диафрагмированные волноводные фильтры / 66876fee-11bd-4fe0-a4c2-c91f2d5b2635

radiations are also observed in the plane. The measured peak gain is about 3.2 dBic for the LHCP radiation and 2.4 dBic for the RHCP radiation. The gain difference could be due to the ohmic loss of the diodes.

V. CONCLUSION

A circularly-polarized microstrip array antenna with conical-beam radiation has been presented in this communication. The antenna is composed of four L-shaped patches which are connected to the ground plane through shorting walls. Dual orthogonal modes can be excited simultaneously from the antenna, and their resonant frequencies can be controlled with the shorting walls. By properly selecting the width of the shorting walls, a left-hand circular polarization operation can be found. Moreover, the polarization sense can be switched to right-hand circular polarization by increasing the number of the shorting walls. A reconfigurable prototype with the ability of electrically switching between left-hand and right-hand circular polarization has also been realized. Experimental results show that the prototype can operate at two different frequencies, and both the operating frequencies have uniform conical-beam radiation patterns with good circular polarization performance. Moreover, the size of the prototype is about 0.47 in side length and 0.08 in height, where is the free-space wavelength corresponding to the lower operating frequency. The compact size and omnidirectional radiation make the antenna be suitable for the applications of present-day wireless communication systems; besides, the property of dual frequency and dual polarization may be required for the system that uses different frequencies for uplink and downlink communications.

REFERENCES

[12]N. J. McEwan, R. A. Abd-Alhameed, E. M. Ibrahim, P. S. Excell, and J. G. Gardiner, “A new design of horizontally polarized and dual-po- larized uniplanar conical beam antennas for HIPERLAN,” IEEE Trans. Antennas Propag., vol. AP-51, pp. 229–237, Feb. 2003.

[13]S. H. Yeh and K. L. Wong, “A broadband low-profile cylindrical monopole antenna top loaded with a shorted cross patch,” Microw. Opt. Technol. Lett., vol. 32, pp. 186–188, Feb. 5, 2002.

[14]C. Delaveaud, P. Leveque, and B. Jecko, “New kind of microstrip antenna: The monopolar wire-patch antenna,” Electron. Lett, vol. 30, pp. 1–2, Jan. 6, 1994.

[15]S. H. Chen, J. S. Row, and K. L. Wong, “Reconfigurable square-ring patch antenna with pattern diversity,” IEEE Trans. Antennas Propag., vol. 55, pp. 472–475, Feb. 2007.

[16]Y. J. Sung, T. U. Jang, and Y. S. Kim, “A reconfigurable microstrip antenna for switchable polarization,” IEEE Microw. Wireless Compon. Lett., vol. 14, pp. 534–536, Nov. 2004.

Design and Characterization of 60-GHz Integrated Lens Antennas Fabricated Through Ceramic Stereolithography

Ngoc Tinh Nguyen, Nicolas Delhote, Mauro Ettorre, Dominique Baillargeat, Laurent Le Coq, and Ronan Sauleau

Abstract—Three integrated lens antennas made in Alumina and built through ceramic stereolithography are designed, fabricated and characterized experimentally in the 60-GHz band. Linear corrugations are integrated on the lens surface to reduce the effects of multiple internal reflections and improve the antenna performance. The lenses are excited by Alumina-filled WR-15 waveguides with an optimized dielectric impedance matching taper in E-plane. The main characteristics of the first two prototypes with corrugations of variable size are compared to those of a smooth lens without corrugation (third prototype). Experimentally their reflection coefficient is smaller than 10 dB between 55 GHz and

27–39, Jun. 1997.

[2]D. Zhou, R. A. Abd-Alhameed, C. H. See, N. J. McEwan, and P. S. Excell, “New circularly-polarized conical-beam microstrip patch antenna array for short-range communication systems,” Microw. Opt. Technol. Lett., vol. 51, pp. 78–81, Jan. 2009.

