STATE-OF-THE-ART MATERIALS FOR KU AND KA BAND MOBILE SATELLITE ANTENNA

RADOMES

TELECOMMUNICATIONS GROUND SEGMENTS WORKSHOP

ESA/ESTEC, NOORDWIJK, THE NETHERLANDS

3-5 OCTOBER 2012

O. Russo (1), A. Colasante (1), G. Bellaveglia (1), F. Maggio (1), L. Marcellini (1), L. Scialino (2),

L. Rolo (3), J.C. Angevain (3), R. Midthassel (3)

(1) Teleinformatica e Sistemi S.p.A., Via di Tor Tre Teste 229, 00155 Rome (IT) E-mails: orlando.russo@t-e-s.it, alessia.colasante@t-e-s.it,

giancarlo.bellaveglia@t-e-s.it, fabio.maggio@t-e-s.it, luca.marcellini@t-e-s.it (2) Space Engineering S.p.A., Via dei Berio 91, 00155 Rome (IT), lorenzo.scialino@space.it

(3) ESA ESTEC, Keplerlaan 1, PB 299, 2200 AG Noordwijk, The Netherlands E-mails: luis.rolo@esa.int, jean.christophe.angevain@esa.int, rolv.midthassel@esa.int

ABSTRACT

Radome design is always a delicate compromise between electrical and mechanical requirements. In the special case of SATCOM mobile applications, the different mobile environments (maritime, aircraft, high speed trains and rubber wheeled vehicles) yield very distinct radome requirements and require individual attention. This paper discusses the preliminary outcomes of an ESA study in which, taking as starting point the shortcomings of the state of the art radomes, several candidate materials and layups are compared in terms of electro-mechanical performance by means of both theoretical and experimental evaluations.

1.1.1.1. INTRODUCTION

The primary function of a radome is to protect an antenna system from the environments encountered in land-mobile, shipboard and airborne applications (rain, winds, ice, aerodynamic pressure). Radomes have to be designed to have minimum impact on the electrical performance of the enclosed antennas but must also have high mechanical strength properties. Unfortunately, these properties are often mutually exclusive and the best solution has to be a clever compromise between mechanical and electrical performance. The growing demand for broadband services within mobile environments is leading to more and more interest towards the investigations of higher capacity systems. As consequence the Ka Band components of SATCOM systems, as well as the radomes, are of wide interest in the commercial market. In this study several radome materials and wall

configurations have been analyzed in the special case of SATCOM mobile applications (land, maritime and avionic) and for different frequency bands ( Ku and Ka). Each of these applications asks for a custom specification in the radome design taking into account the different nature of the environments. Shortcomings and problems areas for antenna radomes are also herein discussed.

2.2.2.2. STATE OF THE ART FOR ANTENNA

RADOMES

The electrical performance of a radome enclosed antenna is always altered by a radome itself. This is due to distortion of the fields near the antenna caused by interactions at the material interfaces as well as from amplitude, phase and polarization changes in the radome material. The radome environment is a primary factor in determining material type, wall design and shape. All radome designs must consider operating temperature, structural loads, vibration, wind, humidity, sand, hail, rain, etc. 2.1.2.1.2.1.2.1. Review of radome materials

Radome design is based on the materials and construction needed to provide adequate safety margins, acceptable weight and good electrical performance. The primary electrical properties of candidate materials are the relative dielectric constant and the loss tangent at the operational frequencies of the radome.

The materials must provide necessary strength over the service temperature and not have fatigue problems from vibration or impacts. Mechanical stresses are produced in radomes by aerodynamic loading due to airflow, acceleration forces and sudden thermal expansion due to aerodynamic heating (thermal shock). The structural and environmental requirements determine other parameters for a candidate radome material and include:

• Mechanical properties, such as flexural and tensile moduli, strength, and hardness;

• Material density; • Water absorption; • Rain erosion (particle impact) resistance; • Resistance to heat and fire; • The variation of both the mechanical and

electrical parameters of the material due to temperature variations.

