected] (G.T.); [email protected] (P.A.) 2 Formerly with Airbus Italy, 00131 Rome, Italy; [email protected] 3 Airbus Italy, Modem & Gateway, 00156 Rome, Italy; [email protected] * Correspondence: [email protected]; Tel.: +31-71-565-4047



Received: 26 February 2020; Accepted: 13 March 2020; Published: 16 March 2020


Abstract: In the paper the development of a fully electronic transmit-receive phased-array antenna system in Ku-band for aircraft communications via satellite is presented. Particular emphasis has been placed in the improvement of the following key elements: a dual-polarization self-diplexing radiating element, a transmit/receive active module with full polarization agility based on a digital vector modulator and a SiGe multinode MMIC. The optimized antenna elements enable a significant improvement towards the realization of a future affordable commercial product for satellite communications.

Keywords: active phased arrays; phased array antennas; MMIC; mobile antenna systems; satellite communications

1. Introduction Broadband connectivity has become essential in everyday life both for business and for entertainment. This, together with the increasing demand of mobile connectivity in locations where terrestrial networks are not available or less efficient, has promoted the development of antennas to be installed on aircrafts, trains, boats and vans to provide interconnection via satellite. Moving from experimental verification to operational conditions, aeronautical broadband services provided by satellites is today a reality; but to be commercially attractive, the services offered via satellite should have a quality comparable to those available via terrestrial networks, should allow the same type of applications and should be offered at an affordable cost. Together with the technical issues to be addressed, a main driver is the cost, which comprises the cost of ground segment, which is related to materials, installation (mainly related to the antenna system) and maintenance of the equipment, the cost of the satellite segment and the overall operations of the system. Focusing on the aeronautical platforms, satellite communication is the only means to provide continuous connectivity over the oceans. Airborne RF communication system applications pose challenging problems to current antenna technology: Interactions with the aircraft aerodynamics impacting drag, lift and controllability; • Influence of the aircraft structure on the antenna performance; • Electromagnetic Compatibility (EMC)/Electromagnetic Interference (EMI) with other RF and/or • electronic systems on board the aircraft; Very wide-angle scanning performance, so as to operate at the extreme latitudes required by • polar routes;

Electronics 2020, 9, 488; doi:10.3390/electronics9030488 www.mdpi.com/journal/electronics Electronics 2020, 9, 488 2 of 15

Compatibility with other satellite systems and with terrestrial communication services (involving • beamwidths, beam pointing accuracy and sidelobes performance); Keyhole problem typical of narrow beam antenna systems employing two axes for tracking a • satellite. The keyhole is caused either by mechanical constraints or position slew rate limitations of the antenna mount [1].

The ideal antenna should actually be high gain, broadband, low profile, highly reliable and low cost. Different choices of the antenna architecture [2] could bring to a fully mechanical [3], fully electronic [4] or “hybrid” (mechanical–electronic) solutions [5,6]. The fully mechanical and hybrid solutions are based on antennas with rectangular or highly elliptical apertures in order to minimize the antenna thickness. They create highly elliptical beams and, because they rely usually only on two motors, it is not always possible to project the most directive cut of the pattern along the equatorial plane. The addition of a third axis permits limiting this keyhole problem. Solutions based on fully electronic steering solve the keyhole issue and permit to maintain a transmit–receive link also when flying near the Equator where respecting the regulatory aspects is particularly challenging. Moreover, the fully electronic antenna, thanks to the graceful degradation (typical of active arrays) exhibits lower costs in terms of maintenance as compared to mechanically steering antennas. Electronically scanning antennas can be flat or conformal. In this development conformal antenna are considered in order to guarantee equalized performances in an entire half sphere. It is important to mention that by limiting the scanning directions with respect to the zenith within a cone with an angular opening of 60 degrees instead of 90 degrees, the antenna thickness could be reduced by a factor of two.

2. Antenna Requirements The main requirements for the Ku-band aeronautical market, in terms of antenna characteristics, are summarized in Table1.

Table 1. Antenna operative requirements.

ITEM Value Frequency band—RF received signal 10.7 - 12.75 GHz Frequency band—RF transmitted signal 14.0 - 14.5 GHz Polarization—RF received signal Dual orthogonal linear or dual orthogonal circular Polarization—RF transmitted signal Single adjustable linear

The operational band is the complete Ku-Band (10.7 - 12.75 GHz in receive and 14.0 - 14.5 GHz in transmit). The realignment of the polarization in respect to the satellite for both receiver and transmit signals, with the capability to operate also with the circular polarization in receiver mode, is required. The minimum antenna requirements in terms of operative field of view (e.g., antenna gain-to-noise temperature [G/T] over RF band in the entire field of view and for every selectable polarization, Equivalent Isotropic Radiated Power [EIRP] over RF band in the entire field of view and cross-polar discrimination (including pointing error)) are reported in the Table2.

Table 2. Antenna performance requirements.

ITEM Value Radiation gain pattern In agreement with [7] 0 < φ < 360 Field of view ◦ ◦ 0◦ < θ < 90◦ G/T >8 dB/K EIRP >43 dBW Electronics 2020, 8, x FOR PEER REVIEW 3 of 15

Table 2. Antenna performance requirements.

ITEM Value Radiation gain pattern In agreement with [7] Electronics 2020, 9, 488 3 of 15 0° < φ < 360° Field of view 0° < θ < 90° Table 2. Cont. G/T >8 dB/K ITEM Value EIRP >43 dBW Number of beams 1 Number of beams 1 Cross polarization discrimination >15 dB Cross polarization discrimination >15 dB

ItIt hashas toto bebe underlinedunderlined thatthat anan antennaantenna inin transmissiontransmission modemode hashas toto respectrespect thethe applicableapplicable regulationsregulations [[7]7] forfor thethe EIRPEIRP antennaantenna patternspatterns expressedexpressed inin termsterms ofof powerpower densitydensity perper bandwidthbandwidth asas shownshown inin FigureFigure1 1..

Figure 1. European Telecommunications StandardsStandards InstituteInstitute (ETSI)(ETSI) regulationregulation maskmask [[7].7].

Moreover,Moreover, complexitycomplexity andand costcost ofof thethe antennaantenna areare otherother keykey factorsfactors toto taketake intointo accountaccount together withwith practicalpractical feasibilityfeasibility andand abilityability toto operateoperate simultaneouslysimultaneously inin receiverreceiver andand transmittransmitmodes. modes.