[3]D. I. Wu, “Omnidirectional circularly-polarized conformal microstrip array for telemetry applications,” in IEEE Antennas Propag. Soc. Int. Symp. Dig., 1995, vol. 2, pp. 998–1001.

[4]J. Takada, A. Tanisho, K. Ito, and N. Ando, “Circularly polarised conical beam radial line slot antenna,” Electron. Lett., vol. 30, pp. 1729–1730, Oct. 13, 1994.

the center frequency (60 GHz), the total antenna loss (including feed loss) is smaller than 0.9 dB and the radiation efficiency exceeds 80%.

Index Terms—Broadband lens, ceramic stereolithography, integrated lens antennas, millimeter wave.

I. INTRODUCTION

Two-and-one-half dimensional manufacturing techniques, like surface and volume micromachining of Silicon or Gallium-Arsenide sub-

and interconnects, and for packaging needs. Among those, polymer [5] and ceramic [6], [7] stereolithography, as well as extrusion freeforming methods [8] are really promising since they do not require the use of molds and cutting tools. These are relatively fast and reliable techniques capable of building truly 3-D structures with a high aspect-ratio and accuracy. For instance they have been involved in the fabrication of polymer-based vertical filters and highcavity resonators [5], periodic arrangements made in various ceramics (Zirconia, BZT, Alumina) such as bandpass filters [7], low-profile electromagnetic bandgap resonator antennas [9], and compact horn antenna arrays [10].

More precisely, stereolithography is an additive layer-by-layer process that allows forming locally solid parts by selectively illuminating polymer photoresists or photoreactive ceramic suspensions. As a consequence it becomes possible to construct all-dielectric 3-D devices with arbitrary complex geometries and/or adjustable refractive index by controlling the volumetric proportions of the composite materials. These concepts have been applied to explore the capabilities of broadband photonic-crystal waveguides [11] and design non-homo- geneous monolithic lens antennas in -band [6].

On the other hand, integrated lens antennas (ILAs) are very attractive for a number of millimeter wave applications (e.g. [12] and references therein). Two main categories of ILAs are generally distinguished, namely i) the extended hemispherical ILAs ([12]–[14]), and ii) the shaped ILAs ([15]–[18]). The first ones are mainly used for beam switching and imaging/sensor applications, whereas the second ones are of particular interest for beam shaping systems. In both cases the lens is usually fabricated using computer numeri- cally-controlled milling/lathe machines. This may result in expensive, delicate—and even challenging—tasks, especially when dealing with electrically-small or strongly shaped lenses, e.g., [19], [20]. For all these configurations, new low-cost fabrication methods are attractive alternatives and must be benchmarked.

In this frame stereolithography techniques seem to be extremely promising. Therefore the purpose of this communication is to assess the feasibility of ceramic stereolithography for manufacturing ILAs at millimeter waves. As an intermediate step towards the application of such methods for the synthesis of ILAs with arbitrary shape and constitution, we have implemented this technology to manufacture simpler ILAs, namely several synthesized elliptical ILAs [13]. To our best knowledge, this work is one of the first ones studying homogeneous ILAs made by stereolithography and providing experimental results in the 60-GHz band.

This communication is organized as follows. The antenna geometry and the corresponding technical choices are explained in Section II-A. The main characteristics of the proposed ILAs are given in Section II-B. The fabrication process and the experimental results are then discussed in Sections III and IV, respectively. Conclusions are finally drawn in Section V.