The radome configuration evolves from these environmental considerations and from the electrical requirements. The basic problem in radome electrical design is the development of a wall configuration that supports the antenna system requirements and is also compatible with the overall mechanical and environmental requirements. Some basic configurations include homogeneous single-layer (monolithic) and multilayer wall construction (Fig. 2.1.1 shows the common wall types). The best configuration for a particular application depends on the mechanical requirements and operating frequency.

Figure 2.1.1. Radome wall construction configuration

A monolithic-wall consists of a single constant-thickness slab of homogeneous dielectric material. Monolithic radome walls are used in shipboard applications where weight is not a problem and simplicity of design and construction is desired. Multilayer structures (sandwiches) are used when a monolithic wall is unacceptable or does not provide adequate performance as bandwidth, weight, stiffness and strength. Common multilayer configurations include the A sandwich, B sandwich and C sandwich (Fig. 2.1.1). The A sandwich consists of three layers: two dense

high-strength skins separated by a lower-density, lower-dielectric core material such as foam or honeycomb. This configuration can provide much higher strength for a given weight than a monolithic wall (higher strength-to-weight ratio). The B sandwich has three layers, reversed to the A sandwich: a dense core material and two lower-density outer materials, which can serve as quarter-wave matching layers. The B sandwich can provide very wide bandwidth at high incident angles. Nevertheless, as low-density outer layers usually have poor structural or environmental properties, it is only used in applications where this is still acceptable. The C sandwich consists of five layers, like two A sandwiches joined together. These additional layers may be used to provide greater strength and stiffness in the mechanical design and also give more degrees of freedom for the electrical design. Modern radomes are manufactured using composite materials such as fiberglass, quartz, kevlar and polyethylene fibres held together with polyester, epoxy and other resins (Fig. 2.1.2). Foam and honeycomb cores are often added between inner and outer “skins” of the radome to function as a low-dielectric constant spacer material providing structural strength and stiffness. It is important that the dielectric constant of the material is low. A low dielectric constant material reduces reflections, which minimize impact to the radiation pattern and insertion loss. The other parameter to take into account for the electrical performance of the radome is the loss tangent. A radome design where a high loss tangent material is used will, unavoidably, lead to high ohmic losses.

Figure 2.1.2. Composite materials: resin and

reinforcement fibre

Self-Reinforced Polypropylene is a novel material that combines the versatility of a 100% thermoplastic with the high performance of a fibre reinforced composite.

High stiffness, high tensile strength and outstanding impact resistance at low density are the remarkable properties of this material. Such a material has been taken into account as a promising candidate for radome applications and experimental tests at material level have been performed to extract mechanical and electrical parameters (reported later in this paper). Applicability of Metamaterial (MTM) radomes has been also investigated for mobile satellite application. It has been checked whether they were able to offer potential improvements from electromechanical point of view or reduced costs. It has been seen during the study that MTM radomes are used in Radar applications with the aim of reducing boresight error. Additional degrees of freedom offered by the MTMs (like the negative index material) have unfortunately a too narrow-band behaviour to be considered in these bands (Ku and Ka). Frequency Selective Surfaces (FSS) are mainly used for antenna Radar Cross-Section (RCS) reduction i.e. for better control of electromagnetic wave transmission (in the operative frequency bands) and scattering (outside the bands of operation). Bandpass radomes are most often constructed from one or more metallic screens that are sandwiched between dielectric slabs. The metallic screens are perforated in a regular pattern such that at the resonant frequency of the structure, the radome passes nearly 100% of the incident power. Outside the passband, nearly all of the incident power is reflected. Nevertheless, in commercial mobile applications, the above mentioned feature of FSS is not of interest as the main task for the radome is protecting the antenna from external environment while keeping an RF transparent behaviour (at least) at the frequency of interest. 2.2.2.2.2.2.2.2. Shortcomings and problem areas

