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A Planar Dual-Band Periodic Leaky-Wave Antenna Based on a Mu-Negative (MNG) Transmission Line

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A Planar Dual-Band Periodic Leaky-Wave Antenna Based on a Mu-Negative (MNG) Transmission Line
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  2370 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015 A Planar Dual-Band Periodic Leaky-Wave Antenna Basedon a Mu-Negative (MNG) Transmission Line Roy B. V. B. Simorangkir and Yongshik Lee  Abstract— In this study, a planar periodic leaky-wave antenna (LWA)that provides a dual-band full-space scanning property is presented. TheLWA is based on a mu-negative (MNG) transmission line that offers anonlinear dispersion characteristic with a very simple structure. The dual-band beam scanning property is achieved by utilizing the  n  = − 1  and n  = − 2 space harmonics to overcome the limitation of the MNG line thatprovides forward scanning only in the n  = 0  space harmonic. Microstripbends are introduced to achieve proper matching and thereby suppressthe open stopband effect at both broadside radiation frequencies, to avoidinterference between two space harmonics, and to ensure efficient radia-tion. The proposed design method is validated by good agreement betweenthe simulated and experimental results for the dual-band LWA that isdesigned to provide full-space scanning with the first and second broad-side radiation frequencies at 4.3 and 8 GHz. The demonstrated total scanangle range of 279 ◦ is the widest range reported for the dual-band LWA.  Index Terms— Composite right-/left-handed, dual-band, full-spacescanning,leaky-waveantenna(LWA),mu-negative(MNG),openstopband. I. I NTRODUCTION Leaky-wave antennas (LWAs) have a unique feature that their beamcan be scanned by varying the frequency of operation. Therefore, arelatively high directivity is achieved with great structural advantagesof extremely simple feeding network, low profile, and low-cost fab-rication. These advantages make planar LWAs suitable for variousapplications that require single or multiple beam scanning, such ashumantrackingradars[1],[2],automotiveradars[3],[4],andreal-timespectrum analyzers [5].Current research efforts focused on LWAs include dual-band oper-ation to enable scanning in two different operating bands [6]–[9]. Oneof the most popular methods for realizing planar dual-band LWAs is toutilize the nonlinear dispersion characteristic of extended compositeright-/left-handed (E-CRLH) technology [10], [11], which success-fully overcomes the limitation of a CRLH line that cannot providefull-space scanning in two different bands [6]–[8]. Nevertheless, anE-CRLH unit cell consists of a number of reactive elements, whichcomplicate the design procedure. In addition, the E-CRLH unit cellsare more vulnerable to parasitic effects not only from the reactiveelements but also from the relatively complex layout and via holesfor shunt connections in microstrip forms. Therefore, nonideal prop-erties are observed, such as imperfect matching [6]–[8] and backsideradiation [8].This study presents a dual-band periodic LWA based on a mu-negative (MNG) transmission line. An MNG line is a variation of theCRLH line [12], which also provides a nonlinear dispersion charac-teristic. However, with an MNG line, this essential characteristic forthe development of dual-band circuits is achieved with a much simpler Manuscript received August 26, 2014; revised January 30, 2015; acceptedMarch 02, 2015. Date of publication March 09, 2015; date of current ver-sion May 01, 2015. This work was supported in part by the Korea EvaluationInstitute of Industrial Technology (KEIT) Research Grant of 2015(10047815)and in part by the National Research Foundation (NRF) of Korea government(MEST) under Grant 2011-0016802.The authors are with the Department of Electrical and ElectronicEngineering, Yonsei University, Seoul 120-749, Korea (e-mail: yongshik.lee@yonsei.ac.kr).Color versions of one or more of the figures in this communication areavailable online at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2015.2410802 structure because no shunt connections are required. Therefore, thedesign and fabrication processes of LWAs are simplified greatly.Furthermore, without any inductors, an MNG line is especially moresuitable for high-frequency applications. In addition, a method isdemonstrated, for the first time, that utilizes two higher order spaceharmonics to achieve the dual-band full-space scanning property. Thisapproach allows tuning the dispersion easily