II.ANTENNA GEOMETRY AND NUMERICAL RESULTS

A. Antenna Geometry

The antenna geometry is represented in 3-D view and side view in Fig. 1(a) and 1(b), respectively. It consists of a synthesized elliptical lens made in Alumina; the dielectric characteristics of Alumina have been measured at 10 GHz using a cylindrical resonant cavity: ,

2 . The reasons why the loss tangent is so small are given in Section III. The same values are chosen at 60 GHz (note that even if the dielectric loss is ten times larger, the impact on the antenna radiation performance will be negligible). The extension length of the lens and its diameter have been defined so that, in the Geometrical Optics approximation, all incident rays that impinge parallel on the front surface of the lens are

Fig. 1. Antenna geometry. (a) 3-D view. (b) Side view. (c) Zoom close to the corrugations. The lens is made in Alumina and is covered by linear corrugations serving as a transition layer to reduce the dielectric contrast at the lens interface. The antenna is excited by a rectangular waveguide with an integrated impedance matching taper in E-plane.

collected at the rear focal point [12], [13]. In contrast to the monolithic configuration previously proposed in [6], the ILAs studied here are excited by a separate external open-ended Alumina-filled WR-15 waveguide. This choice enables one to reduce possible mechanical constraints at the perpendicular junction between the lens base and the feed waveguide. The metallic waveguide and flange have been fabricated by electrical discharge machining. An impedance matching taper in E-plane is integrated inside the waveguide to match the antenna over a broad frequency band ( , ).

Due to fabrication and integration issues, the size of the dielectricfilled waveguide section is the same as the standard one (1.9 mm 2 3.8 mm). We have checked that manufacturing errors or possible excitation of higher order modes do not spoil the antenna performance; using a single-mode waveguide as a primary feed would produce slightly narrower beams, but at the expense of challenging fabrication issues due to the very small size of the waveguide. A rounded flange has been fabricated around the lens base to facilitate the antenna mounting onto the circular ground plane whose surface has been opti- cally-polished ( , ).

It is well known that ILAs made in dense materials (like Silicon or MgO) are desirable to favor power transfer from the feed to the lens. Nevertheless the impedance and radiation characteristics of such lenses are substantially distorted due to the excitation of multiple internal reflections, e.g., [12], [21], [22]. These limitations can be partly overcome by reducing the dielectric contrast at the lens interface, using either conventional matching layers (ML) or caps (i.e., quarter wavelength wave transformers [14], [23]), or optimized ones [16]. Such approaches require the use of—at least—two different materials that must be carefully selected (among those available commercially), fabricated and assembled. Moreover, for a number of shaped ILAs, assembling the lens core with the outer shell is even impossible due to peculiar surface curvatures, e.g., [15], [17]–[19].

To overcome these limitations an alternative solution consists in fabricating an effective material at the lens interface, for instance by drilling thin holes [24] or corrugations in the host medium. This leads to fully monolithic antenna configurations. In this work we use linear corrugations in E-plane [Fig. 1(c)]. Their depth , width and

Fig. 2. Three flat dielectric interfaces illuminated by a plane wave in TE mode.

(a) Geometry of the problems. (b), (c) Reflection coefficients for two angles of

spacing equal 700 , 300 , and 1200 , respectively. These dimensions have been chosen to obtain a relative effective permittivity close to 3. To compare the performance of such corrugations with a standard ML, let us consider the following canonical problem: an infinite flat dielectric interface between Alumina and free space illuminated by a TE-polarized plane wave [Fig. 2(a)]. This configuration constitutes a local approximation of the lens problem as explained in [25]. The magnitude of the reflection coefficient at this interface is represented in Fig. 2(b) and 2(c) (in solid line) as a function of frequency for two angles of incidence: [normal incidence, Fig. 2(b)] and [Fig. 2(c)]. Note that the value of is slightly smaller than the critical angle beyond which total reflection occurs [25]. Comparison with an ideal Alumina/Homogeneous

Fig. 3. Reflection coefficient and maximum directivity computed with the FDTD method for . : reflection coefficient. : directivity. The gray lines represent the of a lens antenna of same size but without

corrugation or coated with an ideal quarter wavelength matching

layer .

ML/Free space interface shows that linear corrugations exhibit very good performance over the whole frequency band of interest.