3.3. Antenna Design TheThe hemi-sphericalhemi-spherical antennaantenna array when mounted on the aircraft is able to guarantee the proper link,link, whatever the the position position of of the the airborne airborne with with resp respectect to to the the satellite satellite is, is,thanks thanks to toits itsshape shape [8]. [ 8In]. Inorder order to tominimize minimize the the complexity complexity of of the the antenna, antenna, the the hemi-spherical hemi-spherical continuous continuous surface has beenbeen approximatedapproximated with flatflat facets.facets. To replace thethe sphere,sphere, thethe icosahedrons,icosahedrons, aa polyhedronpolyhedron withwith twentytwenty equilateralequilateral triangles,triangles, has been selected as starting geometry. Then a new solid ofof 8080 facetsfacets hashas beenbeen generated;generated; where,where, toto thethe originaloriginal vertexes,vertexes, thethe pointspoints ofof middlemiddle edgeedge ofof each triangle have been added afterafter thethe projectionprojection onon thethe spheresphere surface.surface. Half portion of the new solid is covered by sub-arrays and, afterafter furtherfurther refinements, refinements, the the position position of of the the tile tile is adjustedis adjusted to avoidto avoid the the intersection intersection of the of extruded the extruded part whichpart which contains contains the RF the components RF components and the and control the control logic. As logic. a result As ofa result the optimization, of the optimization, the geodesic the hemi-spheregeodesic hemi-sphere with 40 facets with shown40 facets in Figureshown2 in has Figure been 2 selected has been as selected antenna aperture.as antenna Each aperture. triangular Each facetriangular is composed face is bycomposed the same by sub-array, the same i.e., sub-arra the samey, i.e., number the same of radiating number elements of radiating arranged elements in a triangulararranged in lattice. a triangular lattice. TheThe finalfinal optimizedoptimized antennaantenna isis ableable to combine on the same aperture the receiver and transmitter capabilities,capabilities, has has a a diameter diameter of of 84 84 cm, cm, a height a height of 39 of cm 39 and cm anda weight a weight in the in order the of order 90 Kg. of The 90 Kg. number The numberof radiating of radiating elements elementsis 1440 with is 1440an inter-spacing with an inter-spacing of 22 mm, ofwhich 22 mm, corresponds which corresponds to one wavelength to one wavelength in the transmit higher frequency. Each triangular sub-array, indicated as tile, presents 36 radiating elements.

For a fixed beam direction, only a sector composed by 14 tiles is active while all the other elements are turned off. All the radiating elements in the active sector are excited in order to generate an equi-phase wave front perpendicular to the desired beam direction. Electronics 2020, 8, x FOR PEER REVIEW 4 of 15 in the transmit higher frequency. Each triangular sub-array, indicated as tile, presents 36 radiating elements. ElectronicsFor 2020a fixed, 9, 488 beam direction, only a sector composed by 14 tiles is active while all the 4other of 15 elements are turned off. All the radiating elements in the active sector are excited in order to generate an equi-phase wave front perpendicular to the desired beam direction. It is important to note that when projecting the conformal aperture in an arbitrary pointing It is important to note that when projecting the conformal aperture in an arbitrary pointing direction, the triangles are deformed and become irregular; this concept is clear watching at Figure2. direction, the triangles are deformed and become irregular; this concept is clear watching at Figure The resulting sparse array, thanks to this irregular lattice, exhibits a pattern without grating lobes. 2. The resulting sparse array, thanks to this irregular lattice, exhibits a pattern without grating lobes. Moreover, the optimization of the excitation coefficients in amplitude and phase, or in phase only, Moreover, the optimization of the excitation coefficients in amplitude and phase, or in phase only, permits to maintain the side lobes within the spectral density mask imposed by the regulatory aspects permits to maintain the side lobes within the spectral density mask imposed by the regulatory aspects as demonstrated in the paper. as demonstrated in the paper.

Figure 2. Faceted hemi-spherical antenna.

Concerning the antenna electrical architecture, th thee input/output input/output ports are fed with the IF band signal, and converted in a Ku bandband signalsignal byby thethe upup/down/down converter.converter. Each tile is composed by the radiating elements and the transmittransmit/receive/receive modules,modules, directly connectedconnected onon the back. Two dedicated beam forming networks 1:40 for the receiver and the transmitter mode are connected to the tiles and the CPUCPU controlscontrols a a set set of of switches switches in orderin order to involveto involve the selectedthe selected sub-arrays sub-arrays to form to form the antenna the antenna beam. Thebeam. CPU The controls CPU controls the beam the steering beam steer anding the and polarization the polarization alignment. alignment. The trackingtracking capability capability of of the the antenna antenna permits permits to point to point the beam the electronicallybeam electronically in a hemi-spherical in a hemi- fieldspherical of view field (i.e., of 360view degrees (i.e., 360 in azimuth degrees and in azimuth 90 degrees and in elevation)90 degrees with in elevation) negligible degradationswith negligible of thedegradations electrical performance,of the electrical guaranteeing performance, a minimum guaranteeing G/T a of minimum 8 dB/K-1 andG/T anof 8 EIRP dB/K-1 of 43 and dBW. an EIRP of 43 dBW.The results obtained using the simulated pattern of an embedded radiating element are reported in the following.The results The obtained excitation using coe thefficients simulated have beenpattern optimized of an embedded considering radiating the realistic element functionality are reported of the Receive(Rx)/Transmit(Tx) active module (combining the amplitude and phase of the two orthogonal Electronicsin the following.2020, 8, x FOR ThePEER REVIEWexcitation coefficients have been optimized considering the realistic5 of 15 polarizationfunctionality for of eachthe Receive(Rx)/Transmit(Tx) radiating element). active module (combining the amplitude and phase of the twoThe orthogonal Gtypical/T result azimuth is polarization shown transmit in Figure for antenna each3a. The radiating pattern performance iselement). reported matches in Figure the required 5a. The 8 dB black/K in line the represents entire field theof view. maskThe As G/Tthat expected, resulthas been is the shown imposed degradation in Figure in the of optimization3a. the The performance performance process at lower matches(corresponding pointing the required angles to the is 8 dueETSI dB/K to mask thein the different for entire 512 KHzorientationfield bandwidth).of view. angles As expected, The of the corresponding tiles, the which degradation are elevation involved of the tran in pe thesmitrformance beam antenna forming, at pattern lower with pointing is respectshown angles toin theFigure is horizon. due 5b. to the different orientation angles of the tiles, which are involved in the beam forming, with respect to the horizon. The typical azimuth receiver antenna pattern is shown in Figure 3b. During the transmission, all the active devices are able to work at the maximum power level and the expected performance in term of EIRP are reported Figure 4: The minimum guaranteed value is in the order of 43 dBW over the complete antenna field of view. The antenna is also able to control the transmit antenna patterns side-lobe in order to respect the applicable regulatory recommendations such as ETSI in the European countries [7] and the FCC in the United States.