while maintaining thesimplicity of an MNG unit cell. The experimental results of an LWAwith broadside radiation frequencies at 4.3 and 8 GHz are in greatagreement with the simulated results, thus validating the proposedapproach.II. D ESIGN  P RINCIPLE  A. Utilization of Higher Order Space Harmonics in an MNG Line Fig. 1 shows the schematics of conventional CRLH and MNGunit cells, where  C  L  and  L L  are the series capacitance and parallelinductance of the left-handed part, respectively, and  Z  R  and  θ R  arethe characteristic impedance and electrical length of the right-handedtransmission line section, respectively. Compared with a CRLH linefrom which the MNG line was derived, the shunt inductor is elimi-nated in the MNG line. This makes the MNG line particularly simplerin terms of the structure while still allowing manipulation of its nonlin-ear dispersion characteristic to achieve dual-band operation. However,the dispersion diagram of the MNG line consists of a right-handedband above the mu-zero frequency  f  M   = 1 / 2 π √  L R C  L  and a rejec-tion band below it, where  µ <  0  and  ǫ >  0  [12]. This indicates thatwhen an MNG line-based LWA is operated in its  n  = 0  space har-monic, it cannot provide the full-space beam scanning property andradiates only in the forward direction above  f  M  .In this study, the above-mentioned limitation of the MNG-line-based LWA is overcome by operating in its higher order spaceharmonics. When MNG unit cells are cascaded to develop an LWA, itbecomes aperiodicstructure.AccordingtotheBloch–Floquettheorem[13], an infinite number of space harmonics is generated automaticallyby periodic modulation, whose phase constant β  n  of the n th harmonicis given by β  n ( ω ) = ±  β  0 ( ω ) + 2 πn p  , n  = 0 , ± 1 , ± 2 ,...  (1)where  β  0  is the phase constant of the  n  = 0  space harmonic, and  p  isthe period of the structure. Because β  n  is simply a repetition of  β  0  witha period of   2 π , there are higher order space harmonics that also radiatefrom the backward to forward direction in different frequency bands.Therefore, by utilizing two higher order space harmonics, n  = − 1  and n  = − 2  space harmonics, a dual-band periodic LWA can be developedwithout increasing the complexity of the unit cell.  B. Synthesis of an MNG Unit Cell The design equations of an MNG unit cell for dual-band applica-tions can be derived from its dispersion relation as follows: L R  =  Z  R   ( φ 21 − φ 22 )( ω 21 − ω 22 )  (2a) C  R  = 1 Z  R   ( φ 21 − φ 22 )( ω 21  − ω 22 )  (2b) C  L  =  C  R ( ω 21  − ω 22 )( φ 21 ω 22 − φ 22 ω 21 )  (2c) θ R  =  ω 1 √  L R C  R  (2d) 0018-926X © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.  IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015 2371 Fig. 1. Schematic of unit cells in (a) CRLH line ( π − model) and (b) MNG line. where  φ 1  and  φ 2  are the desired phase shifts at the predeterminedfrequencies  ω 1  and  ω 2 , respectively.For the MNG line, the frequency of   φ 1  = 0 ◦ corresponds to a mu-zero frequency of the  n  = 0  space harmonic. Because the MNG lineexhibits a stopband below this frequency, only forward scanning ispossible with its  n  = 0  space harmonic. Instead, in this study, phaseshifts of   φ 1  = − 360 ◦ and  φ 2  = − 720 ◦ are chosen at the two targetfrequencies  ω 1  and  ω 2 , respectively. From (1), the difference betweenthe propagation constants of two successive space harmonics is  2 π .Therefore, once a number of unit cells are cascaded periodically, thefrequencies with phase shifts of  − 360 ◦ and − 720 ◦ in the n  = 0  spaceharmonic will have a phase shift of 0 ◦ in the  n  = − 1  and  n  = − 2 space harmonics, respectively. This automatically transforms  ω 1  and ω 2  into the broadside radiation frequencies in the n  = − 1 and n  = − 2 space harmonics, respectively. Consequently, the two bands are setup where the LWA provides the beam-scanning property. This alsoindicates that the MNG line is designed to operate in its passbandregion.Furthermore, the selection of   ω 1  and  ω 2  controls the scanningspeed of the LWA, as it determines the slope of the dispersion curve.By utilizing the nonlinear dispersion characteristic of the MNG line,the ratio between the two broadside radiation frequencies  r  =  ω 2 /ω 1 can be set up to 2, which can be increased further by choosing ahigher  − φ 2 . Therefore, the limitation of the periodic LWA basedon a perfectly right-handed line is overcome, for which the ratio isfixed at 2.Although  r  can be tuned easily, it may degrade the matching con-dition at the two broadside