B. Antenna Performance

The antenna described in Section II-A is labeled . Its characteristics have been computed with a homemade FDTD solver [12]. Fig. 3 represents its reflection coefficient and directivity at broadside. The antenna is matched over the whole unlicensed 60-GHz band (57–64 GHz) for short range communications in the US, and its directivity varies between 20.2 dBi and 20.8 dBi. Comparison with the of a lens without corrugation or with an ideal quarter wavelength matching layer ( , ) clearly demonstrates the effectiveness of the linear corrugations to reduce the dielectric contrast at the lens interface.

The co-polarization components calculated at the center frequency (60.5 GHz) and at the band edges (57 GHz and 64 GHz) are plotted in Fig. 4 in E- and H-planes. The half-power beamwidth is very stable versus frequency, and the maximum side lobe level does not exceed

16 dB [E-plane, Fig. 4(a)].

III. FABRICATION PROCESS

Stereolithography is a rapid prototyping process based on a space-re- solved laser polymerization, and where objects are built layer by layer [6], [7]. After decomposition of the closed 3D-CAD model into a set a elementary triangles, the object is numerically sliced in layers, and the cross sectional patterns to polymerize in each layer are defined. The specific pattern, including processing parameters (laser power, scanning speed and sequence, focalization) is sent to the automated machine to physically build the object. Fig. 5 represents the experimental set-up used in this work.

A thin layer of organic components (liquid photocurable resin (monomer), binders, etc.) loaded with a high percentage ( 50%) of high-purity Alumina particles is firstly deposited on the working surface. A blade is used here to precisely control the layer thickness (50 ). Then the object first slice is hardened (polymerized) under UV laser exposure controlled by galvanometric X Y mirrors and according to the pre-defined patterns.

Once this first slice is physically fabricated, the working surface goes down along -axis, and another layer of ceramic suspension is deposited above the previous one by the blade. The exposure process is repeated for this second slice which is thus bonded to the previous

Fig. 4. Co-polarization components computed at the center frequency (60.5 GHz) and at the band edges (57 GHz and 64 GHz). (a) E-plane.

Fig. 6. Synthesized elliptical ILA fabricated by ceramic stereolithography. (a) Antenna prototype (after assembly). (b) Cross-section view of the Alumina lens alone. The depth , width and spacing of the corrugations are the following: 700 , 300 , and 1200 for , and 700 , 400 , and 1600 for . (c) Alumina taper.

This technology has been developed by the Centre de Transfert de Technologies Céramiques of Limoges, France [7], [27] and has an average accuracy close to 100 (after sintering). It allows fabricating arbitrarily-shaped 3-D objects (e.g., enclosures) made in ceramic (Alumina, Zirconia) that would be very difficult, or even impossible, to obtain using standard machining techniques. Such a fabrication process has been already applied to produce inhomogeneous devices like EBG woodpile structures [27] and Luneberg lenses [6]. The two major restrictions are the following: i) the enclosures must be ‘open’ so as to remove the non-polymerized paste during the cleaning step of the process, ii) only a single material can be used per fabrication; if multimaterial lenses are required, the different parts must be fabricated separately and then assembled.

Fig. 5. Experimental set-up for fabrication through stereolithography.

layer by laser polymerization. The same process is repeated until the final device is entirely built.

Finally the polymerized solid part is removed from the non-polymer- ized liquid. The recovered object is cleaned, debinded and sintered to obtain its final dimensions and density. The purity of the base Alumina powder and the very high density of the final object (more than 98% of the theoretical value) are the two main reasons why very low loss can be obtained [26], [27].

IV. EXPERIMENTAL RESULTS

Three antenna prototypes have been manufactured using the fabrication process described above. The lenses are stratified in height. The first one [ , Fig. 6(a)] and second one have corrugations of different size: their widths equal 300 for and 400for (fabricating thinner corrugations would be challenging and not reliable since the laser spot diameter is 150 ). The other corrugation dimensions are given in Fig. 6; they have been optimized to minimize the reflection coefficient at the lens interface, as already illustrated in Fig. 2. The third lens antenna has a smooth surface, i.e. no corrugation. The Alumina taper has been fabricated using the same technology [Fig. 6(c)]. Considering two different corrugation widths and depths allows assessing the fabrication limitations in terms of resolution and mimimum size of small 3-D objects, whereas comparing ILA configurations with and without corrugations enables one to highlight the impact of these corrugations upon the antenna performance. Here all lenses have the same external diameter and total height , and are fed by the same impedance matching taper.

