IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014 27

Mirror-Integrated Dielectric Resonator AntennaNan Yang, Kwok Wa Leung, Fellow, IEEE, and Eng Hock Lim, Member, IEEE

AbstractA mirror-integrated dielectric resonator antenna(MIDRA) is proposed. The mirror consists of a glass layer with avery thin light-reflective film coated at its back. It is overlaid ontop of a dielectric resonator antenna (DRA), forming a two-layerDRA when the thin light-reflective film is neglected. To allow thewave to penetrate from the DRA to the glass layer, the film is madeof nonconducting alternate layers of titanium dioxide TiO andsilicon dioxide SiO instead of the traditional conducting silver(Ag) or aluminum (Al) coating. To demonstrate the idea, twocylindrical MIDRAs were designed and fabricated. The first oneis excited in its TM mode by axially feeding it with a coaxialprobe, giving an omnidirectional radiation pattern. For the seconddesign, the broadside HEM mode is excited by using a slot-cou-pled source fed by a microstrip line. Experimental results showthat these two MIDRAs radiate effectively as conventional DRAs.Since the proposed antenna appears to be a mirror, it can be anexcellent hidden antenna that provides practical mirror functions.

Index TermsDielectric resonator antenna (DRA), dualfunctionantenna, glass antenna, hidden antenna.

I. INTRODUCTION

D UALFUNCTION or multifunction antennas have beendeveloped rapidly because they can effectively reduce thesystem size and cut down the overall cost [1]. Recently, dual-function transparent antennas have received much attention dueto their attractive features. They have found a wide range ofapplications, such as on-glass antennas for vehicles or aircraft[2][10], protective [11][13] and light-focusing [13] antennasfor solar cells, light-cover antennas for integrations with lightingsystems [14], [15], decorative antennas that turn antennas intoartworks [16], and invisible RFID reader antennas attached onmirrors of fitting rooms [17].For many years, transparent antennas have been of planar

structures. These designs usually deploy transparent con-ducting oxide (TCO) films such as indium tin oxide (ITO),fluorine-doped tin oxide (FTO), and silver coated polyester(AgHT). TCO films, however, need to compromise between theelectrical conductivity and optical transparency; increasing thetransparency will decrease the conductivity and vice versa. Asa result, these transparent antennas have much lower efficien-

Manuscript received November 27, 2012; revised August 24, 2013; acceptedSeptember 24, 2013. Date of publication October 23, 2013; date of current ver-sion December 31, 2013. This work was supported by a GRF grant from theResearch Grants Council of Hong Kong Special Administrative Region, China(Project No.: 116911).N. Yang and K. W. Leung are with the State Key Laboratory of Millimeter

Waves and Department of Electronic Engineering, City University of HongKong, Kowloon, Hong Kong SAR (e-mail: ee.nanyang@my.cityu.edu.hk;eekleung@cityu.edu.hk).E. H. Lim is with the Faculty of Engineering and Science, Universiti Tunku

Abdul Rahman, 53300 Setapak, Kuala Lumpur, Malaysia (e-mail: limeh@utar.edu.my).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2013.2287007

cies and antenna gains than their metallic counterparts. Thisproblem can be solved by using the glass dielectric resonatorantenna (DRA) [13][16]. It is because transparent DRAs donot need any TCO films to operate. The dielectric constantof glass was measured as 7 from 0.5 GHz to 3 GHz [1].