(a) (b)

Figure 3. G/TG/T vs. elevation elevation angle angle over over the the horizon horizon ( (aa)) and and typical typical azimuth azimuth Rx Rx radiation radiation pattern pattern ( b).

Figure 4. EIRP vs. elevation angle over the horizon (with and without mask constraint).

(a) (b)

Figure 5. Typical azimuth (a) and elevation (b) Tx antenna pattern at 14.25 GHz, 30 degrees elevation over the horizon.

4. Developed Devices The main challenges in the design of the antenna have been: (a) the development of the radiating element, (b) the development of the active module for transmitter/receiver and (c) the design and manufacturing of a digital vector modulator (DVM) able to integrate in a single chip the attenuator and phase shifter features. These three components are described in the following paragraphs.

4.1. Self-Diplexing Radiating Element Design The first criticality in the design of Tx/Rx radiators is the capability to separate the transmitted signal from the received one. A typical approach consists in designing a radiating element covering

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TheThe typicaltypical azimuthazimuth transmittransmit antennaantenna patternpattern isis reportedreported inin FigureFigure 5a.5a. TheThe blackblack lineline representsrepresents thethe maskmask thatthat hashas beenbeen imposedimposed inin thethe optimizationoptimization processprocess (corresponding(corresponding toto thethe ETSIETSI maskmask forfor 512512 KHzKHz bandwidth).bandwidth). TheThe correspondingcorresponding elevationelevation trantransmitsmit antennaantenna patternpattern isis shownshown inin FigureFigure 5b.5b. Electronics 2020, 9, 488 5 of 15

The typical azimuth receiver antenna pattern is shown in Figure3b. During the transmission, all the active devices are able to work at the maximum power level and the expected performance in term of EIRP are reported Figure4: The minimum guaranteed value is in the order of 43 dBW over the complete antenna field of view. The antenna is also able to control the transmit antenna patterns side-lobe in order to respect the applicable regulatory recommendations such as ETSI in the European countries [7] and the FCC in the United States. The typical azimuth transmit antenna pattern is reported in Figure5a. The black line represents ((aa)) ((bb)) the mask that has been imposed in the optimization process (corresponding to the ETSI mask for 512 KHz bandwidth).FigureFigure 3.3. G/TG/T vs.Thevs. elevationelevation corresponding angleangle overover elevation thethe horizonhorizon transmit ((aa)) andand antenna typicaltypical azimuthazimuth pattern Rx isRx shown radiationradiation in patternpattern Figure (5(bbb.).).

FigureFigure 4.4. EIRPEIRP vs.vs. elevation elevation angle angle over over the the hori horizonhorizonzon (with (with andand withoutwithout maskmask constraint).constraint).

((aa)) ((bb))

FigureFigure 5.5. Typical Typical azimuthazimuth ((aa)) andand elevationelevation ((bb)) TxTx antenna antenna patternpattern atat 14.2514.25 GHz,GHz, 3030 degreesdegrees elevationelevation overover thethe horizon.horizon. 4. Developed Devices 4.4. DevelopedDeveloped DevicesDevices The main challenges in the design of the antenna have been: (a) the development of the radiating TheThe mainmain challengeschallenges in in thethe design design ofof thethe antenna antenna havehave been: been: (a)(a) thethe developmentdevelopment ofof thethe radiatingradiating element, (b) the development of the active module for transmitter/receiver and (c) the design and element,element, (b)(b) thethe developmentdevelopment ofof thethe activeactive modulemodule forfor transmitter/receivertransmitter/receiver andand (c)(c) thethe designdesign andand manufacturing of a digital vector modulator (DVM) able to integrate in a single chip the attenuator manufacturingmanufacturing ofof aa digitaldigital vectorvector modulatormodulator (DVM)(DVM) ableable toto integrateintegrate inin aa singlesingle chipchip thethe attenuatorattenuator and phase shifter features. These three components are described in the following paragraphs. andand phasephase shiftershifter features.features. TheseThese threethree componencomponentsts areare describeddescribed inin thethe followingfollowing paragraphs.paragraphs. 4.1. Self-Diplexing Radiating Element Design 4.1.4.1. Self-DiplexingSelf-Diplexing RadiatingRadiating ElementElement DesignDesign The first criticality in the design of Tx/Rx radiators is the capability to separate the transmitted The first criticality in the design of Tx/Rx radiators is the capability to separate the transmitted signalThe from first the criticality received in one. the Adesign typical of approachTx/Rx radiators consists is inthe designing capability a radiatingto separate element the transmitted covering signal from the received one. A typical approach consists in designing a radiating element covering signalthe entire from band the received and then one. connecting A typical a approach diplexer consists or a circulator. in designing However, a radiating this configurationelement covering has the drawback to increase the overall dimensions and the ohmic losses at the Low Noise Amplifier (LNA) input. ElectronicsElectronics 2020 2020, 8, ,x 8 FOR, x FOR PEER PEER REVIEW REVIEW 6 of 615 of 15 thethe entire entire band band and and then then connecting connecting a diplexer a diplexer or ora circulator. a circulator. However, However, this this configuration configuration has has the the drawbackElectronicsdrawback2020 to to,increase9, increase 488 the the overall overall dimensions dimensions and and the the ohmic ohmic losses losses at the at the Low Low Noise Noise Amplifier Amplifier (LNA) 6(LNA) of 15 input.input. A Amore more efficient efficient solution solution consists consists of ofimplemen implementing,ting, for for each each radiating radiating element, element, two two different different ports:ports:A One moreOne for for e Txffi cientTx and and solutionone one for for Rx consists Rx signal. signal. of This implementing, This type type of configurationof configuration for each radiating is called is called element, a self-diplexing a self-diplexing two diff erentsolution. solution. ports: ThisOneThis approach for approach Tx and offers oneoffers forbetter better Rx signal.electrical electrical This performanc typeperformanc of configuratione ande and permits permits is the called the reduction reduction a self-diplexing of theof the manufacturing solution.manufacturing This complexity.approachcomplexity. off ers better electrical performance and permits the reduction of the manufacturing complexity. TheTheThe radiating radiating radiating element element element is realizedrealizis realized ed usingusing using twotwo two didifferentff erentdifferent substrates substrates substrates of of RT RT/Duroidof/Duroid RT/Duroid 5880 5880 with5880 with 0.508with 0.508 mm0.508 mmthicknessmm thickness thickness separated separated separated by aby foam by a foam a withfoam with 1 with mm 1 mm 1 thickness. mm thickness. thickness. StartingStartingStarting from from from the the the ground, ground, thethe the firstfirst first twotwo two metallizations metalli metallizationszations are are dedicatedare dedicated dedicated to to the theto receive the receive receive capabilities: capabilities: capabilities: The ThefirstThe first patch first patch patch has has a has diameter a diameter a diameter of of 8.5 8.5of mm 8.5 mm mm and and and it it is is it fed fed is fed byby abya UT047 a UT047 coaxial coaxial coaxial cable; cable; cable; while while while the the the second second second patch, patch, patch, withwithwith a a diameter diametera diameter of of of10.4 10.4 10.4 mm, mm, implements implements the the stackedstack stacked ed configuration,configuration, configuration, by thebythe the foamfoam foam spacer,spacer, spacer, thatthat that allowsallows allowsto tooperate tooperate operate over over over the the the large large large frequency frequency frequencyband. band. band. The The transmittransm transmit radiating radiatingit radiating configuration configuration configuration uses uses uses a asingle singlea single patch patch patch of of of 7.47.47.4 mm mm mm diameter diameter diameter and and and it it isit is fedis fed fed dire directly directlyctly by by byUT047 UT047 UT047 coaxial coaxial coaxial cable cable cable taking taking taking advantage advantage advantage of of theof the the upper upper upper receive receive receive metallizationmetallizationmetallization for for for the the the ground. ground. ground. The The The Figure Figure Figure 6a6a 6ashows shows shows a a sketch sketcha sketch of of theof the the self-diplexing self-diplexing self-diplexing radiating radiating radiating element element element assembly;assembly;assembly; while while while in in inFigure Figure Figure 6b6b 6bthe the the 3D 3D 3D model model model used used used in in fullin full full wave wave wave analysis analysis analysis is is shown. shown.is shown.