The measured reflection coefficients of the three lens antennas are represented in Fig. 7. In all cases it is smaller than 10 dB from 55 GHz to 65 GHz. The disagreement between experiments and simulations (Fig. 3 for ) is attributed to several manufacturing issues: i) the matching taper and the lens base are slightly bent due to mechanical constraints and internal stress; this is clearly illustrated in Fig. 6(b)

corrugations). : (400 -thick corrugations). : (no corrugation). The lines with symbols represent the theoretical directivities of

, , and , respectively.

asymmetry observed on the measured beams probably comes from the lens deformation mentioned above. Additional experimental results have confirmed that these patterns are very stable between 55 GHz and 65 GHz. The half power beamwidth is not the same in E- and H-planes because of the rectangular waveguide excitation.

The antenna gains are plotted in Fig. 9. They have been measured with the comparison method using a 20-dBi standard gain horn in-band. As expected this figure confirms that the gain variations versus frequency are smaller when using fine corrugations . We can also notice that there are some frequency points (e.g., at 57 or 61 GHz) where the gain of the smooth lens is higher than the one of the corrugated lenses and . This is due to the excitation of resonant modes that increase the -factor of the antenna [28], thus its directivity and gain since the Alumina used here has a very low loss tangent. The phenomena would not appear where higher loss material.

At 60 GHz the gain and directivity of equal 19 dBi and 19.9 dBi, respectively. The 0.9-dB loss can be decomposed as follows (Fig. 7): 0.2-dB metallic loss (FDTD simulations), and less than 0.1-dB return loss, 0.6-dB reflection losses (GO/PO simulations [15]). This reflection loss value is similar to the ones reported in [23], [29]. Fig. 9 also shows that the gain drop (compared to the theoretical directivity) varies experimentally between 0.7 dB (at 61 GHz) and 2.2 dB (at 63 GHz), corresponding to antenna radiation efficiencies comprised between 60% and 85% over a 10-GHz frequency band.

where we can notice small deformations at the left hand side of the lens base, ii) the taper section is slightly smaller than the waveguide cross-section (3.8 mm 2 1.9 mm) to facilitate its insertion into the waveguide. Accumulation of these two defects creates unavoidable air gaps and impedance discontinuities explaining very likely the ripples observed in Fig. 7.

The radiation patterns measured at 60 GHz are represented in Fig. 8. They are in good agreement with the FDTD simulations. The slight

V. CONCLUSIONS

An attractive ceramic-based stereolithography process has been developed to build 3-D monolithic all-dielectric devices at millimeter waves. Its main features and limitations have been assessed through the fabrication and experimental characterization of several 60-GHz integrated lens antennas (ILAs) made in very low-loss Alumina. As one of the prime objectives is to validate all technological steps, simple ILAs have been designed, namely synthesized elliptical lenses. The latter are fed by WR-15 metallic waveguides filled with Alumina. Optimized impedance matching tapers in E-plane are used to match the ILAs over the whole unlicensed 60-GHz band. Corrugations of various sizes have been designed to reduce the dielectric contrast at the Alumina/free space interface. Three prototypes have been manufactured and characterized in impedance and radiation. Measurements have shown that the antenna performances are very

stable over the 55–65 GHz band. The agreement between the experimental and numerical results is very satisfactory despite some fabrication issues (mechanical stress observed along the dielectric tapers and lens bases). In particular, for 19-dBi gain antennas, the total amount of loss at 60 GHz is lower than 0.9 dB, corresponding to a radiation efficiency of 80%.

