диафрагмированные волноводные фильтры / 0b2ab0ae-44e4-40c1-ab19-92ee238a3af2

ABSTRACT  |  Additive manufacturing technologies are currently envisaged to boost the development of a next generation of microwave and millimeter-wave devices intended for, among others, satellite telecommunications, navigation, imaging, radio-astronomy, and cosmology. Due to their excellent electromagnetic and mechanical properties, all-metal waveguide components are key building blocks of several radio frequency (RF) systems used in these application domains. This article reports on the prospects originating from the application of allmetal 3D printing to the manufacturing of high-performance microwave waveguide devices. The technology investigated is the selective laser melting process, where a laser beam is used to fuse metal powder particles spread over a building platform. The complete parts are built by overlapping several constantthickness layers. An overview on process parameters, material properties, and design rules is reported for this technology. The electromagnetic properties of test samples built in Al and Ti alloys have been experimentally characterized. A robust design of Ku/K-band filters aimed at satellite telecommunications has been implemented in several prototypes manufactured in Al. The corresponding measured performance confirm the applicability of the laser selective melting process to the intended applications.

KEYWORDS  | Additive manufacturing; microwave devices; microwave filters; 3D printing

I.  INTRODUCTION

Recently, several research institutions and industries are investigating the applicability of three–dimensional (3D)

Manuscript received June 15, 2016; revised September 12, 2016; accepted October 15, 2016.

O. A. Peverini, M. Lumia, G. Addamo, and G. Virone are with the Istituto di Elettronica e di Ingegneria dell'Informazione e delle Telecomunicazioni, National Research Council of Italy, Turin 10129, Italy. (e-mail: oscar.peverini@ieiit.cnr.it).

F. Calignano, M. Lorusso, E. P. Ambrosio, and D. Manfredi are with the Istituto

­Italiano di Tecnologia, Center for Sustainable Futures Ð CSF@PoliTo, Turin 10129,

Italy.

printing techniques to the manufacturing of all-metal waveguide­ components [1]–[6]. This class of radio frequency (RF) components is widely used in microwave and millimeter-wave antenna-feed chains developed for different applications, including imaging, satellite communica- tions, navigation, earth observation, and scientific surveys.

Indeed, waveguide components exhibit excellent electromagnetic performance along with high-power capability, low-losses, robustness, and repeatability [7]. Examples of waveguide components used in antenna-feed systems are filters and diplexers for frequency discrimination/separation

[8]–[10], ortho-mode transducers (OMTs) for polarization separation/combination [11], [12], polarizers and phase- shifters for implementing differential phase-delays between two polarizations or signals [13]–[15], directional couplers for extracting signals from a common waveguide [16], and feed-horns for illuminating the reflectors [17], [18]. Apart from some specific cases, in most waveguide components, dielectrics are not used to avoid additional performance degradation and manufacturing uncertainties.

Powder bed fusion processes, such as Electron Beam Melting (EBM) and Selective Laser Melting (SLM), build all-metal parts without the use of machining tools and may not require subsequent metal plating (depending on the metal powder used). These technological aspects translate into the following main advantages for RF engineers:

•more efficient development of new components, since prototyping activities have reduced time and costs;

•additional design flexibility, because no mechanical constraints arise from the use of machining tools (free-form fabrication), even if some design restric-

tions also apply to additive manufacturing (AM) processes.

Satellite telecommunications is one of the application domains, where SLM could be a technological solution for the implementation of a next generation of microwave and millimeter-wave components (Section II).

This paper reports on the research activities currently underway at CNR-IEIIT and IIT, focused on the applicability of the SLM technology to the manufacturing of allmetal components. The SLM technology has been selected because of

•the wide range of materials that can be processed, including aluminum, titanium, and steel alloys;

•an admissible manufacturing accuracy that ranges from 0.02 mm to 0.1 mm (depending on the specific material used); and

•the building-chamber dimensions and manufacturing speed that enable the development of RF compo- nents with dimension within a 200 mm x 200 mm x 200 mm envelope.

