IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009 1907
Very Broadband Extended Hemispherical Lenses:Role of Matching Layers for Bandwidth Enlargement
Ngoc Tinh Nguyen, Ronan Sauleau, and Cecilio José Martínez Pérez
Abstract—The design and optimization of very broadband inte-grated lens antennas (ILAs) constitutes one of the future trendsin lens antenna field. To this end we investigate numerically theradiation performance of millimeter wave ILAs coated with mul-tiple anti reflection layers. We propose lens structures of moderatesize (four wavelengths in diameter at the center frequency) andmade from a dense dielectric material (ceramic). They are illumi-nated by two kinds of on-axis primary sources, namely a dielec-tric-loaded metallic waveguide and a patch antenna. This enablesto assess the role of the lens illumination law on the performance ofbroadband ILAs. In particular, we demonstrate that ILAs coatedwith three stacked quarter wavelength matching layers exhibit avery broadband promising features. First their radiation charac-teristics remain very stable over a large frequency band: a 36%relative bandwidth is achieved using dielectric-loaded waveguidefeeds. Secondly very high values of aperture efficiencies (beyond91% over a 21% bandwidth) are obtained using printed feeds. Thetruncation effects of the ground plane and substrate of planar feedsupon the beam characteristics are also studied. We conclude thatthey must be taken into account at the very first stages of the designprocess of ILAs.
Index Terms—Broadband integrated lens antennas, matchinglayers, millimeter wave, substrate lenses.
I. INTRODUCTION
Q UASI-OPTICAL technologies are very attractive atmillimeter and sub-millimeter waves [1]. In particular,many recent studies have highlighted the attractive
performance of lens antennas for various applications such asbroadband communication systems in Ka-band [2], Q-band[3] and V-band [4], Automotive Cruise Control radars andon-board obstacle detection radars in W-band [5], [6], highaltitude platforms [7], [8] and passive imaging systems [9]. Inthese contributions, the lenses are fabricated from homogeneousdielectric materials whose dimensions and shape are designedaccording to the system specifications. Because the relativepermittivity of these materials is usually lower than three, thedielectric contrast with free space is weak enough to neglect
Manuscript received April 21, 2008; revised December 23, 2008. First pub-lished May 02, 2009; current version published July 09, 2009. This work wassupported in part by the Conseil Regional de Bretagne under project CREATE/CONFOCAL.
N. T. Nguyen and C. J. Martínez Pérez are with the Institut d’Electroniqueet de Télécommunications de Rennes (IETR), UMR CNRS 6164, Université deRennes 1, 35042 Rennes cedex, France.
R. Sauleau is with the Groupe Antennes and Hyperfréquences, Institutd’Electronique et de Télécommunications de Rennes (IETR), UMR CNRS6164, Université de Rennes 1, 35042 Rennes Cedex, France (e-mail:[email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TAP.2009.2021884
Fig. 1. Power transfer through a flat infinite dielectric interface as a functionof the normalized wavelength ��� . The interface is illuminated by a planewave under normal incidence and is coated with up to three stacked quarterwavelength matching layers (Table I) designed at the center frequency � �
� �� (28 GHz here). ����: No ML.���: One ML. ����: Three MLs.
TABLE IPERMITTIVITY AND THICKNESS OF THE QUARTER WAVELENGTH MATCHING
LAYERS (MLS)
(in most cases) the effects of multiple internal reflections [10],[11] of order higher than two [12].
However, when dealing with strongly shaped beams (e.g.,[13]) or with dense materials like ceramics [13], [14], Silicon[15], [16] and MgO [17], internal resonances are excited withinthe lens [18]. This may substantially affect not only the feedinput impedance [11], [12], [19], but also the radiation prop-erties in terms of: i) gain reduction; ii) beam distortions; andiii) increase of side lobes. These undesirable effects can be re-duced significantly using either corrugations fabricated on thelens surface [20], [21], or wave transformers made of homoge-neous (e.g., [11]) or artificial [22] dielectrics (note also that thethickness of the dielectric coating is not restricted to a quarterwavelength, e.g., [13], [23]).
