On the development of Antenna feed array for space applications by additive manufacturing technique

Accepted Manuscript Title: On the Development of Antenna Feed Array for Space Applications by Additive Manufacturing Technique Authors: S.S. Gill, Hemant Arora, Jidesh, Viren Sheth PII: DOI: Reference:

S2214-8604(16)30338-4 http://dx.doi.org/doi:10.1016/j.addma.2017.06.010 ADDMA 190

To appear in: Received date: Revised date: Accepted date:

6-12-2016 22-6-2017 27-6-2017

Please cite this article as: S.S.Gill, Hemant Arora, Jidesh, Viren Sheth, On the Development of Antenna Feed Array for Space Applications by Additive Manufacturing Technique (2010), http://dx.doi.org/10.1016/j.addma.2017.06.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

On the Development of Antenna Feed Array for Space Applications by Additive Manufacturing Technique S S Gill, Hemant Arora*, Jidesh, VirenSheth Space Applications Centre – ISRO, Ahmedabad, Gujarat, India *Corresponding Author e-mail: [email protected] Abstract Space agencies are looking for advanced technologies to build light weight and stiff payload components to bear space environment and launch loads. Additive manufacturing (AM) techniques like Direct Metal Laser Sintering (DMLS) is one of the suitable option which can be explored for space applications. This paper highlights the development process of Antenna Feed Array (AFA) using DMLS as an Additive Manufacturing (AM) technique. A high efficiency horn element is used in the array. Such horns are preferred for this development as they are the prime choice for feed elements in High Throughput satellites (HTS) that employ Multibeam Antennas. A brief description of Multibeam antennas along with the RF design process for the high efficiency horn is presented. In the development process, certain design rules of AM are adopted based on consideration to produce self-sustaining structures. AFA realized by DMLS is evaluated by functional testing, vibration testing for space qualification test levels. Test results shows its structural intactness which proves its space worthiness. Procedures are very well established for further development of space payload components. Keywords: Antenna Feed, Additive Manufacturing, DMLS, Space Payload

Introduction: The space payload components are required to be designed and analysed with extreme care because they are irreparable and required to be maintenance-free. Payload components are designed to withstand extreme space environment and launch loads. The components should be light weight and highly stiff to withstand such loads. In order to fulfil these requirements, the payload subsystems are required to be built in a single set up avoiding any alignment errors. Payload assemblies are targeted to have minimum number of parts which will lead to assembly time saving, reducing joints and fasteners, decreasing RF leakage, mass saving and minimizing the alignment issues. To meet the above requirements, new technologies like DMLS [1] is explored for realizing space worthy components. Specifically, for space payload structures which are produced as thin shell and light weight, DMLS is an advantageous option in terms of material and machining time saving, compared to conventional machining techniques. For realization of these parts by conventional techniques 80-90% of material is scooped out whereas DMLS deposits material wherever needed. Producing complex geometries [2] at no added cost is another advantage. Initially few test samples and parts were built with AlSi10Mg powder [4] using DMLS to have insights into the technology. The test samples are qualified for physical and chemical properties, electrical conductivity, outgassing, CVCM (collected volatile condensable materials), thermo-vacuum test and other environmental tests necessary for space use qualification. This paper intends to highlight the development process of a specific Space Payload component viz. Ka band Antenna Feed Array (AFA) using DMLS. AFA is a set of antenna feeds arranged in a 4x4 array as shown in Fig. 1 which will be used for Ka band Antenna communication in ISRO’s GSAT series of Satellites. AFA is designed to build up in a single Page 1 of 13

setup and integrated sub-system with advantage of reduction in time, mass, alignment errors and providing better structural intactness. The realized AFA as built by DMLS is tested successfully for RF functional aspects and Dynamic launch loads to prove their space worthiness as per set specifications.