radiation frequencies. This is because theMNG unit cell lacks the second left-handed component, i.e., shunt L L .Thus, mutual cancelation between the series and shunt branches doesnot occur optimally, which in turn sets up open stopbands at the twofrequencies when the unit cells are cascaded. In this study, microstripbends are utilized to achieve maching at the two frequencies, which isdiscussed next. C. Suprression of the Open Stopband  Periodic LWAs are known to suffer from the open stopband phe-nomenon at the broadside radiation frequency, where most of thesignal is reflected back to the source, and the radiation efficiency dropsdramatically. This is due to the two space harmonics with oppositegroup velocities being coupled with each other inside the radiatingregion [14], or equivalently, the constructive interference of the reflec-tions from the periodic discontinuities when the propagation constantbecomes  βp  = 2 nπ , where  n  = 0 , 1 , 2 ,...  [15]. Because of the openstopband effects, even a very small reflection from a single unit cellis magnified by a factor of   N   [16], where  N   is the total number of cascaded unit cells. Therefore, the reflection at the broadside radiationfrequency must be maintained at the lowest possible level.In this study, bends are introduced in the right-handed transmis-sion line sections of the srcinal MNG unit cell. The circuit modelsof the MNG line before and after introducing 12 bends are shown inFig. 2, where  R p  and  L p  are the parasitic resistance and inductance of the capacitor, and  θ R  =  i  2 θ i . The parasitic reactances  L i  and  C  i associated with the bends can be used as a self-matching network thatcontrols the reflection characteristic of a unit cell at both broadsideradiation frequencies, i.e., to suppress the open stopband effects at thetwo frequencies.To provide a qualitative idea, the circuit simulation results of thetwo MNG lines in Fig. 2 are compared in Fig. 3 for various num-bers of cascaded unit cells. Each unit cell consists of a  C  L  = 0 . 6 pF and  Z  R  = 50 Ω  section with  θ R  = 397 . 44 ◦ , which are obtained from2(a)–2(d) with  φ 1  = − 360 ◦ and  φ 2  = − 720 ◦ at  f  1  = 4 . 3 GHz  and f  2  = 8 GHz , respectively.The results in Fig. 3(a) reveal that the LWA based on the straightMNG line may suffer substantially from the open stopband phe-nomenon at the two broadside radiation frequencies because a con-siderable portion of the input power is reflected back to the source.Furthermore, the antenna suffers from even higher reflections as thenumber of cascaded unit cells is increased, indicating that the antennawill exhibit little, if any, radiation at the two frequencies. On the con-trary, Fig. 3(b)–(d) indicates that with the bends, the open stopbandeffects can be controlled and suppressed to virtually negligible lev-els, so that constant leaky radiation is achieved across either or bothfrequencies.  D. Avoiding Interference Besides the suppression of open stopband effects, another criti-cal role of the bends is avoiding the interference between two ormore space harmonics. This is especially important for the pro-posed antenna, which relies on the operation of higher order spaceharmonics.An example is shown in Fig. 4, which is the dispersion diagram of the proposed dual-band LWA. The proposed antenna is designed bycascading 16 unit cells (Fig. 5) that are optimized through full-wavesimulations in HFSS to provide broadside radiation at the prede-termined frequencies of 4.3 and 8 GHz. The dispersion diagram isobtained by applying a Bloch-wave analysis [17] and the Bloch–Floquet theorem [13] to the full-wave simulated  S  -parameters inFig. 6. The unit cell may be somewhat long, especially at  ω 2 . Thisis because the unit cell is designed to provide a relatively large phaseshift to allow the utilization of its higher space harmonics. In fact, longunitcellsaresomewhatcommonindual-bandLWAs[6],[7].However,a leaky-wave operation can still be achieved as long as the dispersioncharacteristics are maintained even with relatively long unit cells.The physical span  p  of the unit cell in Fig. 5 can be tuned by thebend. When a number of unit cells are cascaded to form a periodicline, the physical span  p  becomes the period  p  in (1). Therefore, thebends can be used to adjust the air line  k 0  p  and control the radiatingregion where | βp | < k 0  p .The first band, where the  n  = − 1  space harmonic radiates, is from3.6 to 5.43 GHz ( C − E ), and the second band, where the  n  = − 2 space harmonic radiates, is from 6.75 to 10 GHz ( F − H ). Betweenthe