Mechanical investigation of a multi-layer reflectarray for Ku-band space antennas

Aerospace Science and Technology 10 (2006) 618–627

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Mechanical investigation of a multi-layer reflectarray for Ku-band spaceantennas

Untersuchung des mechanischen Verhaltens einesMehrschicht-Reflektarrays für eine Weltraum Antenne im Ku-Band

Leri Datashvili a,∗, Horst Baier a, Jose Antonio Encinar b, Herve Legay c

a Institute of Lightweight Structures, Technische Universitaet Muenchen, 85747 Garching, Germanyb Universidad Politécnica de Madrid, ETSI Telecomunicación, 28040 Madrid, Spain

c Alcatel Alenia Space, 26, Av. J. F. Champollion, Toulouse, France

Received 13 February 2006; received in revised form 6 June 2006; accepted 30 June 2006

Available online 2 August 2006

Abstract

A multi layer sandwich panel of printed reflectarray (RA) is a complex composition of many different materials enabling a required performancewith exceptional properties. The major properties of a multilayer RA include the following: the use of array layers of reflective patches formingthe flat structure gives the possibility to focus the electromagnetic waves; the use of more than one reflecting layer is widening the RF bandwidth;the possibility of having two different polarisations and two different contours of the beams.

To achieve the mentioned properties, analytical and experimental investigations have been carried out. A sandwich panel for a three-layerprinted reflectarray for dual polarisation with a different coverage in each polarisation has been designed, manufactured and tested. The measuredradiation patterns of the RA show that gain requirements are fulfilled in a 10% bandwidth. Thickness and flatness accuracy requirements are alsofulfilled. Low dissipative electric losses (0.35 dB) have been achieved, as a result of an appropriate mechanical design and selection of low-lossmaterials for the RA panel.© 2006 Elsevier Masson SAS. All rights reserved.

Zusammenfassung

Eine mehrschichtige Sandwichplatte einer gedruckten Reflect-Array-Antenne ist eine komplexe Konstruktion mehrere verschiedener Werk-stoffe, die vorteilhafte Eigenschaften für die geforderten elektrischen und mechanischen Anforderungen ermöglicht. Die Haupteigenschafteneiner mehrschichtigen Reflekt-Array-Antenne sind: die Verwendung von mehreren Schichten mit einem Muster reflektierender Punkte versehenerFolie, die die flache Struktur bilden, ermöglicht es, die elektromagnetischen Wellen zu bündeln; der Gebrauch von mehr als einer reflektierendenSchicht ermöglich die Ausweitung der RF Bandbreite ebenso wie die Möglichkeit, zwei unterschiedliche Polarisierungen und zwei unterschied-liche Strahlformen zu haben.

Um die erwähnten Eigenschaften zu erreichen, wurden numerische und experimentelle Untersuchungen durchgeführt. Eine Sandwichplatteeiner drei-Schicht Reflectarray-Antenne für zweifache Polarisation mit jeweils unterschiedlichem Übertragungsgebiet wurde entworfen, gefertigtund getestet. Das gemessene Strahlungsdiagramm des Reflektors zeigt, dass die gewonnenen Erfordernisse in einer 10 prozentigen Bandbreiteerreicht sind. Die Anforderungen an die Genauigkeit für Dicke und Ebenheit der Oberfläche sind ebenso erfüllt. Geringe Strahlungsverluste (0.35dB) wurden erreicht als Ergebnis eines geeigneten mechanischen Entwurfs und der Auswahl von strahlungsverlustarmen Werkstoffen für dieReflectarray-Sandwichplatte.© 2006 Elsevier Masson SAS. All rights reserved.

* Corresponding author.E-mail addresses: [email protected] (L. Datashvili), [email protected] (H. Baier), [email protected] (J.A. Encinar),

[email protected] (H. Legay).