Nevertheless, to our best knowledge, the influence of multiplematching layers (MLs) upon the radiation characteristics of di-electric lenses has never been studied in detail, especially in theperspective of wideband operation. Note that in this case the usea single coating remains questionable, and multiple stackedMLs may be necessary to improve the lens bandwidth.
1908 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009
Fig. 2. Generic configuration of broadband integrated lens antennas. The lens is coated with up to three MLs. It is fed either by a dielectric-loaded rectangularwaveguide (a),(b) or by a planar feed (c),(d).
In order to illustrate the key role of dielectric coatings, letus consider, as a canonical configuration, a flat infinite dielec-tric interface ( ) illuminated by a normally incident planewave propagating in free space. The power transmittancethrough this dielectric interface is represented in Fig. 1 as a func-tion of the normalized wavelength. Three cases are compared: i)no ML; ii) one ML; iii) three MLs. The MLs have been designedusing standard rules, e.g., [24]. Their thicknesses and permittiv-ities are given in Table I. It is interesting to notice from Fig. 1that the power transmittance is enhanced significantly ( )over a very wide frequency band when using three layers.Although the assumptions made here (namely, flatness of the in-terface, plane wave illumination, normal incidence, etc.) are farfrom being valid in practice, these results clearly highlight therole of stacked dielectric coatings for the design of broadbandILAs. This constitutes the main frame of this work.
This paper is organized as follows. The generic configurationof the integrated lens antennas (ILAs) under interest is describedin Section II. The ILAs are excited by planar or guided primarysources (we first assume that the ground plane and substrate areof infinite size). The ILA radiation characteristics are given inSection III, and the key role of the lens illumination law is high-lighted and discussed. Then the influence of ground plane andsubstrate truncation is studied in Section IV. Finally conclusionsare drawn in Section V.
II. ANTENNA CONFIGURATION AND NUMERICAL MODELING
We have selected one of the most popular configuration ofILAs—namely the so-called extended hemispherical substratelens [12], [25]—to quantify precisely the influence of dielectriccoatings upon bandwidth enhancement of dielectric lenses. Theantenna geometry is represented in Fig. 2. The lens core is made
NGUYEN et al.: VERY BROADBAND EXTENDED HEMISPHERICAL LENSES: ROLE OF MATCHING LAYERS 1909
in ceramic (Macor, ). In contrast to low-permittivity di-electrics, the use of dense materials favors feed integration [10],[11], [15]–[17] and power transfer from the feed to the lens [25].But this also reinforces excitation of internal resonances, whichstrongly degrades the lens performance. To minimize this dra-matic effect, it is possible to coat the lens core with MLs. In thiswork we restrict our attention to ILAs configurations with up tothree MLs. Their permittivity and thickness are labelledand , respectively (Table I). The MLs have been de-signed to operate around 28 GHz ( ). The externaldiameter of the lens ( ) and the heightof the cylindrical extension ( ) are constant what-ever the number of MLs may be.
In this paper two kinds of primary sources with differentlens illumination law are considered and compared: i) a dielec-tric-loaded metallic rectangular waveguide (3.72 mm 1.86mm) filled with the same dielectric as the core lens material[Fig. 2(a) and (b)]; and ii) a square (1.37 mm 1.37 mm) aper-ture-coupled patch antenna (Fig. 2(c) and (d); ,
, , ) [12],[26]. The feeds are centered on the lens axis. Their geometryis deliberately very simple because emphasis is given to theinfluence of the MLs and not to the design of broadband feeds.Their return loss bandwidth extends from 23–33GHz (36%) for the waveguide feed, and from 26.7–29.2 GHz(10.7%) for the planar feed. The co-polarization componentscomputed at the center frequency (28 GHz) are represented inFig. 3. We have checked that they are very stable as a functionof frequency; in particular the directivity bandwidthof the waveguide and patch feeds equals 54% (20–35 GHz)and 28% (24–32 GHz), respectively. Although thereturn loss bandwidth of the patch is moderate, this does notchange at all the conclusions of that work. Therefore, there isno need to use complex planar (e.g., [27]–[30]) or guided feedswith broad impedance and radiation bandwidth to conduct thepresent study and demonstrate the impact of MLs upon the ILAradiation performance.