Shortcomings and problems areas for antenna radomes can be very different according to the specific selected SATCOM applications (land, maritime or avionic) and frequency band (Ku and Ka). Aircraft radomes are a compromise of the conflict between electrical, structural, environmental and aerodynamic requirements. For aircraft radome an 'unfriendly environment' will include the obvious aerodynamic loads (together with shock and vibration loads) and heating effects. In addition, such effects as rain erosion and lightning strike must receive serious consideration. Other possibilities are static charge build-up and damage due to bird or hail impact. Finally, for aircraft applications, the weight of the radome could be a most important feature. As far as maritime radomes are concerned, the main problem areas, related to the sea environment, are the

salt corrosion and the high humidity that can damage seriously the antenna system along with its components below the radome. For these reasons, maritime radomes should be environmentally sealed, offering protection against the harsh saltwater environment and direct solar loading, thus minimizing component failure due to saltwater exposure and excessive thermal stress. The dynamic force of wave impacts should also be included in the structural analyses for maritime radomes together with the other loads, such as wind, temperature extremes (ice, snow), vibrations, hail, bird impact and shock loads. Thermo-mechanical analyses shall provide a robust radome configuration, with sufficient stiffness and strength (e.g. a monolithic laminate or a sandwich with sufficient thick skins and core) to withstand the environmental loads. Considering radome for land mobile applications (e.g. rubber wheeled vehicles and high speed train), one of the most critical aspect concerns bird and stone impacts. This problem specially arises in case of high-speed train antennas where the high speeds and the concomitant presence of stones coming up from the ground against the radome might seriously damage the radome itself and the antenna system below it (Fig. 2.2.1).

(a) (b)

Figure 2.2.1. High Speed Train Radome: external (a) and interior damage (b) due to stone impacts.

For railway applications, due to the critical environmental conditions, many mechanical (strength, stiffness, impact resistance, etc.) and safety aspects (as the fire behaviour of the selected materials) must be kept under control during the radome design phase. As far as mobile antennas operating in the railway environment is concerned, they suffer from the presence of mechanical vibrations, shocks, pressure wave pulse (generated by trains travelling in opposite directions in a tunnel) as well as the outdoor weather conditions including ice, snow, hail and humidity. The achievement, at Ka-band frequencies, of an adequate radome reliability and mechanical robustness versus the antenna electrical requirement is one of the main difficult design goal to achieve. Presence of water on the radome surface also affects the antenna electrical performance when the vehicle is not moving (stations, prolonged stop, etc.). This is especially true in the Ka band.

Water absorbed by a radome increases the dielectric constant and loss tangent of the wall. Preferred radome materials do not readily absorb water or must be treated with protective hydrophobic coatings. Water absorption also reduces the strength of most composite materials. 3.3.3.3. RADOME THEORETICAL ELECTRO-

MECHANICAL DESIGN

Electrical and mechanical model for radome design are unavoidably affected, like all models, by a certain amount of uncertainty. Especially when dealing with state-of-the-art and innovative materials, a given level of uncertainty must be taken into account for both electrical and mechanical parameters that have to be used to evaluate the performance. This reflects on a preliminary design of the radome in which a model retuning must follow (after an experimental phase). Purpose of the model retuning is the identification of more realistic values for mechanical and electrical parameters as well as the thickness of the radome walls at the end of the manufacturing process. Outcomes of the retuning provide the designer with precious information and feedbacks to understand possible unexpected phenomena that may be occurred during the curing phase. When an unexpected behaviour of the process is well-understood with the help of the retuning, specific countermeasures can be taken and a new curing process can be carried out. As shown in Fig.3.1, the whole radome design with multi-objective optimization and model retuning consists of:

• Analysis of electrical, mechanical, environmental and cost requirements. This first step it is important as all the following phases depend on it.

