PATCH ANTENNA WITH STACKEDSPLIT-RING RESONATORS AS ANARTIFICIAL MAGNETO-DIELECTRICSUBSTRATE

Mikko Karkkainen and Pekka IkonenRadio Laboratory/SMARADDepartment of Electrical and Communications EngineeringHelsinki University of TechnologyP.O. Box 3000FIN-02015 HUT, Finland

Received 10 March 2005

ABSTRACT: Densely packed arrays of split-ring resonators (SRRs) areused as an artificial magneto-dielectric substrate to reduce the resonantfrequency of a �/2 patch antenna. The SRR stacks (also called metasole-noids) embedded into dielectric medium constitute an anisotropic magneto-dielectric substrate, thus allowing a considerable reduction of the resonantfrequency of the antenna. The effects of the magneto-dielectric substrates onimpedance bandwidth are studied numerically and experimentally. We ex-perimentally demonstrate a wider impedance band for an antenna withstacked SRRs embedded in a low-permittivity substrate, as compared withone in a higher-permittivity substrate without SRRs. © 2005 Wiley Periodi-cals, Inc. Microwave Opt Technol Lett 46: 554–556, 2005; Published on-line in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21048

Key words: artificial magnetic materials; FDTD; antennas

1. INTRODUCTION

Based on theoretical considerations, magnetic materials have beenfound to be useful in reducing the resonant frequency of patchantennas while approximately retaining the relative bandwidth[1–3]. Patch antennas with resonant magnetized ferrite substrateshave been studied in [4]. It is known that magnetic materialsavailable in nature exhibit high losses at microwave frequencies.This fact severely limits their applicability at microwave frequen-cies. Recently, artificially engineered magnetic materials havebeen proposed for microwave frequencies in [5–9]. The variousmaterial designs are based on different configurations of split-ringresonators (SRRs).

In this paper, stacked SRRs are used between the patch and theground plane of a coax-fed patch antenna in order to determinehow the resonant frequency can be reduced, hence allowing sizeminiaturization for an antenna operating at a fixed frequency. Thebandwidth of the antenna is also examined. In particular, wecompare the bandwidth of a patch antenna filled with dielectricswith a patch antenna filled with artificially engineered magneto-dielectric material.

2. PATCH-ANTENNA STRUCTURE

For the sake of simplicity, we consider a square patch of size 60 �60 mm fed with a 50� coaxial cable near one edge of the patch.A short coplanar strip of dimensions 4 � 10 mm is attached to theedge of the patch and the feeding probe is connected to the strip,as shown in Figure 1. The distance of the patch from the groundplane is h � 10 mm. To eliminate the effects of a finite groundplane and to better see the effect of the artificial material filling, theground plane is large (30 � 30 cm) for the measurements andinfinite for the simulations.

3. SIMULATED AND MEASURED RESULTS

As a numerical tool, we adopted an in-house finite-differencetime-domain (FDTD) program. The (FDTD) method is useful in

this problem, since the number of unknowns grows quickly asthe number of split rings in the volume increases. In the presentproblem, in which several split rings are under the patch, FDTDhas been observed to be considerably faster than frequency-domain numerical methods, such as the MoM or FEM. Thesplit-rings, although not perfect conductors, are modelled asthin PEC-wires in FDTD, which allows us to simulate severalhundreds of SRRs under the patch antenna with essentiallysimilar computational overhead as that for the empty patch. Thefinite conductivity is not expected to be a critical error source.Rather, the discretization of regions such as the gaps in thesplit rings, where the electric field is strong, is a more delicateissue.

The effects of size, shape, orientation, and the packingdensity of the rings on the S11 characteristics were examined.Qualitatively, increasing the electric size of the rings decreasesthe resonant frequency of the metasolenoids and the antenna upto a certain limit with a certain minimum allowed return-losslevel. Increasing the packing density has a similar effect. Basedon extensive FDTD simulations, we chose the structure illus-trated in Figure 1(b) for manufacturing and measurements. Amanufactured and measured antenna prototype is shown inFigure 2.

Consider the stack of SRRs with alternating gap positions.Each individual ring has a width of 9 mm, and a height of 3 mm.The gap in the longer edge of the ring is 1-mm wide. The stripwidth is 0.2 mm (in simulations, we approximate the narrowstrips as thin wires). The rings are organized in stacks, wherethe gap position alternates. The rings in the stack are locatedat a 0.79-mm distance from each other. According to theoreticalconsiderations, the resonant frequency of the stack is about

Figure 1 The patch antenna under investigation (a) as seen from aboveand (b) with x-oriented SRR stacks, side view

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2.8 GHz. An artificial magneto-dielectric material layer can beconstructed by arranging closely spaced SRR stacks under thepatch. As expected, the orientation of the SRR axis affect theresults, and the x-directed SRR stacks are more strongly excitedin this geometry than the y-directed SRR stacks. This is becausethe current flow is mainly perpendicular to the x-directed SRRstacks.