(a)( a) (b) (b)

FigureFigureFigure 6. 6. Sketch6.Sketch Sketch of of theof the the self-diplexing self-diplexing self-diplexing radiating radiating radiating element element element (a ()a and()a and) and 3D 3D 3Dmodel model model (b ().b ( ).b).

TheTheThe radiating radiating radiating element element element provides provides provides four four four ports ports ports able able able to to operateto operate operate in in dualin dual dual linear linear linear orthogonal orthogonal orthogonal polarizations polarizations polarizations bothbothboth for for for transmission transmission transmission and and and reception. reception. reception. AA Aprototype prototype prototype of of ofthe the the radiating radiating radiating elem element elementent is is shownis shown shown in in Figurein Figure Figure 7a,7a, 7a,while while while the the the engineered engineered engineered radiator radiator radiator is is is shownshownshown in in inFigure Figure Figure 77b.b. 7b. The The hexagonal hexagonal ground ground makesmakes makes supportsu pportsupport functionalityfunctionality functionality and and and it it circumscribes circumscribes it circumscribes a a circle circle a circle of of22 of22 mm 22mm mm of of diameter. ofdiameter diameter

(a)( a) (b) (b) Figure 7. Radiating element prototype (a) and engineered radiator (b). FigureFigure 7. 7.Radiating Radiating element element prototype prototype (a ()a and) and engineered engineered radiator radiator (b ().b). Return loss at Rx and Tx ports and isolation are reported in Figures 8 and 9, respectively. It is ReturnReturn loss loss at at Rx Rx and and Tx Tx ports ports and and isolation isolation are are reported reported in in Figures Figures8 and8 and9, respectively.9, respectively. It isIt is important to note that the isolation between the Rx and Tx ports over the entire Tx band is better than importantimportant to to note note that that the the isolation isolation between between the the Rx Rx and and Tx Tx ports ports over over the the entire entire Tx Tx band band is is better better than than 20 dB. 2020 dB. dB.

The gain for the Rx and Tx radiating elements have been measured vs. frequency. The results are reported in Figure 10a,b. The gain in free space condition is better than 8 dB for the receive ports over the frequency band with VSWR of 2:1; while at transmit ports 7 dB gain are guaranteed.

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Electronics 2020, 9, 488 7 of 15 ElectronicsElectronics 20202020,, 88,, xx FORFOR PEERPEER REVIEWREVIEW 77 ofof 1515

(a) (b)

Figure 8. Return loss of the radiating element (simulation and measurements) at Rx ports (a) and Tx ports (b). ((aa)) ((bb))

FigureFigureFigure 8. 8.8. Return ReturnReturn losslossloss ofofof thethethe radiating radiatingradiating elementelementelement (simulation(sim(simulationulation andandand measurements)measurements)measurements) atatat RxRxRx portsportsports (((aaa))) andandand Tx TxTx portsportsports ( (b(bb).).).

(a) (b)

Figure 9. Isolations of the radiating element (simulation and measurements): (a) at Rx1 vs. Rx2 and Tx1 vs. Tx2; (b) Rx ports vs. Tx ports. ((aa)) ((bb))

FigureFigureTheFigure gain 9. 9.9. Isolations IsolationsIsolations for the Rx ofofof the theandthe radiating radiatingradiating Tx radiating element elemenelemen elementstt (simulation (simulation(simulation have and andand been measurements): measurements):measurements): measured vs. (((aaa)) ) frequency. atatat Rx1 Rx1Rx1 vs. vs.vs. Rx2Rx2Rx2 The and andand results are reportedTx1Tx1Tx1 vs. vs.vs. Tx2; Tx2;Tx2; in Figure( ((bbb))) RxRxRx ports ports ports10a,b. vs. vs.vs. The TxTxTx ports.ports.ports.gain in free space condition is better than 8 dB for the receive ports over the frequency band with VSWR of 2:1; while at transmit ports 7 dB gain are guaranteed. TheThe gaingain forfor thethe RxRx andand TxTx radiatingradiating elementselements havehave beenbeen measuredmeasured vs.vs. frequency.frequency. TheThe resultsresults areare reportedreported inin FigureFigure 10a,b.10a,b. TheThe gaingain inin freefree spacespace conditioncondition isis betterbetter thanthan 88 dBdB forfor thethe receivereceive portsports overover thethe frequencyfrequency bandband withwith VSWRVSWR ofof 2:1;2:1; whwhileile atat transmittransmit portsports 77 dBdB gaingain areare guaranteed.guaranteed.

(a) (b)

FigureFigure 10.10.Measured Measured gain:gain: ((aa)) RxRx radiatingradiating element;element; ((bb)) TxTx radiatingradiating element.element.