It has been shown recently that formulations based on raytracing techniques (e.g., [31]) can not be applied confidently toanalyze ILAs in case of strong internal [32] and/or total [33] re-flections. Consequently, to overcome these limitations, the an-tenna configurations studied here are analyzed using a full-waveapproach, namely a home-made electromagnetic solver basedon the finite-difference time-domain (FDTD) method. The ac-curacy of this tool for the prediction of ILA performance hasbeen already demonstrated numerically and experimentally forvarious complex 3-D lens configurations, e.g., [3], [13], [34].Finally we assume here that all materials are lossless and thatthere is no parasitic air gap between the shells.
III. RADIATION PERFORMANCE WITH AN INFINITE GROUND
PLANE AND SUBSTRATE
A. Waveguide Feed
The co-polarization components of waveguide-fed ILAs arerepresented in Fig. 4 in both principal planes for five frequencypoints ranging between 23–33 GHz. Three lens configurationsare compared: i) no ML [Fig. 4(a)]; ii) one ML [Fig. 4(b)]; and
Fig. 3. Co-polarization components of both feeds computed at 28 GHz. Thefeeds radiate in an infinite half space of ceramic (� � �). (a) �-plane. (b)�-plane. ����: Patch antenna.���: Dielectric-loaded waveguide.
iii) three MLs [Fig. 4(c)]. The cross-polarization component isnot given since its level is smaller than .
In the first case [Fig. 4(a)], we observe fast variations of thebeamwidth and first/far side lobes as a function of frequency.This is due to the strong internal resonances previously men-tioned. Such effects are unacceptable for very broadband quasi-optical receivers (e.g., [15], [16], [27], [28]). As suggested byFig. 1, the results given in Fig. 4 confirm that coating the corelens with one [Fig. 4(b)]—or even three [Fig. 4(c)]—MLs en-ables to improve significantly the frequency stability of the mainbeam, as well as to reduce the average side lobe level. In addi-tion, comparison between Fig. 4(a) and (c) shows that, whenusing three layers, the beam distortions are reduced sub-stantially over the whole bandwidth and the side lobe level islowered by 5–10 dB. In all cases, the reflection coefficient ofthese ILAs remains smaller than from 24–33 GHz.Additional simulation results have demonstrated that the use ofthree MLs enables one to improve greatly the robustness of theantenna characteristics with respect to any uncertainty on the di-electric constants and thickness of the MLs.
B. Comparison Between Guided and Planar Feeds
The same conclusions as above have been drawn when thelens is illuminated by the patch antenna printed on an infinitegrounded substrate [Fig. 2(c) and (d)]. Nevertheless, in con-trast to the previous case (rectangular waveguide), our numer-ical simulations have also shown that the beam axisymmetry isimproved significantly when the lens is coated with MLs. Thiscomes from the intrinsic characteristics of planar feeds (patchantennas, double slot antennas, etc.) whose patterns are nearly
1910 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009
Fig. 4. Influence of the number of MLs upon the ILA radiation patterns in �- and�-planes (waveguide feed with infinite ground plane). (a) No ML. (b) One ML.(c) Three MLs. ����: 23 GHz.���: 26 GHz. ����: 28 GHz.���: 30 GHz.���: 33 GHz.
axisymmetric (Fig. 3) when they radiate into a dense materiale.g., [12], [25].
The variation of the ILA directivity is represented in Fig. 5(a)(waveguide feed) and Fig. 5(b) (printed feed) as a functionfrequency (the directivity values have been computed from 3-Dradiation patterns). In both cases, the ripples observed in thedirectivity curves in absence of dielectric coatings originatefrom the destructive interferences created by the multiple in-ternal reflections occurring within the lens [12], [18]. Howeverwhen the lens core is covered by three MLs, these ripplesalmost disappear, and the ILA directivity approaches itsupper limit, namely the directivity of a uniform fielddistribution of same radiating area as the lens base [dotted linein Fig. 5(a) and (b)]. These results confirm the very broadbandbehavior of ILAs coated with MLs.