1270-9638/$ – see front matter © 2006 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.ast.2006.06.007

L. Datashvili et al. / Aerospace Science and Technology 10 (2006) 618–627 619

Keywords: Reflectarray antenna; Reflectarray mechanical design; Printed reflectarray; Multilayer sandwich; Dual polarisation; Dual coverage; Thermal expansionof thin sheet

Schlüsselwörter: Reflekt-Array-Antenne; Reflectarray mechanischer Entwurf; Gedruckter Reflekt-Array; Mehrschichtiger Sandwich; Zweifache Polarisation;Zweifaches Übertragungsgebiet; αT von flexiblem Material

1. Introduction

Reflectarrays for space applications are being more andmore widely developed and discussed in the literature due totheir remarkable electrical and mechanical properties. Accord-ing to previously known electrical performances, printed reflec-tarrays with contoured beams can be an alternative to the on-board solid shaped reflectors in space applications. The reasonfor that is their lower cost, lower mass and a shorter and sim-pler manufacturing, since the need for complex shaped mouldsis eliminated. Easiness of the accommodation of the flat struc-ture together with the satellite in the launcher compared to thecurved structures is another attractive mechanical property ofthe reflect arrays.

Exceptional electrical performances of the multilayer reflec-tarray (RA) sandwich panel include the following aspects:

– First, using one or several array layers of reflective rectan-gular patches (together with the ground plane) gives thepossibility of forming a flat structure being still able to fo-cus or to shape the reflected electromagnetic beam like aclassical parabolic or shaped reflector does.

– Second, using more than one reflecting layer, widening ofthe RF bandwidth can be achieved.

– Third, there is the possibility of having two different polar-isations and two different contours of the beams just dueto the possibility of independent controlling and optimis-ing the dimensions of the rectangular patches in differentdirections.

Contoured beam reflectarrays for Direct Broadcast Satelliteapplications were demonstrated in [7] and [8] using one andthree layers, respectively, with printed patches of variable size.It was demonstrated in [2] that bandwidth in printed reflec-tarrays can be significantly improved by optimising the patchdimensions in a three-layer configuration. When three-arraylayers are used, the electrical performances can be improved,but at the cost of a higher sensitivity on mechanical tolerancesin the electrical performances.

Therefore, much attention must be put on mechanical de-sign, manufacturing and analysis of the multi-layer sandwichpanel, in order to minimise thermo-elastic distortions and toachieve a high accuracy in the whole manufacturing process.In particular, thickness of each layer, patch dimensions and theroot mean square (RMS) of the panel surface deviations are themost sensitive parameters for the electrical performances, andtheir possible variations must be minimised and evaluated, notonly during manufacturing process, but also in space thermalconditions.

Moreover, dissipative losses are increased when three-arraylayers are used in the panel. Therefore, the selection of low-lossmaterials for the sandwich is essential to reduce the electricallosses to acceptable values for antennas in space applications.

There are few works in the literature giving the discussionon mechanical design of sandwich panel RAs. In [1] the me-chanical design and technology of a panel of the multi-layerreflectarray is described. As for the reflecting surface it uses18 µm-thick copper cladding on 0.025 mm-thick Kapton film.Layers are separated by the use of foam material Rohacell 51.

More work is performed in the direction of deployable re-flectarrays formed with membrane electrical layers supportedwith inflatable frames [4–6]. Membrane reflectarrays show theiradvantages in the aperture length range of above 4–6 meters,when the area becomes large and a mass saving is decisive forthe design. For smaller sizes, stiff sandwich panel approach isreasonable to be used for space applications.

This paper focuses on the mechanical design, analysis andtests of a three-layer sandwich RA breadboard of 1 m in di-ameter, which has been designed to replace a dual gridded re-flector in space applications. The requirements are a contouredbeam in H-polarisation (11.45–12.75 GHz) and a pencil beamin V-polarisation (11.45–11.7 GHz). As an example of cover-age, Europe and east coast of USA are taken respectively. ForH-polarisation the requirements consist of two gain contours of28.5 dBi and 25.5 dBi. For V-polarisation the minimum gainrequirement is 37 dBi in the coverage region. Typical pointingerrors are also taken into account.

In reflector technology, these requirements can be met withdual gridded reflectors, having a different reflecting surface foreach polarisation. However, in the proposed technology, the twoindependent beams, one for each polarisation, are generatedwith a single reflectarray flat panel, and consequently, a sig-nificant reduction in mass and volume is achieved.