This value is much higher than that at optical frequencies( for a refractive index of 1.5) and is sufficient forgood DRA designs. The studies in [13][16] show that DRAsfabricated with glass has not virtual effect on the antenna gainat frequencies below 6 GHz.In this paper, a dualfunction glass DRA that provides an ad-

ditional function of mirror is developed. It consists of a conven-tional DRA and a glass layer on its top. Here, the glass is a di-electric loading of the underlaid DRA, instead of a substrate fortransparent patch antennas as studied in [17]. It is worth men-tioning that as compared with the design of [17], the presentmirror-integrated antenna has no components on its mirror sur-face. Also, it has a much higher antenna gain and efficiency.To provide the mirror function, the glass layer is coated at

its back with a very thin periodic multilayer reflective dielec-tric film [18]. This reflective film is nonconducting, which isneeded to enable energy couplings between the DRA and glass.The antenna is essentially a two-layer DRAwhen the thin reflec-tive coating is neglected. Traditionally, the two-layer DRA isused to broaden the impedance bandwidth. Our objective here,however, is simply to integrate a mirror with the DRA insteadof widening the bandwidth. In fact, since the thickness of glassshould not be too thick for a mirror or multiple images will re-sult, no significant bandwidth enhancement can be made in ourdesign.In this paper, the idea is first demonstrated using an axially

probe-fed cylindrical DRA excited in its omnidirectional endfireTM mode. The DRA, glass, and ground plane have the samecross section to form a single cylinder. In the second demon-stration, the slot-coupled source is used to excite the broadsideHEM mode of the DRA. In this case, a large mirror is usedto show the flexibility of the design.Using ANSYS HFSS, the two cylindrical mirror-integrated

DRAs (MIDRAs) operating at 2.4 GHz were designed forWLAN applications. In each case, a prototype was fabricatedand its reflection coefficient, radiation pattern, and antennagain were measured. To study the effect of the reflective film,a film-free counterpart was also fabricated and measured forthe endfire-mode case, and the results are compared with thoseof the endfire MIDRA. It was noticed that the mirror effectdegrades when observed at a large oblique angle. This problemcan be solved by inserting a piece of paper (or any opaquematerial) between the coated glass and the DRA. It was foundthat the effects of the reflective film and paper on the antennaperformance are not significant and can therefore be neglectedin the design.

0018-926X 2013 IEEE

28 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014

It should be highlighted that the proposed antenna can bepackaged as a useful mirror. Also, it is an excellent hidden an-tenna which helps avoid possible concerns about radiation whenit is in close proximity to people.

II. REFLECTIVE FILM OF MIRRORTraditionally, glass of mirror is coated with a thin silver

(Ag) or aluminum (Al) layer, which will block electromagneticwaves. To let waves penetrate into the glass, a nonconductinglight-reflective film is used instead. In this paper, a stratified pe-riodic light-reflective dielectric film is deployed. It is composedof a periodic succession of alternate layers of titanium dioxideTiO and silicon dioxide SiO , which have refractiveindices of and , respectively. For eachlayer, the former and latter have thicknesses ofand , respectively, where is the free-spacewavelength of the light wave to be reflected. There are 25 layersin total, with both the first and last ones being TiO layers. Thereflectivity of this film at normal incidence is given by [18],

(1)

where and are refractive indices of the media flankingthe film, and is the total number of the layers, i.e.,

in our case. It can be shown that regardlessthe values of and , showing that it is a good reflective filmfor obtaining an excellent mirror.It is known that the sensitivity of light-adapted eyes is gener-

ally maximum when the wavelength is 555 nm. With thiswavelength, the thickness of the film can be calculated as

(2)

Obviously, this thickness is negligibly small as comparedwith the dimensions of the glass and DRA. Therefore, the filmcan be neglected in designing the proposed MIDRA.

III. ENDFIRE TM -MODE MIDRA

A. ConfigurationFig. 1 shows the antenna configuration. A cylindrical DRA

with a radius of , a height of , and a dielectric constant ofis placed on a circular ground plane. The DRA is

excited in its fundamental endfire TM mode using an axialcoaxial probe with a length of and a radius of mm.On top of the DRA is a piece of glass with the same radius anda thickness of . Here, theDRA, glass, and ground plane have thesame cross section so that the composite structure forms a singlecircular cylinder that can be handled conveniently. To obtain amirror, the bottom of the glass is coated with a light-reflectivefilm, which has been discussed in detail in the last section.