Conversely, surface roughness and dimensional accuracy of SLM parts strongly depend on material and process param- eters. As a consequence, a reliable manufacturing of high- quality RF components requires

•an SLM-oriented design, as the one reported in Section V;

•careful setting of the process parameters, e.g., scan speed, laser power, and hatching distance; and

•different post-processing steps, e.g., shot-peening and surface plating.

The description of the SLM process presented in Section III aims at illustrating the process parameters and material properties that mostly influence the quality of parts man- ufactured through SLM. Section IV reports the experimental assessment of the electromagnetic performance of components­ builtinaluminum(Al)andtitanium(Ti)alloys.

Fig. 1.  High throughput satellite network: a relevant application domain for the selective laser melting manufacturing of microwave systems.

Fig. 2.  Ku/K-band antenna-feed system aimed at fixed and broadcast satellite services. The system includes two ortho-mode transducers (OMTs) for dual-band operation. (a) Block diagram.

(b) Flight model (courtesy of Thales-Alenia Space Italia).

The former provides an equivalent surface conductivity of approximately ​20 μΩ​cm and a surface roughness Ra lower than 5 μm​. Finally, the development of waveguide filters is reported as a relevant benchmark for the applicability of the

SLM technology in the microwave field (Section V). In par- ticular, low-pass filters manufactured in aluminum through SLM have been measured to provide reflection coefficient and insertion loss values lower than −25 dB and 0.1 dB in the [12.5, 15] GHz passband. The measured rejection values in the [17.5, 21.2] GHz stopband are higher than 50 dB.

II.  A RELEVANT APPLICATION DOMAIN

Satellite communications is one of the application domains in which direct-metal printing is envisaged to boost advancement in RF equipment. Fig. 1 shows a telecommunication network based on the use of a high-throughput satellite

(HTS) to connect the user terminals to the content provid- ers and data servers [19]. To provide the required services, the antenna-feed systems embarked on board of the satellites have to operate in dual-polarization and multi-carrier regime over multiple and wide bands [20]. As an example, Fig. 2(a) shows the block diagram of a Ku/K-band dualpolarization antenna-feed systems aimed at fixed and broad- cast satellite services (Ku-Tx band: [10.7, 12.75] GHz; Ku-Rx

band: [13.7, 14.5] GHz; and K-Tx band: [17.2, 18.5] GHz). The wideband corrugated horn that illuminates a reflector antenna is connected through a common waveguide to a

K-band ortho-mode transducer. The latter is used to inject thetwoleft/right-handcircularpolarizations(LHCP/RHCP) at the feed-horn port in the K-Tx band. The vertical and hori- zontal polarizations VP and HP in the Ku band are either routed toward or extracted from the common waveguide through the Ku-band bandpass filter and OMT. A Ku-band waveguide diplexer is inserted in each polarization channel to separate the Tx and Rx signals allocated to different fre- quency channels. Fig. 2(b) shows the corresponding flight hardwaredevelopedbyThales-AleniaSpaceItaliaincollabo- ration with CNR-IEIIT [21]. As it can be noticed, to enable machining via conventional techniques (e.g., electrical discharge machining and milling), the antenna-feed chains are manufactured in several parts that are subsequently assembled together. As a consequence, contacting flanges are unavoidable, which in turn force stringent constraints in the electromagnetic design. Indeed, oxidation of contacting flanges can generate high levels of passive intermodulation products that impairs the RX functionalities. Additionally, mass and envelope cannot be optimized because of the use of mounting screws that prevents miniaturization of compo- nents, especially above 30 GHz.

