Comparative study of defected ground structures harmonics rejection ability in a compact hybrid coupler

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Comparative study of defected ground structures harmonics rejection ability in a compact hybrid coupler

  1. 1. INTERNATIONAL JOURNAL OF ELECTRONICS AND International Journal of Electronics and Communication Engineering & Technology COMMUNICATION ENGINEERING 6472(Online) Volume 3, (IJECET) (IJECET), ISSN 0976 – 6464(Print), ISSN 0976 –& TECHNOLOGYIssue 2, July- September (2012), © IAEMEISSN 0976 – 6464(Print)ISSN 0976 – 6472(Online)Volume 3, Issue 2, July- September (2012), pp. 94-106© IAEME: www.iaeme.com/ijecet.html IJECETJournal Impact Factor (2012): 3.5930 (Calculated by GISI)www.jifactor.com ©IAEME COMPARATIVE STUDY OF DEFECTED GROUND STRUCTURES HARMONICS REJECTION ABILITY IN A COMPACT HYBRID COUPLER K. Annaram Professor, Department of Electronics and Communication Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, 626001, Tamilnadu, India dr.k.annaram@gmail.com ABSTRACT In this paper defected ground structures (DGSs) featuring compact size and spurious free passband in the context of 180° hybrid ring couplers are investigated. The design method for miniaturizing the conventional 180° hybrid coupler is derived based on the fractal theory by replacing the circumference of the hybrid ring coupler by Koch fractal curves. The advantage of this miniaturization technique is no needs of any lumped elements, via hole or wire bond, but only microstrip line. According to the fractal theory, almost the same frequency characteristics as those of a conventional hybrid ring coupler are obtained. The experimental circuits were reduced to 32.85% in area with the good frequency characteristics. By etching various DGSs on the back-side of compact hybrid coupler, the harmonics are rejected due to its slow-wave characteristics. Furthermore, to demonstrate its practical viability, compact hybrid coupler with DGSs are designed, simulated, fabricated and measured. This paper also compares the harmonic rejection ability of various shapes of DGSs when these structures are etched in a compact hybrid coupler. Furthermore, to show the improved harmonic rejection ability of the proposed hybrid couplers which are using various shapes of DGSs are compared with the conventional hybrid ring coupler which one is not having DGS by designing all the designs for the same design specifications. Finally the measured and simulated results are compared for all the designs. The comparative study of these results indicates that the harmonic rejection levels of the spurious resonance for the proposed couplers are better than the conventional hybrid coupler at 3fo. It is also observed from the comparative study of measured results the compact hybrid coupler with triangular DGS have better harmonic rejection ability than other designs because it has minimum defected area than other DGSs. In addition, the size of the RF-front-end becomes smaller by using the proposed couplers instead of using filters to reject the unwanted harmonics. Hence this technique can be well suited to design any compact microwave systems with low cost. Keywords: Band-Gap Structure, Defected ground structure, Electromagnetic Interference, Fractals, Frequency Selective Surfaces, Harmonic rejection, Hybrid coupler, Microstrip. 94
  2. 2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME 1. INTRODUCTIONRecently, there has been increasing interest in the use of Monolithic microwave wave integratedcircuits (MMICs) in microwave and millimeter-wave communication systems. In these systems,cost effectiveness, low power consumption, and mass productions are much desired. Microwavemixer is one of the important component in the above systems, which includes radio frequency(RF), intermediate frequency (IF) and local oscillator (LO) stages. It is mainly used for the base-band signal up-conversion and down-conversion of the received signal. In up-convertingmicrowave and millimeter wave communication systems, the most important parameter is LO-to-RF isolation. Since the LO frequency is too close to even overlapping the RF frequency, therejection of the LO signal by a low pass filter is difficult or sometimes even impossible. Similarlyin down-converting microwave and millimeter wave communication systems, the super heterodynereceiver is one of the important component which includes RF, IF (base-band) and LO stages. It hasthe advantages of good stability and sensitivity, high gain and low noise. But it has thedisadvantages of high power consumption, complex circuitry and image frequency problems. Inthis receiver band-reject filters are mostly used to reject the unwanted image frequencies. In boththe up-down converting mixers the radiation of harmonics generated by the nonlinear circuit hasbeen identified as a major problem for microwave and millimeter wave communication systems.These harmonics could radiate freely into the surrounding environment at significant power levelsto interfere with continuous or harm the immediate inhabitants. Thus high performance low-passand band-reject filters are necessary to reject the unwanted harmonics. Finally the size of