B. Measured and Simulated ResultsThe dielectric constant of the glass was measured using an

Agilent 85070D Dielectric Probe Kit. It was found that the di-electric constant is given by from 2.0 GHz to 3.0GHz. A dielectric material of was used to fabricate

Fig. 1. Configuration of the proposed endfire MIDRA.

TABLE IDESIGN PARAMETERS OF THE ENDFIRE MIDRA (UNIT: mm)

Fig. 2. (a) Photo of the MIDRA prototype. (b) Photo showing the image of acoin at normal incidence without the DRA. The paper at the bottom is used to fixthe mirror. (c) Photo showing the semitransparent image for an oblique viewingangle. (d) The image in (c) becomes good when the mirror is backed with anopaque material which is the DRA in this case.

the cylindrical DRA.With these dielectric constants, anMIDRAwas designed at 2.4 GHz using HFSS, with the values of the var-ious parameters listed in Table I.Fig. 2(a) shows the fabricated MIDRA prototype with these

parameter values. The performance of the mirror using the non-conducting reflective film is investigated first. Fig. 2(b) showsthe image of a coin at normal incidence with the DRA removed.With reference to Fig. 2(b), a very clear image is obtained,which can be expected according to the analysis of the film fora normal incidence. However, the mirror becomes semitrans-parent when the viewing angle is large, as shown in Fig. 2(c).The problem can be solved easily by backing the glass with anopaque material, e.g., a dielectric or a piece of paper. Fig. 2(d)

YANG et al.: MIRROR-INTEGRATED DIELECTRIC RESONATOR ANTENNA 29

Fig. 3. Measured and simulated reflection coefficients of the MIDRA and ref-erence antenna. The design parameters are given in Table I.

shows the image when the mirror is backed by the DRA. It canbe observed that a good image is now obtained again.To investigate the effect of the reflective coating, a refer-

ence antenna without any reflective coating was also fabricated.In this paper, the reflection coefficients were measured usingan Agilent 8753ES vector network analyzer, while the radia-tion patterns and antenna gains were measured using a SatimoStarLab system. Fig. 3 plots the measured and simulated reflec-tion coefficients of the proposed and reference antennas. Sincethe constitutive parameters of the coating are not known, theMIDRA cannot be simulated and only its measured result isshown in the figure. In contrast, both measured and simulatedresults of the reference antenna are provided in the figure. Forthe MIDRA, its measured 10-dB impedance bandwidth is 3.7%(2.392.48 GHz), which entirely covers the 2.4-GHz WLANband (2.402.48 GHz). It can be seen from the figure that theresult nearly coincides with that of the reference antenna, im-plying that the effect of the coating is negligible. It can alsobe seen that the measured resonance frequency isslightly higher than the simulated value, which should be causedby experimental imperfections including possible air gaps intro-duced between the different interfaces.To study the effect of the glass on the antenna, the cylindrical

DRA without the glass overlay was also simulated and mea-sured. It was found that in this case the measured resonance fre-quency shifts upwards from 2.43 GHz (MIDRA) to2.62 GHz (DRAwithout glass). In other words, adding the glasslayer will decrease the resonance frequency of the antenna. Thisis reasonable because a larger dielectric volume should have alower resonance frequency.Fig. 4 displays the measured radiation patterns of the

MIDRA, along with the measured and simulated results of thereference antenna. With reference to the figure, the three resultsare in good agreement. It can be observed from the figure thatthe MIDRA has an omnidirectional radiation pattern, which isexpected for the fundamental endfire TM mode. For eachresult, the copolarized field is stronger than the crosspolarizedcounterpart by at least 15 dB, which is acceptable.Fig. 5 compares the measured antenna gains between the

MIDRA and reference antenna. It also displays the simulatedantenna gain of the reference antenna, which is in reasonable

Fig. 4. Measured and simulated radiation patterns of the MIDRA and refer-ence antenna. (a) Elevation plane. (b) Azimuth plane . The designparameters are given in Table I.