Next-generation HTS systems will operate from 30 GHz to 50 GHz and, potentially also at 80-100 GHz, and will be based on multispot architectures implementing space- diversity schemes [22]. Depending on the frequency-reuse factor and covering area, the number of spots required are in the order of hundreds. Interference among adjacent spots is suppressed by exploiting frequency or polarization diversity. Each spot is covered by a beam generated either by a dedicated antenna-feed system or by sharing adjacent feed horns [23]. As a consequence, a high number of very compact and lightweight antenna-feed systems with com- plex geometries, as the one shown in Fig. 2(b), will have to be embarked on board satellites. SLM is expected to be a technological solution for the development of these payloads, since it could enable the manufacturing of monolithic antenna-feed systems with minimum mass and envelope. By exploiting the free-form fabrication of the SLM technology, microwave components could even be integrated in the supporting structures of satellites so that different functionalities (electromagnetic, thermal, and structural) could be implemented in a single part.

III.  SELECTIVE LASER MELTING TECHNOLOGY

A. Process Description

The SLM process begins with the creation of a three- dimensional Computer-Aided Design (CAD) model that is subsequently converted in a STereoLithography (STL) model. The latter is imported in the SLM system software

Fig. 3.  WR51 cavity used for investigating the electromagnetic performance of parts built through selective laser melting. The z-axis corresponds to the building direction (laser-beam direction).

(a) Cross-sectional view. (b) STL model.

used for the preprocessing of the geometry. At this stage, the part is properly oriented on the building platform, and the supporting structures (see Fig. 3(b)) are designed for the overhanging areas that correspond to down-facing part surfaces with no material underneath (see Fig. 3(a)).

Afterwards, cross-sections of a given thickness, called

“slices,” are generated from the STL model with descriptions of the part and supports. For each sliced layer, a laser-scan path is calculated. It defines both the boundary contour and the filling sequence that often is a raster pattern.

After preprocessing, the part layers are sequentially built in the SLM system, one on top of the others, as follows [24].

•The metal powder is spread uniformly over the building platform by a recoater.

•A high-power fiber-laser beam selectively scans the building platform and fully melts the predeposited powder layer following the cross-section of the part.

•The melted particles fuse and solidify to form the part layer.

•The building platform is lowered, and the process is repeated until the entire job is completed.

Once the SLM manufacturing process is completed, the components are cleaned from the residual powder and, still attached to the building platform, undergo a stress-relieving process at high temperature. This procedure is aimed at reducing deformation of parts caused by internal stresses that have arisen during the manufacturing process. After detaching the components from the building platform, their surface quality is improved through shot-peening of the inner surfaces and polishing of the flanges.

The microwave components described in this paper have been built through an EOS M270 Dual-mode system equipped with a Yb-fiber laser (1060-1100 nm wavelength). The maximum laser power is equal to 200 W and the beam- spot size is 100​ μm​. The manufacturing process is carried out within a chamber filled with inert gas (argon), ensuring an oxygen content lower than 0.10 % at room temperature. The building platform is heated at 100°C to reduce thermal

TABLE I  Selective Laser Melting Process Parameters

­specific weight and biocompatibility. This material is ideal for many high-performance engineering applications, such as aerospace and motor racing. The particle size distribution of these powders has been investigated through laser diffraction and a Field Emission Scanning Electron Microscope (FESEM, Zeiss Supra). Both powders are gas-atomized, and hence the particles are spherical in shape, with particle sizes ranging from ​0 . 5 μm to 50 μm​for the Al alloy, and from 5 to​ 40 μm​for the Ti alloy. The average particle size is ​25 μm for

Al and ​20 μm​for Ti. Typical values of the mechanical accu- racy guaranteed for Aland Ti-based parts are ​100 μm and 50 μm, respectively.

stresses that arise during the process. The main SLM system parameters are reported in Table 1, where the hatching dis- tance denotes the distance between two adjacent laser-beam traces. These parameters depend on the specific layer area under exposition, namely:

C. Mechanical Design Rules

The high values of energy that are used in the SLM process to melt the metal particles can give rise to problems like balling, residual stress development, and part deformation [30]. The occurrence of this phenomena can be reduced by

A more detailed description of the SLM process is repor­ ted in [25].