The five x-directed stacks are located periodically, startingfrom 4 mm from the edge. There is a 3-mm air gap between eachpair of adjacent stacks, except between two of them, where the airgap is 2 mm. The simulated (FDTD) and measured �S11� results forthe patch antenna filled with air, dielectrics, or magneto-dielectricsare shown in Figure 3. The measured relative bandwidth forSRR-filled patch is 6.2%, while the simulated bandwidth is 6.6%.The measured resonant frequency is about 6% higher than thesimulated one. The slight differences are most likely due to limi-tations in SRR discretization, since due to computer limitations,only two cells are used to resolve the fields in the capacitive gapsof the split rings.

Concerning the impedance bandwidth of the antenna with theproposed magneto-dielectric substrate consisting of SRRs embed-

ded in a dielectric medium, a reference experiment was performedby finding an average relative permittivity of such a substrate thatyields the same resonant frequency as the antenna with SRRfilling. By varying the number of thin dielectric sheets of two types(�r � 10.8 and �r � 2.33) under the patch, we found that if theaverage permittivity of the reference dielectric substrate is about�ave � 8.5, we obtain the same resonant frequencies (about 1.35–1.36 GHz) for both cases. Then, looking at the impedance band-widths, we can draw conclusions about the utility of the artificialmagneto-dielectric substrate. It is observed from the measuredresults that the impedance bandwidth in the case of the magneto-dielectric substrate (6.2%) is higher than in the case of a puredielectric substrate (5.7%). Our measurements verify that the SRRstacks preserve the bandwidth of the antenna better than thedielectric loading. This fact also suggests that the SRR stacks actas an effective magnetic material equivalent to an enhanced in-ductive loading.

The efficiencies were measured using the Wheeler cap method.The efficiency for an air-filled antenna is 96.5%. For the SRR-filled and dielectric-filled antenna, the efficiencies are 92% and95%, respectively. Dielectric losses, losses in the glue material,and losses in split rings are responsible for the small degradationof the efficiency, as compared with the air-filled antenna.

Since the impedance bandwidth of the patch antenna withinductive (inductance L) and capacitive (capacitance C) loadingsis proportional to �L/C, it would be desirable to achieve strongerinductive loading in order to better preserve the bandwidth. Ascompared with a simple dielectric loading, we have already dem-onstrated a step towards this direction, as the bandwidth compar-isons indicate. Efficiency measurements indicate that losses due toSRRs do not increase significantly and the proposed design isfeasible in this sense. Thus, conductor losses cannot explain muchabout the observed bandwidth characteristics. Alternative SRRgeometries could be investigated to see whether it is possible todesign a structure leading to a wider impedance band, correspond-ing to a higher inductive and lower capacitive effect.

4. CONCLUSION

An artificial magneto-dielectric material composed of stacks ofsplit-ring resonators (SRRs) under a patch antenna has been in-

Figure 2 The measured antenna prototype. [Color figure can be viewedin the online issue, which is available at www.interscience.wiley.com.]

Figure 3 Simulated and measured �S11� for the patch antenna with airfilling, dielectric filling and stacked SRR filling ( x-oriented SRR stacks).[Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

Figure 4 Measured �S11� for the patch antenna with stacked SRR filling( x-oriented SRR stacks), and dielectric filling. The two antennas have beendesigned to operate approximately at the same frequency in order to enablebandwdith comparison

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vestigated numerically and experimentally. We have verified ex-perimentally that as far as the impedance bandwidth of a �/2 patchantenna is concerned, it is better to use a magneto-dielectricsubstrate as introduced here rather than a dielectric substrate. Themeasurements indicate that the antenna efficiency remains highwhen the proposed magneto-dielectric substrate is used as a fillingmaterial, thus validating the feasibility of the proposed design.

REFERENCES

1. R.C. Hansen and M. Burke, Antennas with magneto-dielectrics, Micro-wave Opt Technol Lett 26 (2000), 75–78.

2. S. Yoon and R.W. Ziolkowski, Bandwidth of a microstrip patch antennaon a magnetodielectric substrate, Proc IEEE Antennas Propagat Symp,2003.