Fig. 5. Measured and simulated maximum gains of the MIDRA and referenceantenna. The design parameters are given in Table I.

agreement with the measured result. With reference to thefigure, the gains are maximum at around 2.4 GHz, as expected.It can be seen from the figure that the measured antenna gainsof the MIDRA and reference antenna are very close to eachother, showing again that the effect of the coating is small.The maximum measured gain of the MIDRA is found to be1.1 dBi, which is 0.3 dB lower than that of the reference antenna(1.4 dBi). The gain reduction of the MIDRA should be due tothe loss introduced by the reflective coating.

C. DiscussionsTo confirm that the radiation is due to the MIDRA rather

than the feeding probe, a parametric study was carried out usingHFSS. It was found that the resonance frequency of the MIDRAchanges significantly with different mirror thicknesses and di-electric heights , which is a typical behavior of a DRA mode.It was also found that changing probe length does not signif-icantly affect the resonance frequency but influence matchinglevel. All these results verify that it is a DRA mode rather thana probe mode.The effect of the ground plane was also investigated. Fig. 6

shows the simulated reflection coefficients for different ground-plane radii . With reference to the figure, the resonance fre-quency increases significantly with a decrease of when issmaller than the radius of the DRA ( mm). It is becausethe image of the DRA below the ground plane is not a completeDRA when the ground plane is smaller than the DRA base. This

30 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014

Fig. 6. Simulated reflection coefficients for different ground-plane radii .Other design parameters are given in Table I.

can be easily understood by considering the limiting case thatas , no image can be formed at all and only an isolatedDRA in air results. In this case, the DRA size is only half ofthat in the perfectly imaged case, leading to a higher resonancefrequency and a different impedance level. As can be observedfrom the figure, the change of the resonance frequency becomesmuch smaller when because a more complete imagecan now be obtained. It is expected that the change of the res-onance frequency will be negligible when the ground plane ismuch larger than the DRA.

IV. BROADSIDE HEM -MODE MIDRA

A. ConfigurationThe antenna configuration of the broadside case is shown in

Fig. 7. The cylindrical DR has a height of , a radius of ,and a dielectric constant of . On top of it is a pieceof glass with a thickness of . In this case, a much larger glasswith a radius of mm is used to show the flexibilityof the design. The cylindrical DR is excited in its fundamentalbroadside HEM mode by a rectangular slot printed on a sub-strate having a thickness of mm and dielectric constantof . A 50- microstrip line is printed on the otherside of the substrate. The same type of light-reflective dielectricfilm is used again for the mirror design. To avoid the semitrans-parent problem at large oblique viewing angles, the coated glassis backed with a piece of paper.

B. Measured and Simulated ResultsSince the material of was used up, a material of

was used to fabricate the cylindrical DR. Table II liststhe design parameters of the MIDRA that were optimized usingHFSS.Fig. 8 shows the fabricated prototype. The DR is placed at the

center of the slot, as shown in Fig. 8(a). Fig. 8(b) demonstratesthe mirror effect of the coated glass with paper. With referenceto the figure, the image of the object can be clearly seen withoutany semitransparent problem.To study the effect of the reflective coating and backing paper,

a reference antenna without the coating and paper was also fab-ricated and measured. Fig. 9 shows the measured reflection co-efficient of the MIDRA, along with the simulated and mea-sured results of the reference antenna. As can be observed from

Fig. 7. Configuration of the proposed broadside MIDRA. (a) Top view. (b)Illustration of different parts of the configuration.