InIn FigureFigure 11,, the the measured measured((aa)) radiation radiation patterns of the radiatingradiating element ( inin(bb) ) freefree spacespace configurationconfiguration areare reportedreported for forFigure theFigure the extreme extreme 10.10. MeasuredMeasured frequencies frequencies gain:gain: of((aa) ) the ofRxRx the radiating operatingradiating operating element;element; receive receive (( band,bb)) TxTx band, radiatingradiating while while for element.element. the for transmitting the transmitting band itband shows it shows the central the central frequency frequency in the in same the same conditions. conditions. The reportedThe reported measured measured radiation radiation patterns patterns are comparedare comparedInIn FigureFigure with 11 11with the,, thethe simulationthe measuredmeasured simulationanalysis radiationradiation analysis results patternspatterns results demonstrating ofof demonstrating thethe radiradiatingating an excellent element elementan excellent agreement.inin freefree agreement. spacespace configurationconfiguration areare reportedreported forfor thethe extremeextreme frequenciesfrequencies ofof thethe opoperatingerating receivereceive band,band, whilewhile forfor thethe transmittingtransmitting bandband itit showsshows thethe centralcentral frequencyfrequency inin thethe samesame conditions.conditions. TheThe reportedreported measuredmeasured radiationradiation patternspatterns areare comparedcompared withwith thethe simulationsimulation analysisanalysis reresultssults demonstratingdemonstrating anan excellentexcellent agreement.agreement.

Electronics 2020, 9, 488 8 of 15 Electronics 2020, 8, x FOR PEER REVIEW 8 of 15

(a) E-plane @10.7 GHz (b) H-plane @10.7 GHz

(c) E-plane @12.75 GHz (d) H-plane @12.75 GHz

(e) E-plane @14.25 GHz (f) H-plane @14.25 GHz

FigureFigure 11. E-plane and and H-plane H-plane radiation radiation patterns patterns for for Rx Rx at at 10.7 10.7 GHz GHz (a) ( aand) and (b) ( band) and at at12.75 12.75 GHz GHz (c) (andc) and (d); (d for); for Tx Tx at at14.25 14.25 GHz GHz (e ()e )and and (f (f).). Continuous Continuous blue blue line: line: measured measured results, results, dashed dashed red red line: numericalnumerical simulations.simulations.

4.2.4.2. TransmitTransmit/Receive/Receive ActiveActive ModuleModule DesignDesign EachEach radiatingradiating element element is directlyis directly connected connected with an wi activeth an circuitry active thatcircuitry provides that the provides functionality the offunctionality the Tx/Rx module. of the Tx/Rx The Txmodule./Rx module The Tx/Rx (intellectual module property (intellectual of Airbus property Italy of [9 Airbus]) is able Italy to realign [9]) is able the linearto realign polarization the linear in polarization receive and in in receive transmit and or, in alternatively, transmit or, alternatively, may allow the may generation allow the of generation a circular polarization.of a circular Thepolarization. phase shifters The usedphase for shifters the polarization used for the control polarization are also able, contro at thel are same also time, able, to at form the thesame antenna time, to beam form imposing the antenna the correctbeam imposing relative phase the correct delay relative between phase the elements. delay between The active the elements. modules areThe composed active modules by the are receive composed device by and the the receive transmit device device and the (the transmit assembled device Tx/Rx (the device assembled is shown Tx/Rx in Figuredevice 12is ).shown in Figure 12). The manufactured modules are presented in Figure 13: In particular, the control board sides are shown. Their dimensions are 20.6 by 7 by 37 mm3, with a weight of 15 g.

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Figure 12. Tx/Rx active module connected to the radiating element.

The manufactured modules are presented in Figure 13: In particular, the control board sides are shown. Their dimensions are 20.6 by 7 by 37 mm3, with a weight of 15 g. Figure 14a shows the channel gain behavior vs. the frequency when the attenuator and the phase shifter are set at the nominal value (zero). Figure 14b shows how all the SSPAs (Solid State Power Amplifier) power is movedFigure between12. Tx/Rx theactive two module output connected ports when to the the radiating relative element. phase between the two channels is properly set: The correct rotation of the linear polarization is provided by the opportunely balancedThe power. manufactured Figure modules 12. Tx/RxTx/Rx are active presented module in connected Figure 13: to In the particular, radiating element. the control board sides are shown. Their dimensions are 20.6 by 7 by 37 mm3, with a weight of 15 g. TheFigure manufactured 14a shows the modules channel are gain presented behavior in vs. Figure the frequency 13: In particular, when the the attenuator control board and thesides phase are 3 shown.shifter areTheir set dimensions at the nominal are 20.6value by (zero). 7 by 37 Figure mm , with14b shows a weight how of all15 g.the SSPAs (Solid State Power Amplifier)Figure power14a shows is moved the channel between gain the behavior two output vs. the ports frequency when the when relative the attenuator phase between and the the phase two shifterchannels are is set properly at the set:nominal The correct value rotation(zero). Figure of the linear14b shows polarization how all is the provided SSPAs by(Solid the opportunelyState Power Amplifier)balanced power. power is moved between the two output ports when the relative phase between the two channels is properly set: The correct rotation of the linear polarization is provided by the opportunely balanced power.

(a) (b)

FigureFigure 13. 13. Tx/RxTx/Rx active active module: module: (a) (Txa) Txand and Rx Rx RF RF active active board; board; (b) ( bTx) Tx and and Rx Rx digital digital control control board. board.

Figure 14a shows the channel gain behavior vs. the frequency when the attenuator and the phase shifter are set at the nominal value (zero). Figure 14b shows how all the SSPAs (Solid State Power Amplifier) power is moved between the two output ports when the relative phase between the two channels is properly set:( Thea) correct rotation of the linear polarization is provided(b) by the opportunely balanced power. Figure 13. Tx/Rx active(a) module: (a) Tx and Rx RF active board; (b) Tx and Rx(b digital) control board.

Figure 13. Tx/Rx active module: (a) Tx and Rx RF active board; (b) Tx and Rx digital control board.

(a) (b)

Figure 14. Tx/Rx active module performance: (a) Rx active module measured gain; (b) Tx active module control power allocation measured between the two output ports.

(a) (b)

FigureFigure 14.14. TxTx/Rx/Rx active(a) module performance:performance: ( a)) Rx active module measuredmeasured(b) gain;gain; ((b)) TxTx activeactive modulemodule controlcontrol powerpower allocationallocation measuredmeasured betweenbetween thethe twotwo outputoutput ports.ports. Figure 14. Tx/Rx active module performance: (a) Rx active module measured gain; (b) Tx active module control power allocation measured between the two output ports.