themicrowave and millimeter wave communication systems front-end becomes complex and bigger.Such demand has a significant impact on a number of design issues both at component and systemlevels, namely size-reduction, broadband operation, low power consumption, low noise, andinterference problems. Hybrid ring coupler is one of the fundamental component in both up-down converting mixerswhich are mainly used to mix the RF/LO and IF signals. The most commonly used hybrid couplersare branch-line (90°) and rat-race (180°) hybrid ring couplers. Among the above two a 180° rat-racehybrid ring coupler has wider bandwidth and high isolation than the 90° branch-line coupler [1] -[3].To reduce the size of the microwave and millimeter wave system front-end, the 180° hybrid ringcoupler is an ideal component. It consists of a ring that is 1.5λg in circumference at the centerfrequency with four input/output ports. In order to achieve good port-to-port isolation, the 180°hybrid couplers are widely used in balanced mixers which are mostly used in microwave andmillimeter wave systems. Also it can be used in balanced amplifiers, beam forming array antennasand frequency multipliers, due to their simplicity, wide bandwidth in power dividing distribution,and a high isolation between its two output ports. Numerous publications have addressed the topicof how to reduce the size of the hybrid-ring coupler. Several design techniques have been proposedto enhance the harmonic rejection ability and to reduce its size. In earlier days the hybrid ringcoupler is often implemented only by lumped capacitors and inductors. However, the appropriatecapacitor and inductor in the microwave frequency band are difficult to fabricate [4]. Hirota hasproposed a method for reducing the hybrid ring microstrip coupler length by adding lumpedelements for compensation. The advantage of this method is that the reduced size is arbitrary, butthe main limitation is that the capacitance or inductance which is calculated by this method isusually not equal to the primary value available in the market. Moreover, the extra lumped elementswill increase the cost and complexity of manufacturing. The next category of the miniaturizingtechnique is realized only by planar transmission lines such that the drawback of the methoddescribed above can be avoided. Kim presented a strategy without any lumped elements, but thetotal circumference is fixed at 1.25 λg or 7λg/6 and cannot be reduced further. Some approaches usea phase-inverter structure to replace the half wavelength microstrip between ports 2 and 4, which isthe longest transmission line on a typical ring coupler [5] - [7]. Chang reduced the size of the rat-race coupler by only using microstrip lines. It is achieved by connecting multiple open stubs on theinside of the hybrid ring coupler. The main advantage of this method is that no lumped element orextra transition circuit is needed. Thus, the minimum size is restricted. The meander line technique 95
  3. 3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEMEhas been used for the coupled line structure size reduction. Moreover, due to technology limitations,the size reduction is limited by coupling effects between long parallel sections. To reshape thecircular rat-race coupler the space filling curves are used to reduce the occupied surface area whilekeeping the performance unchanged [8]-[11]. A new rectangular geometry for the conventionalhybrid ring coupler, based on computer aided design (CAD) tool is proposed. This geometry is notonly easy to automatize in a CAD tool, it gives a better concordance between best-match and best-isolation frequencies than the standard design. Generally low pass filter is placed for harmonicrejection in microwave front-end systems. Otherwise; the coupler suffers from the presence ofspurious pass-band at the harmonics of the operating frequency. But this solution increases thecomplexity of the circuit, and the insertion loss [12] - [14]. It is very difficult to integrate theconventional rat-race 180° hybrid ring coupler with other adjoining devices such as filters andamplifiers; etc which has the perfectly matching geometry because of its multiple non-collinearlyaligned ports. Therefore it becomes necessary to use a vertically oriented feeding point. Thisinitiates the use of rectangular shape 180° hybrid ring coupler geometry which overcomes thedivergence problem caused by the circular or radial geometry in view of the port alignment eventhough the size was not miniaturized [15]. Therefore, attempts are continually being made to realize a rectangular shape 180° hybrid ringcoupler with vertically oriented feeding point with minimum in size, and rejection of oddharmonics in microwave and millimeter-wave communication systems. Recently in order toenhance the harmonic rejection ability split ring resonators (SRR), complementary split ringresonators (CSRR), photonic band-gap (PBG), defected ground structure (DGS) techniques areused in monolithic microwave integrated circuits (MMICs) which are mainly used in microwaveand millimeter wave communication systems [16]. Hence this paper experimentally compares the harmonic rejection ability of the various DGSs byetching these structures in a compact 180° hybrid ring coupler. The design concept, compactnessand harmonic response of the proposed designs are illustrated in section 2 and 3. The compact 180°hybrid ring coupler with harmonic rejection is then discussed and verified by measurement resultsfrom a fabricated circuit in section 4 and 5. Finally the concluding remark is given in section 6. 