REFERENCES

[1]L. P. B. Katehi, J. F. Harvey, and E. Brown, “MEMS and Si micromachined circuits for high-frequency applications,” IEEE Trans. Microw. Theory Tech., vol. 50, no. 3, pp. 858–866, Mar. 2002.

[2]M. Elwenspoek and H. Jansen, Silicon Micromachining. Cambridge, U.K.: Cambridge Univ. Press, 1998.

[3]W. Y. Liu, D. P. Steenson, and M. B. Steer, “Membrane-supported CPW with mounted actives devices,” IEEE Microw. Wireless Comp. Lett., vol. 11, no. 4, pp. 167–169, Apr. 2001.

[4]N. Tiercelin, P. Coquet, R. Sauleau, V. Senez, and H. Fujita, “Polydimethylsiloxane membranes for millimeter-wave planar ultra flexible antennas,” J. Micromec. Microeng., vol. 16, pp. 2389–2395, Sep. 2006.

[5]B. Liu, X. Gong, and W. J. Chappell, “Applications of layer-by-layer polymer stereolithography for three-dimensional high-frequency components,” IEEE Trans. Microw. Theory Tech., vol. 52, no. 11, pp. 2567–2575, Nov. 2004.

[6]K. F. Brakora, J. Halloran, and K. Sarabandi, “Design of 3-D monolithic MMW antennas using ceramic stereolithography,” IEEE Trans. Antennas Propag., vol. 55, no. 3, pp. 790–797, Mar. 2007.

[7]N. Delhote, D. Baillargeat, S. Verdeyme, S. Delage, and C. Chaput, “Narrow Ka bandpass filters made of high permittivity ceramic by layer-by-layer polymer stereolithography,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 3, pp. 548–554, Mar. 2007.

[8]X. Lu, Y. Lee, S. Yang, Y. Hao, R. Ubic, J. R. G. Evans, and C. G. Parini, “Fabrication of electromagnetic crystals by extrusion freeforming,” Metamaterials, vol. 2, pp. 36–44, 2008.

[9]Y. Lee, Y. Hao, S. Yang, C. G. Parini, X. Lu, and J. R. G. Evans, “Directive millimetre-wave antennas using free-formed ceramic metamaterials in planar and cylindrical forms,” presented at the IEEE AP-S Int. Symp., San Diego, CA, Jul. 2008.

[10]L. Schulwitz and A. Mortazawi, “A compact millimeter-wave horn antenna array fabricated through layer-by-layer stereolithography,” presented at the IEEE AP-S Int. Symp., San Diego, CA, Jul. 2008.

[11]K. F. Brakora and K. Sarabandi, “Integration of single-mode photonic crystal waveguides to monolithic MMW subsystems constructed using ceramic stereolithography,” presented at the IEEE AP-S Int. Symp., Honolulu, HI, Jun. 2007.

[12]G. Godi, R. Sauleau, and D. Thouroude, “Performance of reduced size substrate lens antennas for millimeter-wave communications,” IEEE Trans. Antennas Propag., vol. 53, no. 4, pp. 1278–1286, Apr. 2005.

[13]D. F. Filipovic, S. S. Gearhart, and G. M. Rebeiz, “Double-slot antennas on extended hemispherical and elliptical silicon dielectric lenses,” IEEE Trans. Microw. Theory Tech., vol. 41, no. 10, pp. 1738–1749, Oct. 1993.

[14]N. T. Nguyen, R. Sauleau, and C. J. Martínez Pérez, “Very broadband extended hemispherical lenses: Role of matching layers for bandwidth enlargement,” IEEE Trans. Antennas Propag., vol. 57, no. 7, pp. 1907–1913, Jul. 2009.

[15]G. Godi, R. Sauleau, L. Le Coq, and D. Thouroude, “Design and optimization of three dimensional integrated lens antennas with genetic algorithm,” IEEE Trans. Antennas Propag., vol. 55, no. 3, pp. 770–775, Mar. 2007.