two bands is a guided-wave region where | βp | > k 0  p  and no radi-ation occurs. By increasing  p , the slope of   k 0  p  can be reduced. Thiswidens the radiating region, which in turn reduces the guided-waveregion. With a further increase in  p , the two adjacent space harmonicsmay overlap. For instance, from 9.9 to 10 GHz ( I − H ) in Fig. 4, boththe  n  = − 2  and  n  = − 3  space harmonics radiate in different direc-tions, causing interference. Therefore, spurious grating lobes may beobserved. The  n  = 0  space harmonic also contributes to radiation ina very narrow band near the mu-zero frequency from 1.16 to 1.2 GHz( A − B ). However, the radiation in this band is negligible owing to thehigh reflection, as shown in Fig. 6.  2372 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015 Fig. 2. Circuit models of cascaded unit cells (a) without bends and (b) with 12 bends.Fig. 3. Circuit simulated results of cascaded MNG unit cells in Fig. 2 forvarious numbers of cells: (a) Original straight unit cell without bends. Bendsoptimized to suppress open stopband (b) effectively at 4.3 GHz only; (c) effec-tively at 4.3 GHz and moderately at 8 GHz; and (d) effectively at both 4.3 and8 GHz. Smooth transitions at both broadside radiation frequencies clearlyindicate that the open stopband is suppressed successfully by thebends, and a dual-band leaky-wave operation with continuous scan-ning properties is guaranteed. Finally, the bends can also control thedirection of the current on the antenna. Therefore, they need to becarefully designed to minimize the cancelation of radiation in thefar field.III. E XPERIMENTAL  R ESULTS For experimental verification, the proposed LWA was fabricatedon a substrate with a relative permittivity of 3.5, a loss tangentof 0.018, and a thickness of 0.76 mm. For the lumped capacitors,0603-sized SMD-type capacitors from Murata were used for the Fig. 4. Dispersion diagram of the proposed LWA.Fig.5. MNGunitcellwithbendsonanRF-35substratefromTaconic.Thefinaldimensions are  p  = 16 . 2 ,  w  = 37 . 4 ,  a  = 3 . 8 ,  b  = 1 . 98 ,  c  = 2 . 9 ,  d  = 4 . 94 , e  = 2 . 12 ,  f   = 0 . 6 ,  g  = 2 . 3 ,  h  = 1 . 59 ,  i  = 2 . 9 ,  j  = 3 . 56 , and  k  = 1 . 71 . Alldimensions are in millimeters.Fig. 6. Measured and full-wave simulated  S  -parameters of the proposed LWA. optimized capacitance of 1.1 pF for the effective suppression of openstopband effects at both broadside radiation frequencies. A photographof the fabricated antenna is shown in Fig. 7. The measurement was  IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015 2373 Fig. 7. Photograph of the fabricated LWA.Fig.8. Normalizedradiationpatternsinthe yz -plane.(a)Firstbandwithbroad-side radiation at 4.3 GHz (3.7–5.5 GHz with 0.2 GHz step). (b) Second bandwith broadside radiation at 8.1 GHz (6.7–10 GHz with 0.3 GHz step). performed from 0.5 to 10 GHz using a 37247 D vector network ana-lyzer from Anritsu. The output port of the antenna terminated witha  50 - Ω  load. Electronic calibration was achieved with a 3653-type Ncalibration kit from Anritsu.The measured and simulated  S  -parameters of the proposed LWAare shown in Fig. 6, which are in good agreement. The shaded rangesdenote the leaky-wave regions that correspond to the first and secondbands in the dispersion diagram in Fig. 4. Excellent matching perfor-mance that maintains the return loss above 10 dB in both leaky-wavebands is achieved, validating the proposed technique of suppressingthe open stopband effects. Furthermore, the decrease in transmissionacross both bands is a clear indication of leaky-wave radiation. Thesudden change in the  S  -parameters around 6.2 GHz is due to thecoupling of two space harmonics in the nonradiating region when βp  = (2 n − 1) π , where  n  = 1 , 2 , 3 ,...  [15]. This is clearly evidentin Fig. 3. However, the radiation performance of the antenna is notaffected because the MNG line is in the guided mode around 6.2 GHz.By considering all possible losses and the parasitic components of the Fig. 9. Measured and simulated directions of the main beam.Fig. 10. Measured and simulated peak gains. lumped capacitor that are obtained experimentally, radiation efficien-cies of up to 46% in the first band and up to 81.5% in the second bandareexpected inthefull-wavesimulations.Thiscanbeincreasedfurtherby increasing the number of unit cells cascaded.The far-field patterns are measured in an anechoic chamber, whichare compared with the simulated patterns in Fig. 8. The measuredbroadside radiation frequencies are 4.3 and 8.1 GHz for the firstand second bands, respectively, where the side lobe levels remainbelow  − 10 dB. As can be seen in Fig. 8, continuous frequency-dependent beam scanning from the backward to forward directionis obtained in each band, validating the engineered dispersion inFig. 