B. Powder Material Properties

In addition to process parameters, the quality and cost of parts built via SLM depend on the properties of the metal powders. The material properties relevant to SLM manufacturing can be subdivided in metallurgical (composition, microstructure), geometrical (particle size and shape), and mechanical-physical properties (flowability, absorption of light) [26]. With respect to the effects of the geometrical properties, it has been observed that fine particles enable the manufacturing of high density parts. To segregate outsized particles, the powder is sieved and poured into the

SLM machine. Spherical particles improve flowability, thus resulting in high mechanical properties [27]. Previous analy- ses show that powder reuse improves flowability without a significant variation of the composition [28], [29]. Produc- tion cost of metallic powders mainly depends on the atomization medium. By changing from gasto water-atomized metal powders, up to 75 percent of the material costs can be saved. However, the use of gas-atomized powder may be recommended for parts with high claims concerning the density and the final quality, at the expense of building costs.

In this study, two alloys that are widely used for SLM have been considered: AlSi10Mg and Ti6Al4V alloys, both provided by EOS Gmbh. The Al alloy is very interest- ing because of its high corrosion resistance, low density (2.68 g/cm3), good weldability, good castability, high thermal, and electrical conductivity. The Ti alloy is a well-known light alloy, characterized by having excellent mechanical properties and corrosion resistance combined with low

of SLM-based components, as described in Section IV. If the component were built aligned along the laser-beam direction (​z-​axis), the two downward facing surfaces indicated in Fig. 3(a) would need to be supported. However, supports inside the parts cannot be removed.

A first solution to both avoid the use of internal supports and simplify the external ones is to properly rotate components with respect to the building chamber axes. In this case, the cavity has been manufactured with the orientation shown in Fig. 3(b). The optimum angles of 47.7° on the xz -plane and 14.4° on the yz -plane have been determined on the basis of a previous study carried out on the design of sup- ports for overhanging structures in Al and Ti [31]. A more effective solution to this problem is to design an AM-ori- ented RF layout so that the component can be built aligned along the ​z​axiswithout the use of internal supports. Indeed, this vertical part-orientation reduces the staircase effect and leads to a better surface roughness. This approach has been applied to the Ku/K-band filters described in Section V-B.

I V.  ASSESSMENT OF THE ELECTROMAGNETIC PERFORMANCE OF SLM-BASED COMPONENTS

The applicability of the SLM technology to the manufacturing of microwave components has been investigated in terms of part accuracy and precision, surface roughness, and electrical resistivity. To this end, the test cavity in standard

WR51 waveguide shown in Fig. 3(a) has been considered.

It consists of two stubs in the E plane connected through a waveguide length L. To ensure a significant sensitivity to manufacturing uncertainties and to the electrical resistivity of the metal walls, the cavity has been designed to operate at

relatively high frequencies (18.0-19.5 GHz) and to have an external quality factor ​Q​E of approximately 120. Moreover, the cavity exhibits one reflection zero and two transmission zeros. The nominal values of the geometrical parameters indicated in the insets of Figs. 4–5 are

The test cavity has been manufactured through SLM in

AlSi10Mg and Ti6Al4V alloys with the orientation inside the machine chamber, as shown in Fig. 3(b). The parts have undergone an electrolytic plating with silver (Ag) of 105 HV hardness. The scattering coefficients of the components have been measured through a vector-network-analyzer setup cali- brated through a trough-reflection-line procedure, providing a measurement uncertainty lower than 1 dB for reflection coefficient values in the order of −30 dB and lower than 0.04 dB for transmission coefficient values of approximately 0 dB.

A. AlSi10Mg Alloy

The scattering matrix of the aluminum test cavity has been measured before and after silver plating. Figures 4(a)–(b)

Fig. 5.  Reflection coefficient (a) and transmission coefficient

(b) of an Ti6Al4V alloy prototype of the WR51 waveguide test cavity shown in the insets. The prototype has been manufactured through selective laser melting and silver-plated. Blue solid line: measurement. Black dot-dashed line: simulation of the nominal geometry (1). Cyan dotted line: simulation of the best-fitting geometry (3).

show the comparison between the measured and simulated valuesofthereflectioncoefficientS11 and transmission coef- ficient S21 of the silver-plated test cavity. The simulations are reported both for the nominal geometry (1) and for the best- fitting geometry, for which the parameter values are

Fig. 4.  Reflection coefficient (a) and transmission coefficient

(b) of an AlSi10Mg alloy prototype of the WR51 waveguide test cavity shown in the insets. The prototype has been manufactured through selective laser melting and silver plating. Blue solid line: measurement. Black dot-dashed line: simulation of the nominal geometry (1). Cyan dotted line: simulation of the best-fitting geometry (2).