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Electronics 2020, 8, x FOR PEER REVIEW 10 of 15 Electronics 2020, 8, x FOR PEER REVIEW 10 of 15 4.3. Digital Vector Modulator (DVM) 4.3. Digital Vector Modulator (DVM) 4.3. Digital Vector Modulator (DVM) A DVM in Ku band to control in amplitude and phase the radiating elements has been designed, A DVM in Ku band to control in amplitude and phase the radiating elements has been designed, manufacturedA DVM in andKu band tested. to Itcontrol consists in amplitude of a MMIC and (Monolithic phase the Microwave radiating elements Integrated has Circuit) been designed, in GaAs manufactured and tested. It consists of a MMIC (Monolithic Microwave Integrated Circuit) in GaAs manufactured(Gallium Arsenide) and tested. technology It consists integrating of a MMIC a 5-bit (Monolithic attenuator Microwave (ATT), a 5-bit Integrated phase shifterCircuit) (PS) in GaAs and a (Gallium Arsenide) technology integrating a 5-bit attenuator (ATT), a 5-bit phase shifter (PS) and a (Galliumserial to parallel Arsenide) converter technology (S2P), integrating as reported a in5-bit Figure attenuator 15. (ATT), a 5-bit phase shifter (PS) and a serial to parallel converter (S2P), as reported in Figure 15. serial to parallel converter (S2P), as reported in Figure 15.

Figure 15. GaAsGaAs digital digital vector vector modulator modulator (DVM) (DVM) MMIC. MMIC. Figure 15. GaAs digital vector modulator (DVM) MMIC.

TheThe DVM DVM development development allows allows to to realize realize T/R T/R Modu Modulesles in Ku band with mechanical dimensions The DVM development allows to realize T/R Modules in Ku band with mechanical dimensions andand interconnections reduced reduced with with respect respect to that available on the market, i.e., separated digital and interconnections reduced with respect to that available on the market, i.e., separated digital attenuatorattenuator and phase shifter (Figure 16).16). attenuator and phase shifter (Figure 16).

Figure 16. Rx module with (left) and without (right) DVM. Figure 16. RxRx module module with with (left) (left) and and without without (right) (right) DVM. DVM.

To realize a S2P on GaAs technology the E-mode and the D-mode transistors should be realized ToTo realize aa S2PS2P onon GaAsGaAs technology technology the the E-mode E-mode and and the the D-mode D-mode transistors transistors should should be be realized realized on on the same wafer. To do this, the TQPED (MMIC technological process) process has been selected. onthe the same same wafer. wafer. To doTo this,do this, the TQPEDthe TQPED (MMIC (MMIC technological technological process) process) process process has been has been selected. selected. A 10 A 10 bit S2P has been customized by TriQuint in line with Airbus Italy requirements summarized in Abit 10 S2P bit has S2P been has been customized customized by TriQuint by TriQuint in line in with line Airbus with Airbus Italy requirements Italy requirements summarized summarized in Table in3. Table 3. Table 3. Table 3. Basic characteristics of the DVM. Table 3. Basic characteristics of the DVM. TableITEM 3. Basic characteristics Value of the DVM. ITEM Value ITEMPhase Shifter CellsValue 180◦, 90◦, 45◦, 22.5◦, 11.25◦ Phase Shifter CellsAttenuator Cells 180°, 0.75, 90°, 1.5, 45°, 3, 6, 22.5°, 12 dB 11.25° Phase Shifter CellsRx Band 180°, 10.7 90°,12.75 45°, GHz 22.5°, 11.25° ÷ Attenuator Cells Tx Band 0.75, 14 1.5,14.5 3, GHz6, 12 dB Attenuator Cells 0.75, ÷1.5, 3, 6, 12 dB Rx Band Command Line 10.7 Clock, ÷ 12.75 Data, GHz LE Rx Band Bias 10.7+5; ÷ 12.755 V GHz Tx Band 14 ÷ 14.5− GHz Tx Band Power Consumption 14< ÷ 35014.5 mW GHz Command Line I/O Return Losses Clock,>12 dB Data, LE Command Line Clock, Data, LE Bias +5; −5 V Bias +5; −5 V EachPower attenuator Consumption and phase shifter cell have been < 350 designed mW separately. The position of the cells in the Power Consumption < 350 mW MMIC chainI/O Return has been Losses properly chosen considering >12 dB the RF performances and the layout arrangement. I/O Return Losses >12 dB

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EachEach attenuatorattenuator andand phasephase shiftershifter cellcell havehave beenbeen designeddesigned separately.separately. TheThe positionposition ofof thethe cellscells inin Electronics 2020, 9, 488 11 of 15 thethe MMICMMIC chainchain hashas beenbeen properlyproperly chosenchosen consideringconsidering thethe RFRF performancesperformances andand thethe layoutlayout arrangement.arrangement. TheThe DVM DVM MMIC MMIC andand thethe standstandstand alone alonealone main mainmain block blockblock have havehave been beenbeen manufactured manufacturedmanufactured on onon a 5aa by55 byby 10 1010 mm mmmm2 tile22 tiletile in inina multi aa multimulti project projectproject wafer waferwafer (MPW). (MPW).(MPW). The TheThe manufactured manufacturedmanufactured MMIC MMICMMIC has hashas been beenbeen mounted mountedmounted on an onon aluminum anan aluminumaluminum carrier carriercarrier and andandtested testedtested by means byby meansmeans of a probe ofof aa probeprobe station. station.station. The pictures TheThe picturpictur of theeses DVM ofof thethe under DVMDVM microscope underunder microscopemicroscope is reported isis reported inreported Figure in17in . FigureFigure 17.17.

FigureFigure 17.17. picturepicturepicture underunder microscopemicroscope ofof thethe manufacturedmanufactured DVM.DVM.

TheThe RF RF measurementsmeasurements are areare hereafter hereafterhereafter provided: provided:provided: Relative ReRelativelative amplitude amplitudeamplitude and andand phase phasephase variations variationsvariations (Figure (Figure(Figure 18), 18),18),measured measuredmeasured insertion insertioninsertion loss loss withloss withwith only onlyonly phase phasephase shifter shiftershifter variation variationvariation and phase andand phasephase error error dueerror to duedue the toto attenuation thethe attenuationattenuation states states(Figurestates (Figure(Figure 19), return 19),19), returnreturn loss port lossloss 1 portport and 1 return1 andand returnreturn loss port lossloss 2 port (Figureport 22 (Figure(Figure 20). 20).20).

((aa)) ((bb))

FigureFigure 18. 18. DVMDVM chipchip performance:performance: ((a (aa)) ) measuredmeasured measured relativerelative relative attenuationattenuation attenuation withwith with 11 1 dBdB step;step; ( (bb)) measured measured relativerelative phase phase with with 11.25 11.25 degrees degrees step. step.step.