4 that utilizes two higher order space harmonics. As the fre-quency increases, the beamwidth of the antenna narrows owing tothe increase in the effective aperture of the antenna, which is atits peak at the broadside radiation frequencies. As the frequency isincreased further, the beam angle approaches the endfire direction,and the beamwidth widens back. Similarly, the aperture efficiencyof the antenna is maximized at the two frequencies of broadsideradiation, but it decreases as the beam angle approaches the endfiredirection.Comparison shows that there is an excellent agreement between thesimulated and measured radiation patterns in majority of both bands.The relatively weak agreement around the band edges of both bandsis due to the effects of the transition between guided and leaky-wavemodes. The discrepancy at the higher band edge of the upper band isalso due to the interference between the  n  = − 2  and  n  = − 3  spaceharmonics. Fig. 9 compares the simulated and measured directions of the main beam in the first and second bands, which are in excellentagreement. In the first band from 3.7 to 5.5 GHz, the antenna changesthe scan angle from − 43 ◦ to 73 ◦ , whereas it scans from − 80 ◦ to 83 ◦ in the second band from 6.7 to 10 GHz.Finally, the measured and simulated peak gains within the leaky-wave regions are compared in Fig. 10, which are in good agreement.  2374 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 63, NO. 5, MAY 2015 TABLE IC OMPARISON OF  P LANAR  D UAL -B AND  LWA S ∗ Simulated results only. The measured gains at the two broadside radiation frequencies are6.7 and 10.3 dBi. The difference in the two gains is due to the dif-ference in the electrical lengths of the antenna in the two bands.The measured results show that 26% of the input power is transmit-ted through the lower band, whereas only 3% is transmitted throughthe upper band. Therefore, the gain, radiation efficiency, and aper-ture efficiency are lower in the lower band than in the upper band.In fact, this is typical in dual-band LWAs [6]–[8]. By cascading moreunit cells, the gain can be increased especially in the lower band.In correspondence with the aperture efficiency and effective apertureexplained previously, the antenna gain increases slowly as the fre-quency increases; however, the gain decreases as it scans toward theendfire direction. The decrease in gain is much faster as it approachesthe higher edge of the first band compared to that of the second band.This is because a guided-wave region is above the first band, whereasthe leaky-wave region of the  n  = − 3  space harmonic is above thesecond band.Table I compares the proposed LWA with previous planar dual-bandLWAs. While the demonstrated scan angle range of 163 ◦ in the secondband almost reaches the theoretical limit of 180 ◦ [18], the total scanangle range of 279 ◦ in the two bands is the widest reported for thedual-band LWA. The maximum gains are 9.5 dBi in the lower bandand 15.1 dBi in the upper band. The rate at which gain changes as thebeam is scanned is 0.20 and 0.13 dB/  ◦ in the lower and upper band,respectively. The two rates are much closer to each other than the rates0.15 and 0.38 dB/  ◦ of the previous dual-band LWA in [7]. Other dual-band LWAs [6], [8] cannot be compared because of the insufficientmeasured data. The gains in both bands can be increased further bycascading more sections.IV. C ONCLUSION A planar periodic LWA based on an MNG transmission line, whichoffers full-space scanning properties in two different bands, was pro-posed in this study. In contrast to previous dual-band LWAs, thedual-band characteristic is obtained by utilizing both the  n  = − 1  and n  = − 2  higher order space harmonics. Therefore, the dispersion char-acteristics of the antenna can be tuned easily with a considerablysimple structure that does not require inductors or shunt connections.The experimental results of a dual-band LWA designed with the firstand second broadside radiation frequencies at 4.3 and 8 GHz, respec-tively, are in good agreement with the simulated results, thus validatingthe proposed method. Most importantly, the antenna offers very widescanning angle ranges in both bands, providing a total scan range of 279 ◦ , which is the widest range reported for the dual-band LWA.R EFERENCES [1] S. T. 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