• Selection of possible candidate materials. On the basis of the requirements and the specific application and band (Ku, Ka) at hand, some materials can be selected as applicable, not-applicable and possibly applicable. The latter maybe worth to be investigated even if they may discharge afterwards.

• Selection of possible candidate radome walls. As for the materials, also for the types of radome walls, some solution might be discharged since the beginning (for mechanical and/or electrical unacceptable performance) and some others worth to be tested.

• Run of multi-objective optimization for each

sandwich radome wall and material. Along with the information about cost-requirements, a multi-objective optimization is carried out with the aim of achieving the best compromise between electrical-mechanical and cost performance for each candidate radome wall and material.

• Selection of most promising solutions. At the end of the multi-objective optimization, the most promising radome solutions are chosen to be manufactured.

• Manufacturing of candidate best radome samples. Flat radome samples of different sizes and number are manufactured for electrical and mechanical tests. Sizes of samples for mechanical tests (to evaluate strength and stiffness parameters) are those foreseen by ASTM standards.

• Experimental Tests at sample level (Electrical and Mechanical). At the end of the test campaign, electrical and mechanical parameters are extracted for each flat radome sample.

• Model Retuning at sample level and critical read of its outcomes. Electrical and mechanical models are retuned to fit the experimental results. The outcomes of this phase are important for the understanding of possible unwanted manufacturing process. Such outcomes help the electrical/mechanical designer in the finer design phase.

• EM model design considering shape of radome • EM model manufacturing • Experimental Tests at EM level (Electrical can

Mechanical) • Comparison Theoretical/Experimental • Model Retuning at EM level and critical read

of its outcomes. As for the test campaign at flat radome samples level, results from the test campaign of the shaped radome are used to retune the EM model. A radome design upgrade follows on the basis of the outcomes of this final step.

Processing ofRequirements

Electrical, Mechanical and Cost Requirements

Selection of candidate Materials and

candidate Types of Sandwich

Multi-Objective Layup Optimisation

Electrical and MechanicalParameters Assessment

(Tests on Radome Samples)

Electrical and Mechanical Model Re-Tuning

EM Model Design

Final Electrical and Mechanical Tests on

Manufactured EM Model

End Of Radome Design

Experimental vs PredictionsComparison and update of

Radome Design

Figure 3.1. Radome Design Phases: Block Diagram

3.1.3.1.3.1.3.1. Electrical Analysis at Sample Level

For the radome design at sample level, electrical performance at sample level it is assumed that the radome sample is perfectly flat and infinitely extended. The incident field is a perfect uniform plane wave linearly or circularly polarized (for Ku and Ka band respectively). For a given radome wall hypothesis (A-Type, C-type etcetera) and for the selected candidate material (e.g. Epoxy-Glass, Epoxy-Kevlar), the thickness of each stratification of the sandwich has been optimized with the aim of having a radome with low Insertion Loss and high Cross-Polar Discrimination. The analysis of multilayer radomes (Fig 3.1.1) is carried out by TeS, according to the model firstly published in a paper by Paris [1] and reported by Balanis [2].

Figure 3.1.1. Flat Multi-layer radome model

3.2.3.2.3.2.3.2. Electrical Analysis at EM Level (Shaped

Radome)

For the electrical design analysis at EM level (shaped radome) the whole system composed by antenna and radome is modelled by using in house developed SW. An excellent results agreement is commonly achieved in the final analysis using GRASP analysis tool by Ticra©. The stratification is the same as that the flat model but the shape of the radome is hemispherical (Fig. 3.2.1) or low profile depending on the specific mobile application.

Figure 3.2.1. Shaped Multi-layer radome model 3.3.3.3.3.3.3.3. Mechanical Analysis at sample level

For the radome mechanical design at sample level, a finite element model of a three points bending test (Fig 3.3.1) has been generated in order to calculate the strength and the stiffness of the investigated radome layups. The mechanical behaviour of the candidate sandwiches has been simulated modelling the composite as a set of different plies of orthotropic materials having different thickness and orientation.

