Breinbjerg, Olav

Kim, Oleksiy S.

A miniaturized planar antenna based on a broadside-coupled split ring resonator excited by an arc-shaped dipole is presented. The excitation dipole acts as a small tuning capacitor in series with a parallel RLC circuit represented by the SRR. The antenna resonance frequency and dimensions a essentially determined by the SRR, while by varying the dipole arm length the input resistance is changed in a wide range, thus matching the antenna to a feed line and compensating for simulation and manufacturing inaccuracies. No additional matching network is required. Theoretically, there is no limit on how small this antenna can be. In practice, the lower bound is set by losses in utilized materials and manufacturing inaccuracies. As an example, an antenna of ka=0.09 was designed, fabricated and tested. Although the initially fabricated antenna prototype had the input impedance of 43 ohms, it was subsequently tuned to 50 ohms simply by cutting out the excessive arm length. This tuning technique is especially useful in practical applications, since it allows the antenna to be tuned in-place and thereby compensate for various inaccuracies as well as for an antenna environment

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Miniaturized Planar Split-Ring Resonator AntennaKim, Oleksiy S.; Breinbjerg, OlavPublished in:IEEE International Symposium on Antennas and PropagationLink to article, DOI:10.1109/APS.2009.5172040Publication date:2009Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Kim, O. S., & Breinbjerg, O. (2009). Miniaturized Planar Split-Ring Resonator Antenna. In IEEE InternationalSymposium on Antennas and Propagation (pp. 1-4). IEEE. DOI: 10.1109/APS.2009.5172040Miniaturized Planar Split-Ring Resonator AntennaOleksiy S. Kim* and Olav Breinbjerglectromagnetic SystemsDepartment of lectrical ngineering, Technical University of Denmark0rsted Plads, Building 8, DK- 8 Kgs. Lyngby, Denmark-mail: osk@elektro.dtu.dkIntroductionA split-ring resonator (SRR) is one of the electrically smallest resonant elementsavailable at microwaves. Its applications range from compact filters to advancedmetamaterials. Besides this, an electrically small antenna can be built using anSRR as it was recently shown in ,where a broadside coupled SRR was excitedby an arc-shaped monopole on a metal ground plane. The size of this self-resonantSRR-based antenna was demonstrated to be as small as AI (ka ~. ), where Ais the free space wavelength, k is the wave number, and a is a radius of a minimumsphere enclosing the antenna. As it is argued in ,an antenna with ka < . canbe considered as an electrically small antenna. Then, a planar SRR-based antennapresented in this paper can be regarded as an electrically very small antenna sinceit has ka < ..Geometry and properties of the antennaA planar SRR-based antenna is composed of a broadside coupled SRR and an ex-citation dipole with arc-shaped arms surrounding the SRR (Fig. ). Both the SRRand the dipole are printed on the same substrate. The broadside coupled versionof SRR is chosen due to its lower RextlA ratio as compared to the edge coupledcounterpart . The antenna can be approximated by a parallel RLC circuit tunedto a resonance with a series connected small capacitor. The parallel RLC circuitand the tuning capacitor correspond to the SRR and the dipole, respectively. Byvarying the length of the dipole arms, which is equivalent to changing the reactanceof the tuning capacitor, the antenna input impedance is tuned in a wide range.(b)Figure : Antenna geometry Figure : Broadside (a) and edge (b)overlapping dipole armsAs shown in , the input resistance at the resonance is inverse proportional to thedipole arm length, while the resonance frequency as well as the size of the antenna978-1-4244-3647-7/09/$25.00 ©2009 IEEEAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 4, 2009 at 09:27 from IEEE Xplore.  Restrictions apply. are governed by the SRR geometry. Reduction of the antenna electrical size can beachieved by using thin substrates with high dielectric permittivity. However, as theSRR gets smaller the total arc length of the curved dipole arms required to tune theantenna to a desired input impedance starts exceeding the perimeter of the SRR.To maintain the required dipole arm length it should either be moved away fromthe SRR, which increases the overall antenna diameter, or the arms overlap. In thelater case there are two options:• design 1: arms are printed on the opposite sides of the substrate (Fig. a);• design 2: arms are printed on the same side of the substrate (Fig. b).Simulations performed with the Ansoft's HFSS reveal that these two configurationsdemonstrate different characteristics.Numerical and Experimental ResultsThe geometry with following parameters is investigated: R ext .6 mm, Rint6. mm, Rdip mm, t . mm, 9 . mm, and w .6 mm. TheRogers substrate with relative dielectric permittivity E is utilized. Theconductivity of metalization is assumed to be .8· 8 S/m. The antenna is directlyfed by a -ohm coaxial feed line, whose effect is subsequently removed by de-embedding the input impedance to the antenna feed point. The antenna designsand are simulated for a range of dipole arm lengths, and the results are summarizedin Fig. ,where the antenna resonance frequency and the input resistance at theresonance are plotted as functions of the angle ex. It is observed that with thenon-overlapping dipole arms (ex < 8 0) both antenna designs resonate at the samefrequency, and this frequency is nearly independent on the angle ex. However, as thearms overlaps (ex > 8 0) the curves start diverging and the resonance frequencystarts varying with the angle ex. This is caused by a parasitic capacitor formedbetween the overlapping dipole arms. The parasitic capacitance is much smaller inthe design and therefore the frequency variation is also weaker. Besides this, bothdesigns yield a wide dynamic range in terms of input resistances the antenna canbe matched to simply by varying the dipole arm length.Since the design exhibits weaker dependence of the resonance frequency and theinput resistance on the dipole arm length and consequently weaker sensitivity tonumerical and manufacturing errors, it is preferable from the practical point ofview. Moreover, it allows tuning functions with respect to the resonance frequencyand input resistance to be rather independent on each other. Thus, the designwas selected for experimental testing. To minimize the diameter of the antennaprototype the dipole width in the overlapping domain was set to w . mm. Withthe clearance of. mm added between the dipole arms and the substrate edge theoverall diameter of the antenna is .6 mm (Fig. ), which at MHz correspondsto ka ~ .According to Fig. ,the ohms input resistance at the resonance corresponds tothe angle ex 0. The prototype, however, was fabricated with little longer arms(ex 0). By cutting the dipole arms from ex ° to ex ° the inputresistance at the resonance was tuned from ohms to . ohms. Simulated andAuthorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 4, 2009 at 09:27 from IEEE Xplore.  Restrictions apply. measured reflection coefficients are compared in Fig. . The relative error for boththe angle ex and the resonance frequency is less than %, which is within marginsset by tolerances on the relative permittivity and thickness of the utilized substrate.Radiation measurements were performed at the DTU- SA Spherical Near-field An-tenna Test Facility and the results are presented in Fig. 6. The radiation pattern cor-responds to that of a Z-directed magnetic dipole. Simulated and measured antennaparameters - angle ex, resonance frequency fa, input resistance at the resonance Ro,bandwidth at - dB ~f-lodB, quality factor Q calculated using ,eq.(86), peakdirectivity D, and efficiency 'fJ - are summarized on Table .ConclusionsA miniaturized planar antenna based on a broadside-coupled split ring resonatorexcited by an arc-shaped dipole is presented. The excitation dipole acts as a smalltuning capacitor in series with a parallel RLC circuit represented by the SRR. Theantenna resonance frequency and dimensions a essentially determined by the SRR,while by varying the dipole arm length the input resistance is changed in a widerange, thus matching the antenna to a feed line and compensating for simulationand manufacturing inaccuracies. No additional matching network is required. The-oretically, there is no limit on how small this antenna can be. In practice, the lowerbound is set by losses in utilized materials and manufacturing inaccuracies.As an example, an antenna of ka ~ was designed, fabricated and tested.Although the initially fabricated antenna prototype had the input impedance ofohms, it was subsequently tuned to ohms simply by cutting out the exces-sive arm length. This tuning technique is especially useful in practical applications,since it allows the antenna to be tuned in-place and thereby compensate for variousinaccuracies as well as for an antenna environment.AcknowledgementsThis work is supported by the Danish Research Council for Technology and Produc-tion Sciences within the TopAnt project (http://www.topant.dtu.dk). Dr. S. Piv-nenko is acknowledged for the antenna radiation measurements.Referenceso. S. Kim and O. Breinbjerg, "Miniaturized self-resonant split-ring resonatorantenna," accepted for publication in Electronics Letters,S. Best, "A discussion on the properties of electrically small self-resonant wireantennas," IEEE Antennas Propag. Magazine, vol. 6, no. 6, pp. -R. Marques, F. Mesa, J. Martel, and F. Medina, "Comparative analysis of edge-and broadside- coupled split ring resonators for metamaterial design - theory andexperiments," IEEE Trans. Antennas Propagat., vol. , no. ,pp. 7 - 8,Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 4, 2009 at 09:27 from IEEE Xplore.  Restrictions apply. A. Yaghjian and S. Best, "Impedance, bandwidth, and Q of antennas," IEEETrans. Antennas Propagat., vol. ,no., pp. 8-401405NI397~>.()cQ)393 g.~LL_~j389~\\•design 1 - ~'-'-o desi n 2 -e- --------.- 385170 175 180 185 190 195 200 205 210Angle a, degrees50250 r-------..-----------------,200E~c: 150~cS.~ 100Q)a:Figure Input resistance and resonance fre-quency vs. dipole arm length.Figure Fabricated antennaprototype (Design ).-35 L....- ........L. ----IL.....- ----'392 394 396 398 400 402 404Frequency, MHz-135 -90 -45 0 45 90 135 180Angle 9, degreeso5,.....-----------------....-----.-20in -5"0f-10t5~0-15simulated:u=201measured:a = 202-5-30co -10"0gf -15.Q~ -20Q)a: -25FigurelossSimulated and measured return Figure 6: Measured radiation patternTable Simulated and measured antenna parametersex fa, MHz Ro, ohms ~f-lodB,MHz Q D, dBi "7, %simulated 0 .7 .8measured 0 6. .8 8 . ±. . ± .Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 4, 2009 at 09:27 from IEEE Xplore.  Restrictions apply. 

Miniaturized Planar Split-Ring Resonator Antenna

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Miniaturized planar split-ring resonator antenna

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Miniaturized Planar Split-Ring Resonator Antenna

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