3. H. Mosallaei and K. Sarabandi, Magneto-dielectrics in electromagnet-ics: concept and applications, IEEE Trans Antennas Propagat 52 (2004),1558–1567.

4. A.D. Brown, J.L. Volakis, L.C. Kempel, and Y.Y. Botros, Patch anten-nas on ferromagnetic substrates, IEEE Trans Antennas Propagat 47(1999), 26–32.

5. P. Ikonen, S.I. Maslovski, S.A. Tretyakov, and I.A. Kolmakov, Newartificial high-permeability material for microwave applications, PIERS2004, Pisa, Italy, 2004.

6. M.V. Kostin and V.V. Shevchenko, Theory of artificial magnetic sub-stances based on ring currents, J Commun Technol Electron 38 (1993),72–83.

7. J.B. Pendry, A.J. Holden, D.J. Robbins, and W.J. Stewart, Magnetismfrom conductors and enhanced nonlinear phenomena, IEEE Trans Mi-crowave Theory Tech 47 (1999), 2075–2084.

8. J.D. Baena, R. Marqes, F. Medina, and Jess Martel, Artificial magneticmetamaterial design by using spiral resonators, Phys Rev B 69 (2004),014402.

9. A.N. Lagarkov, V.N. Semenenko, V.N. Kisel, and V.A. Chistyaev,Development and simulation of microwave artificial magnetic compos-ites utilizing nonmagnetic inclusions, J Magnetism Magnetic Mater258–259 (2003), 161–166.

© 2005 Wiley Periodicals, Inc.

COMPACT BROADBAND U-SLOT-LOADED RECTANGULAR MICROSTRIPANTENNAS

Amit A. Deshmukh and Girish KumarDepartment of Electrical EngineeringI.I.T. BombayPowai, Mumbai–400 076, India

Received 7 March 2005

ABSTRACT: A compact variation of a U-slot-loaded rectangular mi-crostrip antenna—a half U-slot (L-shaped slot) loaded rectangular mi-crostrip antenna—is proposed. The bandwidth of a shorted compactsquare microstrip antenna is increased by cutting a quarter-wavelengthstep U-slot. © 2005 Wiley Periodicals, Inc. Microwave Opt TechnolLett 46: 556–559, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21049

Key words: U-slot rectangular microstrip antenna; half U-slot rectan-gular microstrip antenna; stepped U-slot rectangular microstrip anten-na; compact microstrip antenna

1. INTRODUCTION

A compact microstrip antenna (MSA) is obtained by using eithera higher dielectric constant �r substrate or a shorting post, or by

cutting a slot [1, 2]. The bandwidth (BW) is increased by usingeither thicker substrates with lower �r or multiresonator gap-coupled and stacked configurations [3]. However, these BW im-provement techniques increase the antenna size or its thickness.Broadband MSAs using a resonant slot cut inside the patch in-creases the BW without increasing its volume [4]. The slot addsanother resonant mode near the TM10 resonance frequency ofunslotted rectangular MSAs (RMSAs), thereby increasing the BW.

A broadband configuration using a U-slot cut inside the RMSA,a circular MSA, and an equilateral triangular MSA have beenreported [4–6]. In this paper, a U-slot RMSA is discussed and itseven-mode equivalent, that is, a compact half U-slot (L-shapedslot) RMSA (whose size is reduced by half) is proposed. Avariation of the U-slot, the stepped U-slot RMSA, is proposed andthe BW of a compact shorted square MSA (SMSA) is increased bycutting a half stepped U-slot. All the MSAs have been initiallyoptimized using the IE3D software [7], followed by experimentalverification. For the simulation of all the MSAs, an infinite groundplane has been assumed. However, for the experiments, the size ofground plane is taken to be more than six times the substratethickness in all directions with respect to the patch dimensions soas to reduce the effect of finite ground plane [3].

2. U-SLOT RMSA

A U-slot RMSA etched on a glass epoxy substrate (�r � 4.3, h �0.16 cm, and tan� � 0.02), is suspended over the ground planewith an air gap � of 1.7 cm, as shown in Figures 1(a) and 1(b). Thetotal thickness ht is 1.859 cm (0.06�0). The RMSA was designedto operate at around 900 MHz. The resonant U-slot is locatedsymmetrically with respect to the feed-point axis. The broadband

Figure 1 Suspended U-slot RMSA; (a) top and (b) side views; (c) inputimpedance and VSWR plots (—— measured, – – – simulated); (d) simu-lated radiation pattern at 894 MHz and its (e) gain variation with frequency

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