Moreover, if the primary feed is designed to ensure an effec-tive illumination of the inner lens surface, then the major partof the ILA spherical interface contributes to the refraction ofthe incident rays. This allows reducing internal and total re-flection effects, thus improving the ILA directivity and radia-tion efficiency, as recently demonstrated in [35]. Here, in con-trast to the waveguide feed, the radiation pattern of the patchalone is nearly circularly symmetric (Fig. 3), and its directivity(8.6 dBi) is higher than the directivity of the rectangular wave-guide (6.9 dBi). These two features explain why the directivityof the patch-fed ILA is very close to [Fig. 5(b)] whenusing three MLs. In this case, the aperture efficiency of theILA ( ) varies between 91% and 99% from25–31 GHz. These are very high theoretical values for broad-band lenses with directive beams [3].
NGUYEN et al.: VERY BROADBAND EXTENDED HEMISPHERICAL LENSES: ROLE OF MATCHING LAYERS 1911
Fig. 5. Antenna directivity. (a) Waveguide feed. (b) Planar feed. In both casesthe ground plane is infinite. ����: No ML.���: One ML. ����: Three MLs.� � � � � �: Radiating aperture with uniform field distribution.
IV. EFFECTS OF THE GROUND PLANE AND SUBSTRATE
TRUNCATION
In this section we study the influence of the ground plane andsubstrate truncation upon the radiation performance of broad-band ILAs. To our best knowledge, such a study has never beenconducted in the literature.
Our simulations have shown that the finite diameter of thefeed waveguide [ , Fig. 2(a) and (b)] has a very small im-pact on the ILA patterns. Therefore we only consider printedfeeds, as schematized in Fig. 2(c) and (d). In these figures, “ ”denotes the distance separating the patch and substrate edges. Inpractice, for most planar primary sources, the feed transmissionline must be long enough to accommodate the feed connector.This explains why the microstrip line extends beyond the lensperiphery in Fig. 2(d).
The radiation patterns computed at resonance are rep-resented in Fig. 6 (no ML). They show a dramatic influ-ence of the ground plane size. In particular, the full-halfpower beamwidths computed in - and -planes vary from
for , tofor the infinite case.
The very strong distortions observed in -plane for positive ele-vation angles (slight beam squint, higher side lobes, beamwidthenlargement) are due to the non-symmetry of the primary feedin this plane, as well as to the pronounced effects of multipleinternal reflections. Covering the inner lens with MLs enablesone to improve strongly the beam quality (Fig. 7): with threeMLs, the forward radiation becomes almost symmetric (even in
-plane for small values of “ ”), and the back radiation and side
Fig. 6. Influence of the ground plane and substrate truncation (planar feed)upon the ILA radiation patterns at resonance (28 GHz). (a)�-plane. (b)�-plane.There is no ML. ����: Infinite case.���: Finite case (� � ��� ��). ����: Finitecase (� � � ��).
lobe levels are reduced by 10 dB in average. These data clearlyhighlight the crucial effect of the truncation of the ground planeand substrate upon the lens performance. Their finite size mustbe absolutely taken into account during the design process ofdense ILAs. Finally, the variation of the maximum antennadirectivity is represented in Fig. 8 as a function of frequency.Two lens configurations are compared: “no ML” (solid line),and “one ML” (dotted line). In both cases, the ground planeextension “ ” equals 1.6 mm or 2 mm. The beam broadeningeffect due to the finite size of the ground plane, as well as thebandwidth improvement induced by anti reflection coatings,are confirmed. For small ground planes, the directivity of ILAscoated with one ML is reduced by 1–2 dB compared to the oneobtained with an infinite ground plane.