IL_PhIL_Ph variation variation - -SN:A1 SN:A1 -4-4

-5-5

-6-6

-7-7

-8-8

-9-9

-10-10

-11-11 IL [dB] IL [dB] -12-12

-13-13

-14-14

-15-15

-16-16

-17-17

-18-18 10.610.6 11 11 11.4 11.4 11.8 11.8 12.2 12.2 12.6 12.6 13 13 13.4 13.4 13.8 13.8 14.2 14.2 14.6 14.6 FrequencyFrequency (GHz) (GHz) ((aa)) ((bb))

FigureFigure 19.19. DVMDVM chipchip transmissiontransmission performance:performance: ((aa)) measuredmeasured insertioninsertion loss—onlyloss—only phasephase shiftershifter Figure 19. DVM chip transmission performance: (a) measured insertion loss—only phase shifter variationvariation (32(32 states);states); ((bb)) phasephase errorerror duedue toto thethe attenuationattenuation states,states, referrefer toto 00 dBdB andand 00 phasephase state.state. variation (32 states); (b) phase error due to the attenuation states, refer to 0 dB and 0 phase state.

1

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Input Return Loss Output Return Loss 0 0

-5 -5

-10 -10

-15 -15

-20 -20

-25 -25 RL [dB] RL [dB]

-30 -30

-35 -35

-40 -40

-45 -45

-50 -50 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 Frequency (GHz) Frequency (GHz) (a) (b)

Figure 20. DVM chip return loss performance: (a) port 1; (b) port 2. Figure 20. DVM chip return loss performance: (a) port 1; (b) port 2.

The performances of the 11.25°, 90° and 180° phase shifter cells in the DVM MMIC were very The performances of the 11.25◦, 90◦ and 180◦ phase shifter cells in the DVM MMIC were very good in terms of relative phase shift: An average error of 1° has been measured. Vice versa, the good in terms of relative phase shift: An average error of 1◦ has been measured. Vice versa, the measurement of the 22.5° and 45° cells shows a phase shift higher than the target value (8° and 4°, measurement of the 22.5◦ and 45◦ cells shows a phase shift higher than the target value (8◦ and 4◦, respectively). It has to be highlighted that the phase errors of the 22.5° and 45° cells in the stand alone respectively). It has to be highlighted that the phase errors of the 22.5◦ and 45◦ cells in the stand alone main blocks is the half respect thethe same cellscells insideinside thethe DVMDVM MMIC.MMIC. The performanceperformance of of the the attenuator attenuator cells cells agree agree with with the expected the expected ones, exceptones, except for the phasefor the stability phase vs.stability gain vs. variation gain variation in Tx band in Tx which band presentedwhich presented an undesired an undesired phase phase shift for shift attenuation for attenuation higher higher than 17.25than dB.17.25 However, dB. However, such deviation such deviation for the specificfor the parameter specific parameter is generally is alsogenerally present also incommercial present in devicescommercial and devices is not considered and is not significant considered for significant the specific for project. the specific project. The insertion loss loss of of the the reference reference state state was was about about 11.2 11.2 dB dB and and flat flat in inband. band. Considering Considering the thecombination combination of the of thetwo two COTS COTS (Commercial (Commercial of-t of-the-shelf)he-shelf) component component employed employed in in the the antenna demonstrator we experienced an overall insertion lo lossss of 13 dB. Taking into account the inductive eeffectffect of thethe wirewire bondingbonding interconnections,interconnections, thethe returnreturn losseslosses ofof bothboth portsports areare belowbelow 1010 dB.dB. Finally, the power consumption of the S2P, designed by TriQuint, was 250 mW. This could be drastically reduced (up to 120 mW) withwith thethe implementationimplementation ofof somesome identifiedidentified technicaltechnical solutions.solutions.

5. Test Campaign Results The activity has been finalizedfinalized with an exhaustive test campaign dedicated to evaluate first,first, the performance ofof thethe singlesingle radiatingradiating elementelement in in active active configuration configuration (Figure (Figure 21 21),), second, second, considering considering it asit as a demonstrator a demonstrator of a of linear a linear sub-array sub-array of 5 active of 5 radiatingactive radiating elements elements embedded embedded within the within complete the tilecomplete composed tile composed by 36 dummy by 36 radiators dummy inradiators triangular in1 triangular lattice (Figure lattice 22 ).(Figure 22).

Figure 21. Active radiating element in free space configuration. configuration.

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Figure 22. Active antenna demonstrator. Figure 22. ActiveActive antenna antenna demonstrator. The radiation pattern of the active radiating element in free space configuration have been The radiation pattern of the active radiating element in free space configuration have been measuredThe radiation in several pattern cases where of the the active impinging radiating RF waveelement in linearin free polarization space configuration was characterized have been by measured in several cases where the impinging RF wave in linear polarization was characterized by a measureda defined alignmentin several casesangle where with respect the impinging to the radi RF atingwave element in linear axes. polarization Setting thewas proper characterized amplitude by defined alignment angle with respect to the radiating element axes. Setting the proper amplitude and aand defined phase alignment coefficients, angle the with active respect radiating to the radielematingent elementwas able axes. to Settingalign correctly the proper the amplitude received phase coefficients, the active radiating element was able to align correctly the received polarization andpolarization phase coefficients, providing a verythe activegood discriminationradiating elem betweenent was the able two to orthogonal align correctly polarizations. the received Figure providing a very good discrimination between the two orthogonal polarizations. Figure 23a reports polarization23a reports providingthe received a very cross good polar discrimination component between over the the complete two orthogonal operating polarizations. band, when Figure the the received cross polar component over the complete operating band, when the attenuator and the 23aattenuator reports and the the received phase shiftercross polarcontrols component are correct overly set the for completea copolar operatingsignal with band, the polarization when the phase shifter controls are correctly set for a copolar signal with the polarization rotated by 155 degrees; attenuatorrotated by and155 degrees;the phase while shifter Figure controls 23b showsare correct the samely set behaviorfor a copolar for the signal transmit with channelthe polarization over the while Figure 23b shows the same behavior for the transmit channel over the entire frequency band. rotatedentire frequency by 155 degrees; band. while Figure 23b shows the same behavior for the transmit channel over the entire frequency band.