Fig.1 CAD Model of Ka band Feed Cluster

Multi Beam Antennas: The need for higher power and bandwidth primarily driven by multimedia based services has resulted in a new class of satellites called High Throughput satellites which essentially uses the same spectrum multiple number of times. The frequency spectrum is a scarce resource not only for terrestrial communications but also for satellite communications. In order to increase the usage of the available spectrum referred to as frequency re-use, the same spectrum is used in a different polarization. Spatial separation between antenna footprints using the same spectrum is another method adopted for re using the allotted frequency band. In general High Throughput satellites have the intended coverage region illuminated by multiple spot (narrow) beams as opposed to a single large footprint in conventional satellites. Essentially the beams with very high gain are used to populate the coverage area. The spot beam has a narrow half power beam width and can be as low as 0.4 deg for a typical Ka band applications. Spatial and polarisation separation in the spot beams results in tremendous amount of frequency re-use. An HTS satellite would have at-least 5 to 10 times the capacity of conventional satellites. To realise such spot beams, these satellites have typically employ multiple reflectors with four being the preferred number. The narrow spot beams are achieved with a very large size of the reflector resulting in a high gain and hence better link performance for high data rate applications.

DMLS Technique: DMLS is a bottom - up additive technique where components are built from bottom to top, layer by layer depositing powder and sintering it. Principle of DMLS is based on layer by layer deposition in which a thin layer of the powder material is applied on the platform to build desired component. The process is to melt down thin layers of 20 to 60µm of metal powder with a programmed Laser beam. Laser beam fuses the layer of powder at exactly the spots defined by software. The platform is then lowered and another layer of metal powder is applied. Once again the material is fused so as to bond with the layer below at the predefined points resulting in a complex part. Every single fused layer creates a new micro segment of the final part and can therefore be monitored. Basic working principle of DMLS is shown in Fig. 2.[3] Page 2 of 13

Fig. 2 Principle of DMLS Technique

Design of AFA for “Additive Manufacturing”: While designing AFA, Design for Additive Manufacturing (DFAM) considerations are adopted to minimize the support by generating self-sustaining overhang areas. An illustration for producing unsupported areas is made in Fig.3. Orientation of the component for building the final shape is an important aspect in DFAM. Support will be built under unsupported/overhang portions of the same material. In order to avoid support generation on overhangs, the part should be redesigned to be self-supporting or secondary processes may be required to remove supports. Design rules for additive manufacturing adopted are given as: a) Flanges in AFA are modified to become self-supporting b) Supports in AFA are generated in such a way that they can be removed easily by machining. c) Sharp corners are rounded off or filleted as required d) Supports are designed in such a way that it should not hinder with the functional aspect

Fig.3 Illustration for producing various unsupported sections [2] AFA is built up with consideration of described design rules. Basic design of AFA begins with RF design which gives a requirement of cone shaped cluster (4x4). RF design of a single feed with dimensional requirement of diameter and length at each section is shown in fig. 4 and array of 16 feeds is shown in fig. 5. Dimensional accuracy and geometrical tolerances like diameter, length, parallelism, and orthogonality are important factors for RF functional Page 3 of 13

performance. Topology of AFA is optimized to gain variable density profile to achieve maximum stiffness at low mass. Unsupported flange of each feed horn is made self-sustaining to avoid supports getting generated from bottom. Feed horns are built with variable thickness from 0.8 to 1.3 mm for proving the process capability with maintaining its structural integrity.

x

Fig. 4 Image of RF Design requirement of a single Feed

Fig. 5 Image of RF design of 4x4 Feed Array

Fig. 6 CAD model of Mechanical design of 4x4 Feed Array Feeds are of conical in shape with inner diameter varying from 10.43 to 38.4 mm over a length of 137.8 mm. Mechanical Design of 4x4 AFA for built with DMLS of overall size Page 4 of 13

233.4 x 233.4 x 137.8 mm is shown in fig.6. The mass of DMLS built AFA is 1.3 Kg which is substantially lesser than the mass assembly of individual feed horns of same size and material assembled together to form a feed cluster. Total mass of assembly made by individual horns is 2.28 kg which includes mass of feeds, base & top plates and fasteners. The lesser mass of DMLS built AFA is because the top and bottom solid plates are merged with flanges which do not require any fastening interfaces. Time taken for building DMLS feed cluster is approximately thirty-nine hrs which is also comparatively lesser than the manufacturing of individual feed horns and assembling it to form a cluster.