2. Electrical and mechanical concepts of the reflectarrayantenna

2.1. Electrical definition of the reflectarray

The reflectarray antenna configuration is shown in Fig. 1.An elliptical reflectarray with axes 1036 mm and 980 mm hasbeen chosen, the same size as the dual gridded reference an-tenna. Two feeds are defined for V- and H-polarisation, respec-tively, see Fig. 1. The period is defined as 14 × 14 mm (0.6λ at12.75 GHz and 0.53λ at 11.45 GHz). Both, feed locations andperiod for the reflective elements have been chosen to avoid theappearance of grating lobes at any angle of incidence.

For the contoured beam in H-polarisation, a phase-only syn-thesis technique has been applied to obtain the phase distribu-tion on the reflectarray surface. A progressive phase has been

620 L. Datashvili et al. / Aerospace Science and Technology 10 (2006) 618–627

Fig. 1. Geometry and the periodic cell of a three-layer reflectarray.

considered for the pencil beam in V-polarisation. To ensuregood performance in the required frequency bands, a config-uration with three stacked patch arrays is used (Fig. 1). Thephase shift on each polarisation is controlled by respective sidedimension of the rectangular patches. The dimensions of therectangular patches are optimised to achieve the required phaseshift for each beam at several frequencies in the working band,as described in [3].

2.2. Reflectarray mechanical concept and key properties

The important features of the reflect array are generatedtogether with the full panel – the mechanical structure. Thismeans that all the efforts for characterising RF and thermo-mechanical properties of the panel also serve for characteris-ing the multilayer composition and vice versa. The reflectarrayantenna (Fig. 2) for the verification of RF and mechanical con-cepts consists of the following parts: RA sandwich panel, panelsupport structure and feed horn with an arm. The arm is linkedto the panel support structure in order to enable a full rotation ofthe reflectarray antenna structure during the RF measurementsand to provide the breadboard with two different positions ofthe feed horn.

Key properties of the RA breadboard are:

• Aperture, mm elliptical, 1036 × 980.• Operational frequency, GHz 11.45–12.75.• Polarisation/coverage dual/dual.• Panel thickness and deviations, mm 37.464 ± 0.053.• Mass of the RA panel, g 2250.• First eigen-frequency (free-free),

measured (calculated), Hz 146 (156).• Surface flatness RMS accuracy at RT, mm 0.25 (best fit).

Fig. 2. Reflectarray antenna breadboard.

Fig. 3. Cross section through height and a detail of the panel.

Fig. 4. The stiffened electrical layer.

The multilayer sandwich panel of the RA consists of threereflecting (also called ‘electrical’) layers B, C and D (Fig. 3)which include Kapton-copper foils with a Kevlar/resin stiff-ening layer on one (top/front) side only (Fig. 4). Reflectivelayers are separated using Nomex honeycomb separators. Allthese mentioned layers together with the ground plane (layer Ain Fig. 3) form the so called ‘electrical sandwich’. The stiff-ening sandwich with carbon fibre reinforced plastic (CFRP)


Table 1Materials and Lay-up of the RA panel

Layer Component Fibredirection

Av. thick.[mm]

S Carbon T300/Cyanate 0/90◦ 0.375Ester LTM 123 ±45◦

0/90◦Core Nomex honeycomb 30A Carbon T300/LTM 123 0/90◦ 0.361

±45◦0/90◦

Separator Nomex honeycomb 2B Kapton/Copper – 0.09959

Kevlar 49/LTM 123 0/90◦Separator Nomex honeycomb 2C Kapton/Copper – 0.0997

Kevlar 49/LTM 123 0/90◦Separator Nomex honeycomb 2D Kapton/Copper – 0.1001

Kevlar 49/LTM 123 0/90◦

facesheets is used to support the reflecting layers of the elec-trical sandwich from the rear side. One of the CFRP facesheets(layer A) serves as a ground plane at the same time so it belongsto both electrical and stiffening sandwiches simultaneously.