These values have been determined to recover the amplitude and phase responses of the measured S11 and S21 parameters over the entire band. The de-embedding problem is wellposed because variations of different geometrical param- eters cause significantly different effects in the frequency response. Indeed, parameters h​i and w​i (i = 1, 2​) mostly con- trol the position of the transmission zero and the quality factor of the corresponding stub, respectively. The length L of the cavity sets the position of the reflection zero.

As it can be inferred, the manufacturing uncertainty is within ±​ 0 . 1​mm, which is the tolerance typically guaranteed for the AlSi10Mg powder. The equivalent surface resistivity before and after silver plating of the cavity is about​20 μΩcm and 8 μΩ​cm, respectively. These values take into account both the bulk resistivity of the material and the surface roughness.

The latter can be reduced to Ra values lower than 5​ μm by properly setting the parameters of the shot-peening pro- cess, as discussed in [32]. It is worth reminding that the bulk resistivity of Al and Ag at ambient temperature are approxi- mately 3 μΩcm and 1 . 5 μΩcm, respectively.

Because of mechanical errors, the manufactured test cavity exhibits narrower bandwidth, and hence its measured external quality factor ​Q​E increases to a value of approximately

250. The measured values of the unloaded quality factor​Q​U, that accounts for the internal losses, are 2245 and 1295 for the component with and without silver plating, respectively.

B. Ti6Al4V Titanium Alloy

The comparison between the measured and simulated scat- tering coefficients of the titanium test cavity after silver plat- ing are reported in Fig. 5(a) – (b). In this case, the parameter

As expected, the manufacturing accuracy of the SLM process increases when using a strong metal, like Ti. For this material, mechanical tolerances of the SLM process are in the order of

± 0.05 mm. In spite of a lower surface roughness, the Ti-based

Fig. 7.  EDS element analysis of the silver-plated surface of the Ti6Al4V test cavity. Lighter areas correspond to higher concentrations of the highlighted element. (a) SEM image. (b) Ag concentration map.

(c) Ni concentration map. (d) Ti concentration map.

component without silver plating exhibits very high values of electrical resistivity (in the order of​200 μΩ cm).

Hence, coating of the internal waveguide channels is mandatory. To assess the quality of the electrolytic silver plating, the test cavity has been cut along the mid surface. Subse- quently, the internal waveguide surfaces have been subjected to energy dispersive x-ray spectroscopy (EDS) through a scanning electron microscope (SEM). The top and side images of the surface are reported in Fig. 6. Fig. 7 shows the concentra- tion maps of the Ag, Ni (nickel), and Ti elements. The Ni sub- layer of roughly 2-μ​ m​thickness guarantees a good adhesion of the 3-4 μm​thick Ag-layer over the Ti-alloy substrate. The Ag- layer thickness is approximately double that of the skin depth value in the Ku/K bands. The good quality of the silver-plating process leads to the same equivalent electrical resistivity exhib- ited by the coated Al prototype. The measured external quality factor Q​E is approximately 175, whereas the measured values of the unloaded quality factor ​Q​U​ are 2147 and 423 for the com- ponent with and without silver plating, respectively.

Fig. 6.  Field-emission scanning-electron microscope images of the silver-plated surface of the Ti6Al4V test cavity. (a) Top view

(b) Side view.