Comparison with Conventional methodology: Sixteen horns Antenna Feed Array realized by DMLS methodology is compared with realization by conventional manufacturing techniques as described in Table 1. Table 1: Comparison Table of DMLS feed array realized with Conventional methodology and DMLS methodology  









Conventional Methodology All sixteen horns fabricated separately,  assembled with a common base and top plate with fastening joints. Lot of weight penalty due to additional  mounting interface on feed horn flanges, base and top plates and fasteners. Time of realization of Feed array assembly including manufacturing, dimensional inspection, assembly is more than 60 hrs. All parts are machined in different setup. Therefore, alignment issues are obvious and correction made to maximum possible extent with lot of rework. Last minutes changes in design are hard to implement and are time consuming , may not be able to meet the project schedules Tolerances achieved up to ± 20u with surface finish of Ra less than 1.5u.



DMLS Methodology All horns Feed array realized as a single part with no need of separate base and top plate and without any joints. Light weight mass optimized structurally stiff feed array is built up with at least saving of 40-50% mass as compared to Conventional methodology. Time of realization of DMLS feed array is 39hrs.



Alignment errors are very less as whole subsystem is built-up in a single setup.



It has quick development time, easily adaptive in any last minute changes in design.



Tolerances achieved up to ± 30u with surface finish of 5u Ra.

RF design of Multimode horn As described, the allotted frequency is reused amongst adjacent beams thus increasing the net throughput of the satellite. Each beam is typically realised with the help of a feed horn illuminating the reflector and is referred as the Single Feed per beam configuration. However multiple feeds can also be shared to realise a beam. The position of the beam is defined by the coverage requirement and directly sets an upper limit on the size of the feed diameter. Severe space constraints thus result in the focal plane of the feed array owing to the constraints on feed Page 5 of 13

pattern imposed by performance requirements. A typical requirement for such a feed is that the Edge taper (the amount in dB by which amplitude of the feed pattern reduces at the edge of the reflector) is around 10- 12 dB. In order to realise such an edge taper, the feed should be able to maximise the radiation from the physical aperture and that is quantified by the term Aperture efficiency. Conventional feeds used in typical reflector based applications are the corrugated horns, choked horns or Potter horns. However, such horns have very poor aperture efficiency in the order of 50 –70 % depending on the bandwidth. In order to achieve the Edge Taper required of such Multibeam Antennas, the diameter of the above mentioned feed horns would have to be very large. However, the space constraints in the focal plane prohibit the use of such horns. To achieve the requisite efficiency a special type of feed horn is employed. The defining feature of such horns is that it not only employs the fundamental mode of propagation but also higher order modes. The higher order modes are generated within the RF region with the help of special features within the feed horn interior in addition to the fundamental TE11. Specifically, the higher order TE1, n modes are excited by which the phases between the various modes are adjusted. The generation of the modes are effected by circularly symmetric slope discontinuities of specific diameters extending for a pre-determined length. [7] The use of such high efficiency horns results in a significantly narrower feed pattern, which helps to reduce Side Lobe levels.

RF Performance Evaluation: The RF performance test is carried out on the realized AFA by measuring a host or parameters such as Return loss, isolation between the elements in the array, radiation patterns and peak gain parameters over the frequency band of 27 to 29 GHz. The Return Loss and Isolation values are measured using standard Vector Network Analysers and the feed pattern is measured at Anechoic Chamber. The Anechoic chamber consist of a transmit antenna illuminating the feed cluster. The power levels received by the horns are measured for various angular orientations. Such a measurement gives the spatial (angular) variation of the feed pattern. AFA was assembled on a mounting fixture for radiation testing at anechoic chamber with required alignment as shown in fig.7. Measured RF results are compared with predicted results and are quite encouraging. The measured normalized radiation pattern of DMLS feed array at 28GHz frequency is shown in fig. 8. The measured Return loss and Isolation for two adjacent horns are shown in fig. 9. The Return Loss and Isolation values are better than 24 dB and 60 dB in the band of interest. The maximum and minimum gain of the feed cluster was 20 dB and 19.7 dB, which corresponds to a worst case aperture efficiency of nearly 74 %. The predicted aperture efficiency was nearly 83.5 %. The finite surface roughness of the interior of the horn contributes to increase the loss component of the horn.