In Table 1 materials and lay up of the RA panel are given.Along with the fibre orientation in the layers, the average mea-sured thicknesses of stiffened electrical layers and the nomi-nal thickness of separators are given in the table. In betweenof all separators and all layers there are LTM123 resin layers(∼ 70 g/m2) for bonding the honeycombs to the layers.

The manufacturing process of the RA breadboard with thesandwich panel, support structure and the arm for the horn in-cludes the following major steps:

– Pre- and post-curing of carbon/LTM123 layers S and A (seeFig. 3 for layers numbering) and assembling of the stiffen-ing sandwich.

– Pre- and post-curing of each Kevlar/LTM123/Kapton/cop-per layer and packing against moisture uptake.

– Bonding of the honeycomb separators to the Kevlar face-sheets and assembling of the panel.

– Bonding of the support structure adapters to the panel andcutting of the panel.

– Assembling the RF test configuration of the breadboard us-ing the support structure and the horn arm (Fig. 2).

The manufacturing process ensures high accuracies of thethickness of each Kevlar composite layer, of the separation dis-tance between the electrical layers and of the positioning of thefeed horn relative to the RA panel.

2.3. Sensitivity of electrical performances on mechanicalparameters of RA

Tolerance errors in manufacturing process and thermo-elastic distortions in the patch dimensions, layer thickness andcomposition, will produce a change in the phase of reflection,and consequently a degradation of the electrical performances

of the antenna. The influence of the different geometrical pa-rameters on the radiation patterns has been studied by electro-magnetic simulation, and the maximum deviations have beendefined. The most important conclusions are summarised to-gether with the comments on achieved manufacturing accura-cies as:

• In the sandwich configuration, the real thickness of Kevlarlayers must be known with high accuracy (better than±0.010 mm) to be taken into account during the electri-cal design. This is a very stringent requirement that obligesto a very accurate manufacturing process for flatness, uni-formity and repeatability of each individual layer. Duringthe manufacturing, the average thickness repeatability of±0.004 mm was achieved. Standard deviation of the thick-ness of each Kevlar/resin layer over the surface is in therange of 0.012 mm with the minimum and maximum ab-solute deviations of ±0.024 mm.

• For patch dimensions, both manufacture tolerances andthermal distortions must be kept less than ±0.050 mm. It isa really strict requirement, since the accuracy of the photoetching process for the copper/Kapton film in most casesis in the range of ±0.040 mm. So patch thermal expan-sion compensation was performed using Kevlar compos-ite layers keeping the patch thermal elongations below of±0.010 mm.

• For a thickness of honeycomb layers, both manufactur-ing tolerances and thermal distortions must be kept lessthan ±0.1 mm, and their dielectric constant (DK) must bedetermined within the accuracy of ±0.01. An aerospacequality of honeycomb cutting ensures the mentioned ac-curacy of the thickness. The honeycomb average thicknessof 2.042 mm and the standard deviation of 0.020 mm weremeasured for the RA sandwich panel. The same accuracywas maintained during the sandwich assembling process.Through-thickness thermal expansion of the separators isinsignificant.

• Out of plane deviation RMS of the panel of 1 mm do notproduce appreciable effect in the electrical performances.Nevertheless, 0.25 mm RMS of manufacturing deviationsand 0.425 mm RMS of thermal orbital deformations arecalculated for the RA sandwich panel.

All the achieved accuracies allow concluding that the man-ufactured sandwich panel meets most of the accuracy require-ments.

3. The material aspects of the reflectarray

3.1. Panel RF requirements and selection of base materials

Selection of the materials to form the complete multilayersandwich panel and especially the electrical layers is connectedto the certain design complexities. There are two major require-ments defining and constraining properties of the materials tobe selected:


• The low RF loss design requirement.The number of electrical layers (3 + ground plane) makesthis requirement even stronger. No single conventionalepoxy system can meet this requirement because of theyare characterised by very high loss factor.

• Thermo-elastic stability of the sandwich.Copper patches in electrical layers produce high thermaldeformations due to their high coefficient of thermal ex-pansion (CTE) and Yong’s-modulus. To compensate thesedeformations a composite material with close to zero oreven negative CTE is required at each electrical layer.