2. MINIATURIZATIONFractals: In recent years the beauty of fractals has attracted wide interest among mathematiciansand microwave researchers. A number of techniques for generating fractal shapes were developedand used to produce miniaturized circuits. The most commonly used fractal shapes are Koch curve,Sierpinski gasket, Cantor dirt and Hilbert curves. These fractal shapes are self similar in structure, aportion of the fractal geometry always has the same shape as that of entire structure. Themicrowave circuit size can be reduced by increasing the perimeter of the shape as the iterationincreases, while still being confined in the same area due its space filling property. By using theseproperties the most commonly used techniques are construction and iteration function which arepopularized by Mandelbrot, for generating fractal shapes. This paper focused only Koch fractalswhich are very useful for designing MMICs because these are simpler than other fractals because ofits structure. According to Mandelbrot’s generalization, the Koch construction consists of recursivelyreplacing edges of an arbitrary polygon (called the initiator) by an open polygon (the generator),reduced and displaced so as to have the same end points as those of the interval being replaced. Forexample in figure.1, ao is length of the initiator and Ko is the generator transformation. The ad-hociterative function system (IFS) algorithm is used to construct the Koch curves as follows: a12 b= (1) ao 96
  4. 4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME The initiator length ao corresponds to the λg/4 length. In order to construct the generator, narbitrary transformations Ko, K1, K2…..Kn are applied successfully as follows K n+1 = ao ( K n ) = Ua p K ( n ) (2) where p=1, 2, 3…..n; K n+1 = an1 ( Kn ) U an 2 ( Kn ) .... U anp ( Kn ) (3) After applying the above transformations the Koch curves generators physical parametersshould satisfy the equation a11 = a12 = a13 which are shown in Figure.1 because fractals are selfsimilar. In this paper, Koch fractal with iteration factor (b) 0.25 is chosen to design the proposedminiaturized hybrid coupler. The geometrical properties of Koch fractals for various iterations areshown in Figure.2 [17] [18]. Figure.1. Initiator and Generator Figure.2 Koch fractal for various iterations 3. HARMONIC REJECTIONDGS: Many techniques were reported in order to suppress the unwanted harmonics such as PBG,SRR, CSRR and DGSs in microwave and millimeter wave communication systems, which hasperiodic array of defects. These periodic and non-periodic defects enhance the harmonic rejectionability of a microwave and millimeter wave circuits. This harmonic rejection enhancement is veymuch needed in many microwave and millimeter wave circuits such as power amplifier, planarantennas, power divider, hybrid couplers and filters. The research on PBG and EBG structures wereoriginally used for enhancing the harmonic rejection ability in the stop-band. However, thesestructures are very difficult to model because of its too many design parameters also causesradiation from the periodic defects in microwave and millimeter wave communication systems.Hence DGSs have gained significant interest in microwave and millimeter wave circuits forharmonic rejection.Various shapes of DGSs are shown in Figure.3 which is very useful for designing proposed coupler.The DGSs should be etched in the proposed compact hybrid coupler ground plane to reject theharmonics. An etched defect in the ground plane disturbs the shield current distribution in theground plane. This disturbance can change the characteristics of a transmission line such as linecapacitance and inductance. It can be seen that employing the proposed DGSs increases the seriesinductance to the microstrip line. This effective series inductance introduces the cut-offcharacteristic at a certain frequency. In addition, the DGS has a self resonant frequency at the stop-band. Due to this self resonant characteristic of the DGS section, the proposed coupler can provide 97
  5. 5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEMEan attenuation pole in the upper stop-band. Due to this attenuation pole, the stop-band is wider thanthat of conventional coupler. These attenuation poles can be explained by parallel capacitance withthe series inductance. This capacitance depends on the etched gap below the conductor line [19].The physical parameter extraction method for various DGSs and its equivalent are described by LCcircuit as shown in Figure. 4. a) Square-head DGS (b) Dumb-bell DGSc) Arrow-head DGS (d) Fractal DGS Figure 3. Schematic of DGSs Figure.4. Equivalent Circuit of DGSThe capacitance Cp and inductance Lp are computed by 5 fc Cp = F (4) π  f o2 − f c2    250 Lp = H (5) C p (π f o2 )where f c is the cut-off frequency of the band reject and f o is its pole frequency. At any frequencyf < f o , the parallel circuit behaves as an inductor and its value is Lp Leq = H (6)   f 2  1 −      fo    It is observed that Leq is frequency dependent, which is inconvenient for the design of anymicrowave circuits. However, variation in Leq is not very rapid for the frequency below fc. Theparameter extraction method is used to find the equivalent physical parameter of the DGSs. The 98