V. CONCLUSION
The role of multiple stacked quarter wavelength matchinglayers (MLs) on the radiation performance of broadband inte-grated lens antennas (ILAs) made in dense material has beenstudied numerically around 28 GHz. Compared to ILAs withoutML, it has been shown that the use of three MLs enables to de-sign very broadband lenses with strongly enhanced radiationcharacteristics, namely: i) improvement of beam quality andsymmetry; ii) enhancement of the aperture efficiency and for-ward-to-backward ratio; iii) reduction of the beam distortionsdue to ground plane and substrate truncation. We would like alsoto emphasize that such lens configurations combine the mainadvantages of high- ILAs (enhanced power transfer from theprimary source to the lens, integration of active feeds at the
1912 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 57, NO. 7, JULY 2009
Fig. 7. Finite ground plane and substrate (planar feed, � � �����). Influenceof the number of matching layers upon the ILA radiation patterns at resonance(28 GHz). (a) �-plane. (b) �-plane. ����: No ML.���: One ML. ����: ThreeMLs.
back of the lens) and low- ILAs (broader bandwidth, weakmultiple internal reflections, improved power budget). The extracost due to the fabrication of the dielectric coatings remains ac-ceptable since they are axisymmetric and can be manufacturedeasily using standard molding processes or computer-numeri-cally-controlled fabrication techniques. Moreover, comparisonbetween the ILA performance when the lens is fed either by aplanar feed or by a waveguide, has highlighted the key role ofthe lens illumination law to optimize the antenna characteris-tics. Finally, we have quantified the individual influence of theMLs and truncation effects upon the beam distortions over avery wide frequency range.
ACKNOWLEDGMENT
The authors are grateful to CNRS/IDRIS, Orsay, France, forthe access to their high-performance computing platforms.
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Ngoc Tinh Nguyen was born in Binh Dinh, Vietnam,in 1982. He received the electronic and telecom-munication engineering degree from PolytechnicUniversity, National University of Ho Chi MinhCity, Vietnam, in 2005 and the Master degree ofTelecommunications in Radiofrequency and Mi-croelectronic from the University of Nice-SophiaAntipolis (UNSA), Nice, France, in 2006. He iscurrently working toward the Ph.D. at the IETR,University of Rennes 1, France.
His main fields of interest include the analysis andoptimization of lens antennas for microwave and mm-wave applications.
Ronan Sauleau (M’04–SM’06) graduated inelectrical engineering and radio communicationsfrom the Institut National des Sciences Appliquées,Rennes, France, in 1995. He received the Agrégationdegree from the Ecole Normale Supérieure deCachan, France, in 1996, and the Doctoral degree insignal processing and telecommunications and the“Habilitation à Diriger des Recherche” degree fromthe University of Rennes 1, France, in 1999 and2005, respectively.
From September 2000 and 2005, he was an As-sistant Professor at the University of Rennes 1, where, since November 2005,he has been an Associate Professor. His current fields of interest are numer-ical modeling, millimeter-wave printed and reconfigurable (MEMS) antennas,lens-based focusing devices, periodic structures (electromagnetic bandgap ma-terials and metamaterials) and biological effects of millimeter waves. He has re-ceived four patents and is the author or coauthor of 59 journal papers and morethan 150 contributions to national and international conferences and workshops.
Dr. Sauleau received the 2004 ISAP Conference Young Researcher ScientistFellowship (Japan) and the first Young Researcher Prize in Britany, France,in 2001 for his research work on gain-enhanced Fabry-Perot antennas. InSeptember 2007, he was elevated to Junior member of the “Institut Universi-taire de France”. He was awarded the Bronze medal by CNRS in 2008.
Cecilio José Martínez Pérez was born in Cartagena,Spain, in 1982. He received the Master degree inTelecommnications Engineering from PolytechnicUniversity of Cartagena (UPCT), Cartagena ,Spain, in 2007. His final project was carried out in2006–2007 at IETR, University of Rennes 1, France.
His research activities include lens antenna anal-ysis and optimization for microwave and mm-waveapplications.