[16]J. R. Costa, C. A. Fernandes, G. Godi, R. Sauleau, L. Le Coq, and H. Legay, “Compact Ka-band lens antennas for LEO satellites,” IEEE Trans. Antennas Propag., vol. 56, no. 51, pp. 251–1258, May 2008.

[17]B. Barès, R. Sauleau, L. Le Coq, and K. Mahdjoubi, “A new accurate design method for millimeter-wave homogeneous dielectric substrate lens antennas of arbitrary shape,” IEEE Trans. Antennas Propag., vol. 53, no. 3, pp. 1069–1082, Mar. 2005.

[18]R. Sauleau and B. Barès, “A complete procedure for the design and optimization of arbitrarily-shaped integrated lens antennas,” IEEE Trans. Antennas Propag., vol. 54, no. 4, pp. 1122–1133, Apr. 2006.

[19]B. Barès and R. Sauleau, “Electrically-small shaped integrated lens antennas: A study of feasibility in Q-band,” IEEE Trans. Antennas Propag., vol. 55, no. 4, pp. 1038–1044, Apr. 2007.

[20]A. Rolland, R. Sauleau, M. Drissi, and L. Le Coq, “Synthesis of reduced-size smooth-walled conical horns using BoR-FDTD and genetic algorithm,” IEEE Trans. Antennas Propag., submitted for publication.

[21]A. V. Boriskin, A. Rolland, R. Sauleau, and A. I. Nosich, “Assessment of FDTD accuracy in the compact hemielliptic dielectric lens antenna analysis,” IEEE Trans. Antennas Propag., vol. 56, no. 3, pp. 758–764, Mar. 2008.

[22]A. V. Boriskin, G. Godi, R. Sauleau, and A. I. Nosich, “Small hemielliptic dielectric lens antenna analysis in 2-D: Boundary integral equations versus geometrical and physical optics,” IEEE Trans. Antennas Propag., vol. 56, no. 3, pp. 485–492, Feb. 2008.

[23]M. J. M. van der Vorst, P. J. I. de Maagt, A. Neto, A. L. Reynolds, R. M. Heeres, W. Luingue, and M. H. A. J. Herben, “Effect of internal reflections on the radiation properties and input impedance of integrated lens antennas—Comparison between theory and measurements,” IEEE Trans. Microw. Theory Tech., vol. 49, no. 6, pp. 1118–1125, Jun. 2001.

[24]K. Sato and H. Ujiie, “A plate Luneberg lens with the permittivity distribution controlled by hole density,” Electron. Commun. Jpn. (Part I: Commun.), vol. 85, no. 9, pp. 1–12, Apr. 2002.

[25]D. Pasqualini and S. Maci, “High-frequency analysis of integrated dielectric lens antennas,” IEEE Trans. Antennas Propag., vol. 52, no. 3, pp. 840–847, Mar. 2004.

[26]N. M. Alforda and S. J. Penn, “Sintered alumina with low dielectric loss,” J. Appl. Phys., vol. 80, no. 10, pp. 5895–5898, Nov. 1996.

[27]T. Chartier, C. Duterte, N. Delhote, D. Baillargeat, S. Verdeyme, C. Delage, and C. Chaput, “Fabrication of millimetre wave components via ceramic stereoand microstereolithography processes,” J. Am. Ceram. Soc., vol. 91, no. 8, pp. 2469–2474, 2008.

[28]A. Rolland, M. Ettorre, L. Le Coq, and R. Sauleau, “Axis-symmetric resonant shaped dielectric lens antenna with improved directivity in-band: Performance and comparison with an extended hemispher-

ical lens,” IEEE Trans. Antennas Propag., submitted for publication.

[29]X. Wu, G. V. Eleftheriades, and T. E. van Deventer-Perkins, “Design and characterization of singleand multiple-beam mm-wave circularly polarized substrate lens antennas for wireless communications,” IEEE Trans. Microw. Theory Tech., vol. 49, no. 3, pp. 431–441, Mar. 2001.