Figure 3.3.1. Finite Element Model of a three points

bending test

The candidate layups are then compared in terms of stiffness, strength and weight. In the three points bending configuration the sandwich stiffness is calculated as the ratio between the applied load and the centerline deflection while the strength is evaluated using the Failure Index associated to the Tsai-Wu criterion. Once the stiffness and strength global properties for the sandwich under test have been calculated, an additional comparison can be done in terms of global weight.

3.4.3.4.3.4.3.4. Mechanical Analysis at EM level

For the mechanical analyses at EM level, a Finite Element model of the whole radome can be modelled and analyzed considering the constraints and the external loads (e.g. wind pressure, vibrations) for the selected application. For high speed applications (e.g. high speed train) a Computational Fluid Dynamic simulation can provide the generated Static Pressure Distribution (Fig. 3.4.1) acting on the radome surface. A detailed structural analysis can provide the generated radome displacement and the stress distribution in order to calculate the margin of safety.

(a) (b) Figure 3.4.1. Computational Fluid Dynamic Simulation (a) with the associated Static Pressure Distribution (b)

A Normal Mode Analysis together with an Impact Analysis or alternative experimental impact evaluation (Fig. 3.4.2) can provide the necessary information to avoid possible radome resonance and to verify the impact resistance of the selected radome wall layup.

(a) (b)

Figure 3.4.2. Normal Mode Analysis (a) and Impact Analysis (b)

4.4.4.4. SAMPLES ELECTRICAL AND

MECHANICAL CHARACTERIZATION:

EXPERIMENTAL RESULTS

4.1.4.1.4.1.4.1. Electrical tests at material level

Candidate materials have been all characterized electrically deriving the value of dielectric constant by using a calibration kit in WR90 waveguide (Fig. 4.1.1.). Good agreements have been found between estimated/declared by datasheet values and measured values. In Fig. 4.1.2 the case of self-reinforced polypropylene is given. The manufacturer had provided a value of dielectric constant of 2.18. The estimated value from experimental results (2.20) using the Nicolson-Ross (NR) method [7] is quite in line with such value. Also the other methods gave essentially the same result. In the same picture, the black curve represents the derived dielectric constant using the NR method implemented by TeS Teleinformatica e Sistemi. It can be seen that the green and the black curve gave overlap significantly.

(a) (b)

Figure 4.1.1. WR90 measurement setup (a); Self-Reinforced Polypropylene in sample holder (b)

8 8.5 9 9.5 10 10.5 11 11.5 12 12.52.15

2.16

2.17

2.18

2.19

2.2

2.21

2.22

2.23

2.24

2.25

Frequency [GHz]

Die

lect

ric C

onst

ant

FM

NR

PM

NR TeS

Figure 4.1.2. Self-Reinforced Polypropylene: Electrical Characterization in WR90, TeS's NR method included

4.2.4.2.4.2.4.2. Electrical tests at sandwich level

At sandwich level, five candidate samples were selected and built in the form of 400mm x 400mm square panels. Electrical testing was carried out by means of the Low Frequency Material Characterization System available at ESTEC (Fig. 4.2.1.), which is built-up with two quasi-optical modules assembled inside the Anechoic Room of the ESTEC’s Compact Antenna Test Range (CATR). The S-parameters of each panel were measured at different frequency and incidence angle values. Preliminary results show that model predictions fits very well with the measurements, provided that a fine retuning of each dielectric layer size is carried out and measured dielectric constants are used. The overall experimental results are not yet available at the issue date of this paper.