The lowest RF loss factor compared to the other resin sys-tems was found in Cyanate Esters and is around 0.005. CyanateEster LTM 123 (from ACG Ltd.) was selected as a resin systemwith a DK of 2.7. Low moisture absorption is another usefulfeature of this selected resin system, which significantly re-duces expansion/contraction of the structure due to moistureloss. Kevlar 49 plane weave fabric with a negative CTE of thefibres was selected for the thermal deformation compensationat electrical layer level.

Based on these considerations a Kevlar fabric reinforcedCyanate Ester LTM123 composite is used to stiffen the Kapton-copper reflective layers against thermal loads. For the backsidesandwich of the panel the same resin and T300 carbon fibre fab-ric have been used, one facesheet of which serves as a groundplane of the reflectarray. By this the use of a metal ground planewas avoided which would contribute, as analysed, in a highthermal deformations and high mass of the panel. For valida-tion of the use of a carbon fabric as a ground plane separateRA sandwich was built and electrical tests were performed. Re-sults of the tests confirmed that CFRP could be used as a groundplane with an acceptable value of dissipative losses of 0.1 dBi.The use of high modulus carbon fibres (e.g. M40j) can furtherreduce these losses.

A Nomex honeycomb is used to separate the electrical lay-ers. Note that the panel did not have to be subjected to the vac-uum tests. Therefore the vent-holes were not important to havein honeycomb at this stage. For the space hardware of coursethis point cannot be omitted (e.g. triax honeycomb solves thisproblem easily). For bonding of the layers to the honeycombseparators a thin film of LTM 123 resin was used.

3.2. Thermo-mechanical material characterisation

To enable FE modelling and analytical verification of themechanical design of the RA panel an extensive test programwas set for the selected materials and components of the struc-ture.

Standard tensile tests were performed to measure Kevlar/resin and carbon/resin composite elastic properties. Results aresummarised in Table 2. All specimens were manufactured withan average fibre volume fraction of 63%.

In order to obtain effective material properties for the com-plete electrical layer, which consists of a Kevlar/resin/Kapton/copper composition, special CTE measurements were per-formed. The measurement method using the vertical dilatome-

Table 2Measured material properties of RA sandwich layers

Materials E1[MPa]

G1,2[MPa]

ν1,2 CTE[∗10−6/◦C]

Kevlar/LTM 123 31100 1550 0.066 −0.225Carbon/LTM 123 57100 3800 0.04 1.95Nomex honeycomb 28.3

Fig. 5. Specimens for CTE measurements (a) and measurement configurationin vertical dilatometer (b).

ter enables high accuracy measurements of the length variationusing the inductive displacement transducer (measuring toler-ance 0.02 µm). This is very important since the materials arehaving a very low CTE. In principle, measurements can be per-formed in the temperature range from −200 ◦C to +500 ◦C.Temperature is measured using thermocouples. For the spec-imens of RA materials thermal strain was measured over thetemperature range from −150 ◦C to +150 ◦C.

The specimens for CTE measurements of the mentionedcomposition have a peculiar design (Fig. 5(a)). The basic idea


Table 3CTE measurement results of stiffened electrical layers

Layers of Kevlar/LTM 123/Kapton/copper B C D

CTE [×10−6/◦C] 4.4 4.01 3.93

of the design is to form such a structure that can maintain itsshape and disposition under the vertical dilatometer conditionsto enable pure thermal deformation measurements. Specimensare formed by rolling thin flexible material layers in cylin-ders of ∼8 mm diameter. The rolled shapes are fixed usingKevlar ropes (Fig. 5). After putting the specimen into the ver-tical dilatometer tube a thin quartz plate is placed on top of thespecimen to support the push rod of the dilatometer (Fig. 5(b)).This design of specimens for CTE measurements is already be-ing successfully used for many other thin and flexible structuralmaterials.

In Table 3, a list of CTE measurement results obtained usingthe above-described method is given. CTE of all three electri-cal layer specimens were measured in order to understand aninfluence of different copper contents within the layer.