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Fig. 7 DMLS built AFA under test in Anechoic Chamber

Fig.8 Measured radiation pattern of 3D printed horn feed at 28GHz frequency

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Fig. 9 Measured Return Loss and Isolation of two adjacent horns

Space Qualification: Various tests under space qualification scheme have been carried out on DMLS built samples made from AlSi10Mg powder. The details of tests are given in table 2. Table 2: Test and Characterization for Space Qualification 1

2

3 4 5 6 7

Chemical Analysis

Elements

By Specified Spectro Value [8] (%Wt) 11 9.0-11.0 0.15 ≤ 0.55 0.004 ≤ 0.05 0.039 ≤ 0.45 0.3 0.2-0.45 0.011 ≤ 0.05 Balance Balance Specified Value

Si Fe Cu Mn Mg Ni Al Hardness Test Measured Value 70 HRB 68±7 HRB 2.78 gm/cc 2.70 ± 0.27 gm/cc Mass Density 0 163.22 W/m 173± 10 W/m0 C Thermal C Conductivity Test 320.396 MPa 335± 20 MPa Tensile Test (UTS) 70 ± 10GPa Modulus of Elasticity 80.6 GPa Surface Treatment (Silver Plating and Black Anodising)

Deviation

-------Remarks --

-+0.6 Successfully qualified on DMLS built samples

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8

Thermal Cycling Test

9

Thermo Vacuum Test

10

Vibration Test

No degradation observed. No degradation observed Successfully undergone Qualification levels of 17.5g

Dynamic Characterization: Dynamic stability is an important aspect of designing space born structures. Dynamic stability relies on stiffness of structure which needs to be taken care at design stage. Dynamic stiffness of structure is designed in such a way that the first natural frequency should be at least √2 times the excitation frequencies of launch vehicle in order to avoid dynamic coupling. Apart from natural frequencies, modal effective mass and mode shapes of components at modal frequencies are considered in design of space based sub-systems [5]. Structural integrity of AFA is a critical parameter which need to be evaluated for launch loads. AFA is structurally analysed [6] using ANSYS simulation software for predicting fundamental modal frequencies and random responses at critical locations up to qualification test specifications. AFA structure is modelled with 3D elements with fixed boundary conditions at the bottom flanges of individual feed horn. Normal mode and random response analysis are carried out. First modal frequency is predicted at 458.1 Hz and amplifications observed are within limits. First modal frequency is well above the threshold frequency of 100 Hz for spacecraft borne subsystem which shows the adequate structural stiffness of AFA. Result obtained for first mode is shown in fig. 10 and random response is shown in fig. 11.

Fig. 10 First natural frequency mode shape Page 9 of 13

Fig. 11 Random Response at 17.5 grms Realized AFA is subjected to qualification level Sine and Random vibration test in parallel and normal direction to mounting plane. Vibration Test specifications are tabulated in Table 3.

Fig. 12 AFA mounted on vibration table A photograph of DMLS built AFA on vibration table is shown in fig. 12 and one of the results plots is shown in fig. 13. The first natural frequency obtained is 501.7 Hz which is more than the predicted first modal frequency. Therefore, DMLS based feed cluster proves to be a structurally stiff DMLS built AFA to withstand/survive launch dynamic loads.

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Table 3: Vibration Test Specifications PSD (g2/Hz) (Qualification Level) Frequency (Hz)

20 – 100

Normal

to

Parallel to

Mounting Plane

mounting plane

+ 3 dB/octave

+

3

dB/octave 100 – 700

0.28/octave

0.1/octave

700 - 2000

- 6 dB/octave

-

3

dB/octave Overall RMS

17.5 g

11.8 g

Duration

120 secs

120 secs

Fig.12 Screen shot of Vibration Results Non-destructive test reveals no physical damage to AFA post vibration test and no internal cracks are detected which represents the structural intactness of DMLS feed array. No change or degradation in RF performance is observed post vibration.