V.  MICROWAVE FILTERS MANUFACTURED VIA SELECTIVE LASER MELTING TECHNOLOGY

Waveguide filters are largely used in microwave and millim- eter-wave systems, such as the dual-polarization dual-band antenna-feed system shown in Fig. 2. The manufacturing of filters is a challenging task because of the high standing waves developing inside the components. This phenomenon leads to high sensitivity to mechanical tolerances and high losses. Furthermore, stringent constraints apply to the mechanical design of filters aimed at high-power applications [33].

For these reasons, filters are considered in this paper as a strongly relevant benchmark for the SLM fabrication of waveguide components. In particular, the authors have addressed the development of Ku/K-band filters meeting electrical requirements typical of satellite communications.

Fig. 8.  Statistical distribution of the maximum reflection coefficient in the passband [12.5, 15.0] GHz (red bars) and the maximum transmission coefficient in the stopband [17.5, 21.2] GHz (blue bars) for the two WR51 waveguide filters shown in the insets.

(a) Seventh-order E-plane iris configuration. (b) Fifth-order E-plane composite step/stub configuration.

On the basis of the results reported in Section IV, Albased filtersmay notrequirecoating ofthe internalsurfaces, thus facilitating the manufacturing of complex geometries in single-part components. Consequently, the Al-based SLM process has been initially considered for which results are reported in this article.

have been designed to exhibit a reflection coefficient lower than −30 dB in the [12.5, 15.0] GHz passband and a trans- mission level lower than −45 dB in the [17.5, 21.2] GHz stopband.

These configurations have been subjected to sensitivity statistical analysis, based on the simulation of several thousands of realizations affected by geometrical uncertainties uniformly distributed on the interval [-0.1, 0.1] mm.

Fig. 8(a) and (b) reports the probability mass function of the maximum reflection coefficient in the passband and of themaximumtransmissioncoefficientinthestopbandforthe two filters. While mechanical tolerances do not significantly affect the transmission coefficients of both configurations, they have a disruptive influence on the reflection coefficient of the iris configuration. Indeed, for this filter architecture, the mean value of the S11 probability-mass function is approxi- mately −12 dB against a value of −26 dB for the step/stub configuration, The main reason for the low sensitivity to geo- metrical tolerances provided by the latter filter relies on the combination of the resonant behavior of stubs with the wide- bandcharacteristicsofsteps.Onthecontrary,theirisconfigu- rationreliesonintense constructive/destructiveinterferences occurring between the irises. These phenomena are strongly frequency-dependent and increase sensitivity of RF performance to mechanical ­tolerances.

B. Fifth-Order Ku/K-band Filter

According to the previous results, the E-plane step/stub layout has been selected for bread-boarding through SLM. Seven prototypes have been manufactured with the same orientation on the building platform used for the test cavity (see Fig. 3(b)). The envelopes of the measured scattering coefficientsS11 and S21 ofthesevenAlsamplesarecompared

A. Electromagnetic Robust Design

As reported in Section IV-A, the manufacturing uncer- tainty of the Al-based SLM process is approximately

± 0.1 mm. This tolerance may prevent the successful implementation of Ku/K band filters, unless an electro- magnetic robust design is adopted. To better quantify this aspect, two different filtering structures are compared in Fig. 8, namely a standard seventh-order E-plane iris configuration [34] (inset of Fig. 8(a)), and a fifth-order E-plane filter based on the composite step/stub configura- tion recently reported in [33] (inset of Fig. 8(b)). The filters­

Fig. 9.  Reflection coefficient S11 and transmission coefficient S21 of the fifth-order filter shown in the inset of Fig. 8(b). The magenta/cyan-filled area is the envelope of the measurements of seven prototypes. The red and blue solid lines correspond to the

predicted performance. The red and blue vertical dot-dashed lines mark the passband [12.5 15.0] GHz and stopband [17.5 21.2] GHz.

Fig. 11.  Sixth-order Ku/K-band filter. The z-axis corresponds to the building direction (laser-beam direction). (a) STL model.

(b) Cross-sectional view.

Fig. 12.  Contour maps of the electrical figure-of-merits of a WR51 stub resonator tilted by 45° as a function of its height ​h​and width ​w​.