Results show that the effective CTE of the Kevlar/Resin/Kapton/copper composition decreases by decreasing the coppercontent in the layer because of the cooper having high stiffnessand CTE is a biggest source of thermal deformations. Gener-ally, the thermal deformations of the copper patches are wellsuppressed by the use of a Kevlar fabric since low CTEs weremeasured. These results enabled to calculate effective engineer-ing constants of the composition for the use in the analyses ofthe sandwich panel addressed in the following.

4. Thermo-elastic analysis of the reflectarray andexperimental mechanical verification

4.1. Parametric study and design optimisation of the RAmultilayer sandwich panel

A parametric finite element (FE) model of the RA sandwichpanel was established in ANSYS software using layered solidelements with all layers of the RA panel modelled as homoge-neous layers. A variation of all parameters has been performedfor studying the influence of each of them on the RA panelsurface deviation RMS. The most important parameters arebackside support sandwich core thickness, Kevlar/epoxy lay-ers thickness and fibre volume fraction. These parameters allowgood adjustment and tuning of the resulting RMS of the out ofplane deformations of the panel surface.

Copper patch layer thickness influences very much the RMSand, since it is the significant source of deviations the minimumthe better.

Suppression of the patch thermal deformations using singleKevlar/resin plies at both sides of the Kapton-copper layer givesbetter results for surface RMS. However, reduction of mass andRF losses can be achieved by putting the Kevlar/resin layer onone side only.

So, a design optimisation has been performed for the RApanel in order to identify accuracy capabilities of the panel for

both options (option with one Kevlar layer and option with twoKevlar layers at each electrical layer) under different thermaland moisture load cases. Both deformations (RMS) and weightwere to be minimised.

The following continuous and discrete variables were con-sidered:

– thickness of the core of the backside sandwich;– fibre volume percentage in each woven fabric ply with re-

spective changes of the stiffness characteristics and thick-ness of the ply;

– fibre directions in each stiffener −0/90◦ or +/−45;– materials in the facesheets of the backside stiffening sand-

wich, T300 and/or M40J.

In total 18 variables for the one Kevlar layer option and 24variables for two Kevlar layer option were considered. 3 ther-mal and 3 moisture load cases were defined. Both include uni-form load and two directional through thickness gradients.

In Fig. 6, RA panel design optimisation results are shownthat are obtained using an evolutionary algorithm. There aretwo Pareto fronts for the two different designs (1 and 2 Kevlarplies) shown in figure. Upper right points represent individualsfrom the first generations and lower left points mean more re-cent. The front is shifted to right for the second design, what islogical since it contains the weight of three additional plies. Butit offers a lower RMS limit (Fig. 6).

However, comparing the results with the same scale for themass (between e.g. 2.4 and 2.8 kg/m2) it becomes clear thatthe second alternative provides no better result than the firstone in the interesting weight region. The first option with oneKevlar/resin layer on one side of the Kapton film is satisfyingthe RF requirements having lower losses and less RMS. Basedon these results the individual with a backside core thickness of30 mm is chosen for the design of the panel.

A sensitivity analysis has been performed showing that 10%deviation of core thickness and fibre volume fractions (alsomeans respective thickness variation) in different layers influ-ence the total RMS value significantly – in the range of 20%for each variation. Fibre directions have no remarkable influ-ence on the total RMS.

4.2. Reflectarray surface deviation analyses

The established FE model was used to carry out RA surfacedeviation RMS analyses. As already mentioned, all layers ofthe laminate were considered as homogeneous layers includingthe layers of copper patches.

For the model’s material properties practically all valueswere experimentally measured. Some missing properties of thelayers were calculated via two different methods.

• First method includes the use of detailed modelling ofa piece of the laminate with Kapton/copper bonded onKevlar/resin ply.

• Second method is to model all materials as a continuouslayer with known properties of the components. The varia-


Fig. 6. Designs of optimal goal compromises with one (a) and two (b)Kevlar/resin plies.

tion of the thickness of the copper continuous layer is per-formed until the resulting CTE of the laminate matches themeasured one for Kapton/copper/Kevlar/resin specimens.