TABLE IIDESIGN PARAMETERS OF THE BROADSIDE MIDRA (UNIT: mm)

the figure, the measured 10-dB impedance bandwidth of theMIDRA is 10.2% (2.322.57GHz), covering the entire 2.4-GHzWLAN band. The simulated result of the reference antenna is9.4% (2.322.55 GHz). It can be observed from the figure thatthe measured result of the MIDRA agree very well with that ofthe reference antenna, implying that coating and paper can beneglected in the design process. With reference to the figure, themeasured resonance frequency is, again, slightly

YANG et al.: MIRROR-INTEGRATED DIELECTRIC RESONATOR ANTENNA 31

Fig. 8. (a) Photo of the slot-fed DRA prototype. (b) Photo of the proposedMIDRA with a piece of opaque paper at back, from which a clear image of theglass swan can be observed and no semitransparent issue can be found from anoblique viewing angle.

Fig. 9. Measured and simulated reflection coefficients of the MIDRA and ref-erence antenna. The design parameters are given in Table II.

Fig. 10. Measured and simulated radiation patterns of the MIDRA and refer-ence antenna at 2.44 GHz. (a) Elevation plane. (b) Elevation plane.The design parameters are given in Table II.

higher than the simulated frequency due to experimental imper-fections as mentioned before.Fig. 10 shows the radiation patterns of the MIDRA and refer-

ence antenna at 2.44 GHz. From the figure, broadside radiationpatterns are observed for both antennas, as expected. The resultof the MIDRA almost coincides with that of the reference an-tenna, further showing that the coating and paper has no virtualeffect on the antenna performance. For each antenna, the copo-larized field is stronger than the crosspolarized counterpart byat least 20 dB. It can be seen from the figure that the measuredand simulated radiation patterns of the reference antenna are ingood agreement.Fig. 11 shows the boresight gains of the MIDRA and refer-

ence antenna. With reference to the figure, the measured results

Fig. 11. Antenna boresight gains of the MIDRA and reference antenna. Thedesign parameters are given in Table II.

Fig. 12. Simulated bandwidth and resonance frequency as a function of mirrorthickness . Other design parameters are given in Table II.

of the two antennas are, again, very close to each other. It canbe found from the figure that the measured gains of the antennasare 4.5 dB over the WLAN band (2.42.48 GHz), which is0.5 dB lower than the simulated result.From the results of Figs. 911, it is obvious that the effects

of the reflective film and backing paper on the antenna perfor-mance are negligibly small, simplifying the MIDRA design.The effect of the mirror (glass) thickness on the impedance

bandwidth is studied in Fig. 12. With reference to the figure,the bandwidth increases with an increase of , which can be ex-pected from the knowledge of the two-layer DRA. As discussedbefore, however, cannot be too large or the image will be un-clear. The effect of on the resonance frequency of the antennawas also studied and displayed in the same figure. As can be ob-served from the figure, the resonance frequency decreases withincreasing , which is consistent with the fact that a larger res-onator has a lower resonance frequency.

V. CONCLUSIONAn MIDRA has been proposed and investigated for the first

time. It consists of a conventional DRA and an overlaid glasslayer. The back of the glass layer is coated with a nonconductinglight-reflective film to provide the mirror function. Both endfireand broadside MIDRAs have been studied in this paper. In theformer case, a cylindrical MIDRA fed by an axial coaxial probewas designed, fabricated, and measured. It is excited in its fun-damental endfire TM mode. A reference antenna without thereflective coating was also fabricated to investigate the effect of

32 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 1, JANUARY 2014

the coating on the antenna performance. It has been found thatthe effect of the coating is small. Therefore, the coating can beneglected when designing the antenna, making the design veryeasy.For the broadside design, the slot-coupled source is used to

excite the fundamental broadside HEM mode of the DRA. Insome applications, it may be required that the size of the mirrorbe much larger than the DRA. In this case, the coating regionoutside the DRA can be covered by an opaque material (e.g.,paper) to obtain a good mirror at different viewing angles. Ithas been shown in this paper that reflective coating and backingpaper have negligible effects on the antenna performance andcan therefore be neglected in the modeling.Finally, it should be mentioned that the MIDRA simply ap-

pears as a mirror which can be widely used in our daily life. Itcan also be used as an excellent hidden antenna to avoid pos-sible concerns of radiation.