(a) Frequency of the transmission zero ​f​0​. (GHz). (b) Bandwidth at half maximum ​B = f0 /QE​(GHz). (c) Maximum amplitude (dB) of the reflection coefficient in the passband [12.5, 15.0] GHz.

C. Sixth-Order Ku/K-Band Filter

TheE-planecompositestep/stubfilterhasbeenredesignedto investigate the manufacturing quality ensured by a different ori- entation in the SLM building chamber. According to the design rules outlined in Section III-C, the waveguide propagation axis has been aligned with the building direction (see Fig. 11(a)). To avoid using supports for the internal surfaces, the stubs have

been tilted downwards, as shown in Fig. 11(b). The minimum tilting angle of the downward facing surfaces has been set to 45° to minimize the risk of warping. Tilting of stubs by angles smaller than approximately 45° does not degrade the electrical performance of the stubs. Figure 12 shows the contour maps of the main electrical figure-of-merits of a WR51 stub resonator tilted downwards by 45° as a function of its height h​ ​and width w​ .

The figure-of-merits are the frequency of the transmission zero, the bandwidth at half maximum, and the maximum amplitude in dB of the reflection coefficient in the passband. A stub not tilted downwards exhibits almost the same electrical performance.

In this filter design, a more demanding transmission coefficient (​≤−50​dB) has been implemented by adopting a sixth-order configuration. First, two prototypes have been manufactured in Al through two distinct SLM jobs to assess repeatability of the manufacturing process. The measured scattering coefficients are shown in Fig. 13(a). The two pro- totypes provide almost identical performance, but they are affected by a systematic error. Indeed, the measured data exhibit a frequency shift caused mainly by a nonoptimum beam-offset of the laser during the SLM manufacturing process.

For this reason, a tuning of the SLM process parameters has been carried out. Several WR51 lines have been succes- sively manufactured and tested. The process parameters (mainly beam-offset, hatching and scan speed) have been adjusted in order to minimize the difference between the measured and theoretical values of the phase shift introduced by the lines.

Since the vector-network-analyzer setup provides a measurement accuracy of the phase response in the order of

0.1–0.2°, the waveguide dimension ​a​(see inset of Fig. 4(a)) can be de-embedded with an accuracy better than 0.01 mm.

The tuning process has been repeated until reaching a manufacturing error lower than 0.08 mm. Subsequently, two additional filter samples have been built. The measured performance reported in Fig. 13(b) prove the effectiveness of the process tuning procedure, although a residual systematic enlargement of the waveguide dimensions has still to be compensated. Nevertheless, all the filter prototypes, as they are, meet typical requirements set in satellite communica- tions, specifically ​S​11≤−25​dB and ​S​21≤−50​dB.

One prototype has been silver-plated also for this filter topology. The detailed view of the insertion loss of both the three Al samples and the Ag sample is reported in Fig. 14.

The values of equivalent electrical resistivity are those used in the simulation of the fifth-order filter described in the previous section. The weight of the filter and the mass sav- ing achieved by using SLM instead of conventional machining techniques are almost equal to the values reported for the fifth-order filter in Section V-B.

Fig. 14.  Transmission coefficient S21 of the sixth-order filter prototypes. The blue and cyan lines correspond to the measured and predicted values of the three Al samples without silver-plating. The red and magenta lines correspond to the measured and predicted values of the sample with Ag coating. The red vertical dot-dashed lines mark the passband [12.5 15.0] GHz.

V I.  CONCLUSION

The bread-boarding of Ku/K bands filters reported in this paper proves that selective laser melting is a promising technology for the manufacturing of all-metal waveguide com- ponents. An RF procedure for the fine tuning of the process parameters has been introduced to improve manufacturing accuracyofAl-basedcomponents.Furtherresearch­activities

will focus on the development of mechanical/electromag- netic-coupled designs aimed at implementing waveguide components with enhanced accuracy in Al, Ti, and other metal alloys. The designs conceived will take advantage of the free-form fabrication capability of AM technologies while enabling an easy postprocessing of the internal chan-

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