The second method has produced more accurate results, sothe effective material properties of the electrical layer were de-termined using the second method.

To calculate effective thickness of the resin for honeycombbonding as a uniform thin layer a microstructure modelling wasperformed. Note that the resin is actually placed only alongthe honeycomb edges. The analysis resulted to a 0.025 mm ef-fective thickness of the resin, which is much less compared to0.06 mm calculated by the resin layer weight of 70 g/m2.

Two worst thermal load cases were defined for the surfaceaccuracy analysis: 1. Top surface −230 ◦C and 2. Bottom sur-face −230 ◦C, all referenced to zero and combined with thecorresponding through thickness gradient determined accord-ing to the backside core thickness. These load cases include

Fig. 7. RMS vs. backside sandwich core thickness for worst case loads.

Fig. 8. Heat-map recorded with thermography and transferred to the FE model.

manufacturing thermal loads (−60 ◦C) also and are still conser-vative.

The maximum RMS of thermal deformations of the multi-layer sandwich panel as it would be exposed to the extreme lowtemperatures (load case 2) is equal to 0.425 mm for a backsidecore thickness of 30 mm (Fig. 7). Electromagnetic simulationsdemonstrated that this value of RMS do not produce any re-markable negative effect in the electrical performance of theantenna.

For the electrical tests, which are performed at room tem-perature the optimised value of RMS of panel thermal deforma-tions is calculated for the manufacturing thermal load only. Thisload causes much less deformations than the load case 2 sinceit is about the quarter of the later one and implies no throughthickness thermal gradient. The RMS equals to 0.127 mm for achosen core thickness of 30 mm.

4.3. Experimental thermo-mechanical verification

Two kind of thermo-elastic tests were carried out for sand-wich panel design verification. The first test was performedat uniform temperatures and the other included also a throughthickness temperature gradient. Thermography recording of theRA panel’s surface temperatures gave the possibility to trans-fer the heat map to the established FE model for the simulation


Table 4Thermal deformations RMS comparison

Experiment[mm]

Simulation[mm]

Delta[%]

RMS 0.107 0.102 4.3Avg. max Uz 0.182 0.193 6.0

Table 5Comparison of the eigen-frequencies according to FEM and test results

Analysis [Hz] Experiment [Hz] Delta [%]

156 146 6.85

(Fig. 8). Surface shape deviations were measured using pho-togrammetry with measurement accuracy of 20 micrometers.

Not optimised RMS of surface thermal deformations forboth, simulation and experiment, are calculated and show agood correlation. In Table 4, RMS values for uniform tempera-ture test are given along with the simulated values. The averagesof maximum out of plane deformations (measured and calcu-lated) are compared also in the table. The difference is in therange of 6%.

Another type of verification test performed on the RA sand-wich panel is the experimental modal analyses. In Table 5,a comparison of the measured and simulated eigen-frequenciesis given for free-free conditions. The air effect to the first eigen-frequency, calculated to be around 11 Hz, was taken into ac-count by adding to the measured value. Measurements and sim-ulation correlate well in this case as well.

5. Electrical tests and electrical design verification

Radiation patterns of the RA antenna have been measuredin a planar near field system for both polarisations. In Fig. 9the RF test setup is shown for H-polarisation configuration.The measured patterns in gain are superimposed to the requiredcoverages in Figs. 10 and 11, as a function of the angular co-ordinates u–v in the reflectarray coordinate system previouslyshown in Fig. 1.

5.1. V-polarisation

Fig. 10 shows the gain patterns at extreme frequencies. Thegain requirement of 37 dBi is fulfilled for the whole frequencyband from 11.05 to 12.1 GHz, which is wider than required.Table 6 shows the maximum gain measured at the SMA in-put and includes all the losses. Maximum cross-polar is themaximum gain of the cross-polar radiation. The cross-polar lev-els in the coverage for H-polarisation are always lower than−5 dBi, which corresponds to a cross-polar isolation better than30 dB

5.2. H-polarisation

In Fig. 11, the measured co- and cross-polar patterns areshown for H-polarisation at central frequency. Table 7 showsthe maximum values of gain and cross-polar levels. Note

Fig. 9. Reflectarray RF measurements in an anechoic chamber.