Further Investigation: Further investigation on AlSi10Mg DMLS built samples for weld ability, Electro Discharge Machining (EDM), Electro Discharge wire cut and CNC milling is under progress for its suitability for producing many space based subsystems which are being realized at present by conventional manufacturing techniques. Many space based subsystems which are functionally not critical are straight away replaced with DMLS built mass optimized stiff structures. One example is mounting bracket for Antenna feed array which is required to be structurally stiff Page 11 of 13

to intact the position of AFA. These traditionally designed brackets are bulky and are very good candidate for redesigning to suit DMLS and reduce drastic weight using topographic optimisation techniques.

Conclusion: Additive Manufacturing is definitely the technology of tomorrow. To explore additive manufacturing for space application, an effort has been made in development of Ka band feed cluster by DMLS technique with material AlSi10Mg powder. Various design iterations have been made in the finalization of configuration in order to gain large stiffness with low mass. Certain design rules are adopted with consideration of additive manufacturing design guidelines. DMLS technology is very well demonstrated in development of Ka band AFA. The feed cluster has successfully undergone space environment loads at qualification level and RF tests. First natural frequency obtained is 502Hz which is well above the threshold frequency for Space payload systems. Maximum Von Mises stresses (3 Sigma) obtained under Random vibration is 50.7 MPa which has adequate margin. RF performance test shows quite satisfactory results at 27-30 GHz. Therefore, Ka band AFA for Satellite antenna is developed with stateof-the-art DMLS technique using AlSi10Mg powder and its space worthiness is proved and established. On the same guidelines, many other subsystems are in development stage which are targeted for futuristic communication satellites projects.

Acknowledgements: Authors wish to express their gratitude towards Tapan Misra, Director, SAC-ISRO, Ahmedabad, DD – MESA, GD MSFG, GD ASG, GM PFSF for encouraging and guiding us to carry forward this work. Special thanks are due to Late Shri P Bhar , who initiated DMLS work at SAC. Authors also wish to thank various SAC facilities - AMF/ASG, SSD/STG, PMF/MSSG, QCMF, STPD/ENTSG without whose support it would not have been possible to complete this work. Special thanks are due to the team at Wipro Infrastructure Engineering, Bangalore, for their valuable suggestions and manufacture of the feed cluster.

References: [1] G. Thomas, D. Robert, “Direct Metal Laser Sintering – Identification of process phenomena by optical in-process monitoring”, Laser Technik Journal, 1/2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim [2] B.P. Conner, G. P. Manogharen, A. N. Martof, L. M. Rodomsky, C. M. Rodomsky, D. C. Jordan, J. W. Limperos, “Making Sense of 3D printing: Creating a map of additive manufacturing products and services”, Additive Manufacturing, Vol. , Oct. 2014, pp 64-76. [3] A. R. R. Bineli, A. P. G. Peres, A. L. Jardini, R. M. Filoh, “Direct Metal Laser Sintering (DMLS): Technology for Design and Construction of Microreactors”, 6th Brazilian Conference of Manufacturing Engineering, April 11-15, 2011, Caxias do Sul, RS, Brazil. [4] N. T. Aboulkhair, N. M. Everitt, L. Asheroft, C. Tuck, “ Reducing the porosity in AlSi10Mg parts processed by Selective Laser Melting”, Additive Manufacturing, Vol. , Oct. 2014, pp 77-86.

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[5] Dave S. Steinberg, “ Book on Vibration Analysis of Electronic Equipment, Third Edition, John Wiley & Sons, Inc. publications. [6] C. Herzfeld, “ Analysis of 3-D printed structural components for Cube Satellites”, excerpt from proceedings of the 2014 COMSOL conference in Boston. [7] K.K Chan. S Rao “Design of high efficiency circular horns feeds for multi-beam reflector applications “ IEEE Trans on A&P Vol 56, pp 253-58, Jan -2008 [8] Material Data Sheet of EOS Aluminium AlSi10Mg material, EOS GmbH- Electro Optical Systems, AD Weil/05.2014

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