ACKNOWLEDGMENTThe authors would like to thank the anonymous reviewers for

their useful comments. They would also like to thank Mr. M. S.Leung for taking the photos in this paper.

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[14] K. W. Leung, Y. M. Pan, X. S. Fang, E. H. Lim, K. M. Luk, andH. P. Chan, Dualfunction radiating glass for antennas and lightcoversPart I: Omnidirectional glass dielectric resonator antennas,IEEE Trans. Antennas Propag., vol. 61, no. 2, pp. 578586, Feb. 2013.

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Nan Yang was born in Yangling, Shaanxi, China,in 1987. He received the B.Sc. and M.Eng. degreesin electronic engineering from Zhejiang University(ZJU), Hangzhou, Zhejiang, China, in 2008 and2012, respectively. He is currently working towardsthe Ph.D. degree at City University of Hong Kong.From 2012 to 2013, he was a Research Assistant at

City University of Hong Kong. His research interestsinclude microwave andmm-wave circuits and dielec-tric resonator antennas.

Kwok Wa Leung (S90M93SM02F11) wasborn in Hong Kong in 1967. He received the B.Sc.degree in electronics and the Ph.D. degree in elec-tronic engineering from the Chinese University ofHong Kong, in 1990 and 1993, respectively.From 1990 to 1993, he was a Graduate Assistant

with the Department of Electronic Engineering,the Chinese University of Hong Kong. In 1994, hejoined the Department of Electronic Engineeringat City University of Hong Kong (CityU) as anAssistant Professor. Currently, he is a Professor and

an Assistant Head of the Department. From January to June, 2006, he was aVisiting Professor in the Department of Electrical Engineering, The Pennsyl-vania State University, State College, PA, USA. His research interests includeRFID tag antennas, dielectric resonator antennas, microstrip antennas, wireantennas, guided wave theory, computational electromagnetics, and mobilecommunications. He was an Editor for HKIE Transactions and a Guest Editorof IET Microwaves, Antennas and Propagation. He has served as an AssociateEditor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION (TAP)and IEEE Antennas and Wireless Propagation Letters.Prof. Leung is a Fellow of HKIE. He received the International Union of

Radio Science (USRI) Young Scientists Awards in 1993 and 1995, awardedin Kyoto, Japan and St. Petersburg, Russia, respectively. He received Depart-mental Outstanding Teacher Awards in 2005, 2010, and 2011 and the CityU Re-search Excellence Award 2013. He was the Chair of the IEEE AP/MTT HongKong Joint Chapter for the years of 2006 and 2007. He was the Technical Pro-gram Chair, 2008 Asia-Pacific Microwave Conference, Hong Kong, the Tech-nical Program Co-Chair, 2006 IEEE TENCON, Hong Kong, and the FinanceChair of PIERS 1997, Hong Kong. He has been rated as Outstanding Asso-ciate Editor of TAP and received IEEE TRANSACTIONS Commendation Cer-tificates twice in 2009 and 2010. He has been the Editor-in-Chief of the IEEETRANSACTIONS ON ANTENNAS AND PROPAGATION since August, 2013. He is aDistinguished Lecturer of the IEEE Antennas and Propagation Society.

Eng Hock Lim (S05M08) was born in Selangor,Malaysia. He received the B.Sc. degree in electricalengineering from National Taiwan Ocean Univer-sity, Keelung City, in 1997, the M.Eng. degree inelectrical and electronic engineering from NanyangTechnological University, Singapore, in 2000, andthe Ph.D. degree in electronic engineering from CityUniversity of Hong Kong in 2007.Since 2008, he has been an Assistant Professor at

the Universiti Tunku Abdul Rahman, Kuala Lumpur,Malaysia. His current research interests include mul-

tifunctional antennas and microwave components.Dr. Lim is an Associate Editor of IEEE TRANSACTIONS ON ANTENNAS AND

PROPAGATION.