Fig. 10. Measured gain patterns for V-polarisation at 11.05 GHz and 12.10 GHz.

that the values in gain are around 9 dB below those forV-polarisation, because the reflectarray patches are used toproduce a shaped wide beam for European coverage in H-polarisation, while a pencil beam with maximum gain is gen-erated in V-polarisation. The gain patterns practically fulfil therequirements, not only at central frequency as shown in the fig-ure, but also in the band 12.7–13 GHz in close agreement withthe simulated radiation patterns, but with a small shift in the fre-quency band to higher frequencies (the RA panel was designedfor the 11.45–12.75 GHz band). The frequency shift observedin H-polarisation and not in V-polarisation is the result of asmall anisotropy of the honeycomb produced by the hexagonal


Fig. 11. Measured co- and cross-polar contour patterns for H-polarisation at12.1 GHz.

Table 6Measured gain for V-polarisation

Frequency[GHz]

Gain[dBi]

Maximum cross-polar[dBi]

11.050 38.536 1.29611.250 38.944 4.00111.450 39.097 5.21111.575 39.408 5.70711.700 39.617 5.59211.900 39.658 5.85412.100 39.382 5.470

cells, not considered in the design, because the dielectric con-stant is lower in H-polarisation, with electric field across theribbon.

The cross-polar levels are always lower than −8 dBi in thecoverage associated to V-polarisation, which corresponds to across-polar isolation better than 45 dB.

The reflectarray was also tested in a spherical near fieldsystem at 12.1 GHz to measure the directivity. The losses areobtained as the difference between measurements of gain anddirectivity, and they are equal to 0.4 ± 0.18 dB. These lossesare slightly higher than in a conventional dual-gridded reflec-tor antenna (0.10 dB for the front shell and 0.20 dB for the rearshell), but they are very low when compared with other results

Table 7Measured gain for H-polarisation

Frequency[GHz]

Gain[dBi]

Maximum cross-polar[dBi]

11.250 29.039 4.59711.450 30.454 3.67011.700 30.855 4.37112.100 31.195 3.88212.500 31.619 3.38012.750 31.002 2.81113.000 30.633 2.639

reported in reflectarrays. The reduction in dissipative losses waspossible due to the selection of both the appropriate low-lossmaterials and the design with one Kevlar skin in each electri-cal layer, as demonstrated in the mechanical design. A furtherreduction can be achieved by using other types of low-loss ma-terials in the sandwich configuration, based on quartz-fibres andquartz-honeycomb.

6. Conclusions

For a multi-layer sandwich reflectarray antenna, electricaland mechanical investigations were carried out and the latterone was presented in more details in the paper. The electricaland mechanical designs, analyses, manufacturing and thermo-elastic characterisation as well as RF tests were performed. Thereflectarray meets RF and mechanical requirements for the Ku-band space applications.

The reflectarray mechanical concept allows having the me-chanical and the electrical design processes uncoupled. Theestablished FE model enabled to perform high accuracy sim-ulations based on the measured material properties, especiallybased on the CTE measurement of the highly flexible thinspecimens of the electrical layers. Correlation between thermo-elastic and modal tests and respective simulations is in the rangeof 4–7%.

The sandwich panel of the RA antenna shows exceptionalelectrical and mechanical performances. Both electrical andmechanical design and analyses have been successfully val-idated. The RMS surface deformations of the panel do notproduce any deterioration of the electrical performances of theantenna. The electrical performances show that the two linearlypolarised independent beams are achieved, with the requiredwide bandwidth (10%), low dissipative losses and satisfactorycross polarisation levels. Mechanical and electrical results pre-sented in this paper show that the proposed concept for thereflectarray panel could be a valid alternative to conventionalsolid reflector antennas in space applications.

Acknowledgements

This work has been supported by the European SpaceAgency under the contract No. ESTEC/16919/02/NL/JA. TheAuthors gratefully acknowledge a support of Mr. G. Toso.


References

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Mechanical investigation of a multi-layer reflectarray for Ku-band space antennas

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