Figure 4.2.1. ESTEC’s low frequency material

characterization setup

4.3.4.3.4.3.4.3. Mechanical tests at material level

Specific mechanical tests have be performed according to ASTM standards in order to estimate the stiffness and strength properties of the candidate materials. For composite materials, there is very often a lack of reliable data, hence an accurate experimental characterization of the materials is required. This helps the designer in choosing the more appropriate material as well as to development an analytical or numerical procedure suitable to precisely assess the stress field in the sandwich panel and compare this with the limits imposed by the failure criteria related to the different modes of failure. Self-Reinforced Polypropylene material, due to its promising electrical properties along with its low density, has been tested according to ASTM standards (e.g. Fig. 4.3.1 and Fig. 4.3.2) in order to extract strength and stiffness parameters needed for Finite Element analyses.

Figure 4.3.1. Short Beam Test (ASTM D2344)

(b)

(a) (c)

Figure 4.3.2. Tensile Test (ASTM D3039): experimantal set-up (a); material sample before (b) and

after test (c) 4.4.4.4.4.4.4.4. Mechanical and thermal tests at sandwich

level

The candidate layups for each application (land, maritime and avionic) and band (Ku and Ka) have tested at sandwich level (according to ASTM standards) in order to measure the real stiffness and strength parameters. A four points bending test (Fig. 4.4.1.), performed with an high span allows to evaluate the sandwich flexural behaviour providing its global stiffness that can be evaluated from the slope of the Load-Midspan Deflection curve (Fig. 4.4.2.)

Figure 4.4.1. Four Points Bending Test setup (ASTM D7250)

Figure 4.4.2. Four Points Bending Test (ASTM D7250):

Load-Midspan Deflection curve A flatwise Tensile Test (ASTM C297) can provide information about the load transfer between the skins and the core to check the quality of core to facing bonding (Fig. 4.4.3).

Figure 4.4.3. Flatwise Tensile Test setup (ASTM C297)

At the end, thermal tests will be conducted using ESA-LIVAF (Little Vacuum Facility) in order to evaluate the sandwich transverse thermal conductivity.

Figure 4.4.4. ESA-LIVAF for Transverse Thermal Conductivity evaluation.

5.5.5.5. CONCLUSIONS

In the frame of an ESA study about antenna radomes for mobile applications (land, maritime and avionic), TeS Teleinformatica e Sistemi s.r.l. has identified and compared several state-of-the-art materials and stratifications. The whole radome design approach is described. A multi-objective optimization along with one ore more retuning phases are used to achieve a realistic model of the design and extract information about the curing process. At this first part of the study, the main difficulties consisted in the impossibility to directly gather accurate information about electrical and mechanical parameters for the analysis. This reflected on an experimental phase (mechanical and electrical) at the end of which a model-retuning can be carried out. At the end of the first part of the study for each mobile application (land, maritime and avionic) and band (Ku, Ka) the most promising candidate sandwich has been selected and manufactured to be put under electrical and mechanical test. 6.6.6.6. ACKNOWLEDGEMENTS

The authors would like to acknowledge the contribution to this work which was made by Elena Saenz and Juan Sanz-Fernandez for the radome samples free space electrical characterization using ESTEC quasi-optics measurement system (ESTEC Antenna Section). 7.7.7.7. REFERENCES

1. Paris D. T. (1970) Computer-Aided Radome Design, IEEE Trans. Antennas Propagat., AP-18, pp. 7-15, Jan.

2. Balanis C. A. (1989) Advanced Engineering Electromagnetics, John Wiley & Sons.

3. D. J. Kozakoff, (2010) Analysis of Radome-Enclosed Antennas, Boston: Artech House

4. MIL-HDBK-17 - Composite Materials Handbook

5. O.O. Ochoa - J. N. Reddy (1992) Finite Element Analysis of Composite Laminates, Kluver Academic Publisher

6. Z. Gurdal, R.T. Haftka, P. Hajela (1999) Design and Optimization of Laminated Composite Materials, John Wiley & Sons

7. A.M. Nicolson, G.F. Ross (1970) Measurement of the Intrinsic Properties of Materials by Time-Domain Techniques, IEEE Trans. Instrumentation and Measurements, IM-19, pp. 377-382, Nov.