(a) (b) (a) (b) FigureFigure 23.23. Example of cross polarization discrimination overover thethe frequencyfrequency band:band: (a)) Rx;Rx; ((b)) Tx.Tx. Figure 23. Example of cross polarization discrimination over the frequency band: (a) Rx; (b) Tx. FigureFigure 2424aa showsshows thethe measuredmeasured radiationradiation patternpattern inin receivereceive whenwhen aa misalignmentmisalignment ofof 3030 degreesdegrees hashas beenbeenFigure correctedcorrected 24a shows byby the thethe Tx Tx/Rxmeasured/Rx ActiveActive radiation Module.Module. pattern in receive when a misalignment of 30 degrees has beenTheThe samecorrectedsame performanceperformance by the Tx/Rx hashas beenbeenActive providedprovided Module. forfor thethe activeactive radiating element operatingoperating inin transmittransmit mode:mode:The Figure Figure same 24b 24performanceb shows shows an an hasexample example been providedof of radiation radiation for patte the pattern activern for for radiatinga transmitted a transmitted element polarization polarization operating rotated in rotated transmit of of15 15mode:degrees degrees Figure with with respect24b respect shows to tothe an the nominalexample nominal axes.of axes. radiation pattern for a transmitted polarization rotated of 15 degreesFigureFigure with 2222 respect shows shows to the thethe manufactured nominalmanufactured axes. antenna antenna demonstrator demonstrator which which implements implements the triangular the triangular array geometryarrayFigure geometry in 22 the shows realin the dimensions thereal manufactureddimensions and representing and antenna represen the demo tileting of nstratorthe the tile hemi-spherical ofwhich the hemi-spherical implements phased arraythe phased triangular shown array in arrayFigureshown geometry2 .in A Figure subset in composed2. the A realsubset dimensions of composed five radiating and of five elementsrepresen radiatingting is connected the elements tile of to the theis connected Txhemi-spherical/Rx active to the modules phased Tx/Rx and activearray the shownradiationmodules in patternandFigure the has2. radiation A been subset measured pattern composed forhas di ofbeenff erentfive meas radi beamatingured steering elementsfor different positions is connected beam and polarizationsteering to the positions Tx/Rx alignments. active and modulespolarizationFigure and 25 alignments. thea shows radiation the measuredpattern has radiation been meas patternured whenfor different the excitation beam steering coefficients positions have beenand polarizationapplied to steer alignments. the beam at 10 degrees without the recovery of a polarization misalignment (30 degrees); while in Figure 25b it reported the same measurement but with the recovery of the polarization angle.

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Electronics 2020, 9, 488 14 of 15 ElectronicsElectronics 2020 2020, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 1414 of of 15 15

(a) (b)

Figure 24. Measured pattern after polarization alignment: (a) Rx signal (11.7 GHz) impinging with 30 degrees misalignment with the nominal; (b) Tx signal (14.0 GHz) transmitted with 15 degrees

polarization rotated. ((aa)) ((bb))

FigureFigure 24. 24. Measured Measured pattern pattern after after polarization polarization polarization alignment: alignment: alignment: ( a(a) ( )aRx )Rx Rx signal signal signal (11.7 (11.7 (11.7 GHz) GHz) GHz) impinging impinging impinging with with with 30 30 degrees30degrees degrees misalignmentmisalignment misalignment withwith with thethe the nominal;nominal; nominal; (b(b) ( )b Tx)Tx Tx signalsignal signal (14.0(14.0 (14.0 GHz)GHz) GHz) transmittedtransmitted with with 15 1515 degrees degrees polarizationpolarization rotated. rotated.

(a) (b)

Figure 25. Measured Rx pattern of the active demonstrator: (a) only steering coefficients applied; (b) steering and polarization recovery coefficients applied.

Figure 25a shows((a athe)) measured radiation pattern when the excitation((bb)) coefficients have been applied to steer the beam at 10 degrees without the recovery of a polarization misalignment (30 FigureFigure 25. 25. Measured Measured Rx Rx pattern pattern of of the the active active demonstrator: demonstrator: ( a(a) )only only steering steering coefficients coefficients applied; applied; ( b(b) ) degrees);Figure while 25. Measured in Figure Rx pattern25b it ofreported the active the demonstrator: same measurement (a) only steeringbut with coe ffithecients recovery applied; of the steering(steeringb) steering and and and polarization polarization polarization recovery recovery recovery coefficients coefficients coefficients applied. applied. applied. polarization angle. Figure 26 shows the measured radiation pattern in transmit mode when the excitation FigureFigure 26 25a25a shows showsshows the thethe measured measuredmeasured radiation radiationradiation pattern patternpattern in transmit whenwhen the modethe excitationexcitation when the coefficientscoefficients excitation coehavehaveffi cients beenbeen coefficients are applied to provide the beam steering of −10 degrees and a polarization rotation of 30 appliedareapplied applied toto steer tosteer provide thethe beambeam the beam atat 1010 steering degreesdegrees of withoutwithout10 degrees thethe recoveryrecovery and a polarization ofof aa polarizationpolarization rotation misalignmentmisalignment of 30 degrees. (30(30 degrees. − degrees);degrees); whilewhile inin FigureFigure 25b25b itit reportedreported thethe sasameme measurementmeasurement butbut withwith thethe recoveryrecovery ofof thethe polarizationpolarization angle. angle. FigureFigure 2626 showsshows thethe measuredmeasured radiationradiation pattepatternrn inin transmittransmit modemode whenwhen thethe excitationexcitation coefficientscoefficients are are applied applied to to provide provide the the beam beam steering steering of of − −1010 degrees degrees and and a a polarization polarization rotation rotation of of 30 30 degrees.degrees.

Figure 26. Transmit measured pattern when applying excitation coefficients relevant to 10◦ steering Figure 26. Transmit measured pattern when applying excitation coefficients relevant to− −10° steering angle and recovering 30◦ polarization misalignment. angle and recovering 30° polarization misalignment. 6. Conclusions

The reported technological solutions constitute the base of on-going developments for affordable phased-arrayFigureFigure 26. 26. antennasTransmit Transmit measured formeasured commercial pa patttternern aeronauticalwhen when applying applying satellite excitation excitation communications. coe coeffifficientscients relevant relevant Similar to to − −10°10° developments steering steering angleangle and and recovering recovering 30° 30° polarization polarization misalignment. misalignment.

Electronics 2020, 9, 488 15 of 15 are currently ongoing at Airbus Italy targeting higher frequencies (i.e., Ka-band) as well as space-borne phased-arrays.

Author Contributions: Conceptualization, A.C., P.R. and M.A.; Validation, A.C. and M.A.; Supervision, G.T. and P.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by ESA/ESTEC ARTES 5.1 “Advanced Antenna Concepts for Aircraft In-Flight Entertainment” project, Contract No. 19508/05/NL/JA. Conflicts of Interest: The authors declare no conflict of interest.

References

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