  6. 6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEMEshunt capacitance and series inductance will be implemented by employing the open stub and shortstub respectively. The length reduction of long DGS slot is achieved by creating the slot heads atboth ends DGSs. The following parameters are used to characterize the band-stop performance ofthe DGSs (1) Linear dimension of a slot (2) Area/radius of a slot head (3) Relative control of cut-off frequency fc and attenuation pole frequency fo by changing the dimension of a slot (4) Sharpness factor fc/foThe slot-head area basically controls the inductance Lp whereas; the width (s) of connecting DGSslot controls the capacitance. The separating distance (d) between the slot heads has influence onboth inductance and capacitance. Various dimensions involved in formation of various DGSs headshave a different degree of control on fc and fo [20] - [22].To design a circuit with any one of DGSs, the physical dimension of the DGS unit is obtained fromLp and Cp values by full wave simulation and optimization is performed by using Agilent ADS. Inthis paper, DGSs which are used for rejection of harmonics in a compact 180° hybrid coupler,because• The structures are simple to design and fabricate• The stopband is very wider and deeper The researchers have commented that for the equal area of slot head, any shape of slot can beused. However, an equal area only ensures equal equivalent inductance and not the identicalresponse of the DGS circuit elements. The shape, size, and orientation of a slot can have aninfluence on performance of the coupler and other neighboring circuits. In this paper, weinvestigated the performance of a compact hybrid coupler by etching various shapes of DGSs onthe ground plane of a compact hybrid coupler.4. DESIGN AND FABRICATIONBy considering the above advantages of DGSs, these DGSs are used to construct a compact hybridcoupler. In order to validate our idea, a rat-race hybrid coupler with various shapes of DGSs aredesigned, fabricated and measured. The layout of the proposed hybrid coupler with DGSs which aregenerated by using Agilent advanced design system software is illustrated in Figure.5. The centerfrequency of the stopband (3fo) for the DGS is determined by the resonance frequency of each DGSunits. The proposed compact coupler with DGSs is designed at the operating frequency 2.44GHz(fo) and the center frequency of stopband (3fo) at 7.32GHz. Both conventional and compact hybridcouplers were designed and implemented on an inexpensive FR4 substrate (εr = 4.3, h = 1.6mm). Aprototype of a conventional and proposed hybrid couplers with DGSs were fabricated which areshown in Figure.6. The optimized physical dimensions of the hybrid couplers and DGSs aresummarized in Table.1 and Table.2 to 5. 99
  7. 7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME a) With square-head DGSs (b) With dumb-bell DGS (c) With arrow-head DGSs (b) With fractal DGSs Figure .5. Layout of the compact hybrid couplers with DGSs Table 1. Dimensions of the hybrid couplers Width (mm) Length (mm) Area mm2 Type of coupler Feed Ring Feed Ring Conventional hybrid coupler 2.909 1.497 16.899 17.417 68.35×59.19 Proposed hybrid couplers 2.909 1.497 7.5 9.826 30 ×44.293 Table 2. Dimensions of square-head DGSs Parameter Dimensions of DGS (mm) Area of the square-head DGS (a) a = 1.5 Spacing length between square-head (d) d = 4.3 Spacing width between square-head (s) s = 0.5 Table 3. Dimensions of dumb-bell DGS Parameter Dimensions of DGS (mm) Radius of dumb-bell DGS (r) r = 1.5 Spacing length between dumb-bell heads (d) d = 4.3 Spacing width between dumb-bell heads (s) s = 0.5 Table 4. Dimensions of an arrow-head DGS 100
  8. 8. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July September (2012), © IAEME July-September Parameter Dimensions of DGS (mm) Area of an arrow-head DGS (b) b=3 Spacing length between an arrow arrow-heads (d) d = 4.3 Spacing width between an arrow- -heads (s) s = 0.5 Table 5. Dimensions of fractal DGSs Parameter Dimensions of DGS (mm) Area of the fractal DGS (a) a = 3.1 Spacing length between fractal ( (d) d = 4.3 Spacing width between fractal edges (a3) a3 = 0.5 (a) Front-side view b) Back-side view with (c) Back-side view with square- head DGSs dumb umb-bell DGSs 101
  9. 9. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEMEd) Back-side view with (c) Back-side view with arrow- head DGSs fractal DGSs Figure 6. Fabricated prototypes of compact hybrid couplers with DGSs5. RESULTS AND DISCUSSIONTo compare the harmonic rejection ability of the DGSs, compact hybrid coupler with DGSs and theconventional hybrid ring coupler were designed and fabricated which one is not having DGSs withthe same design specifications. Finally the measured and simulated results are compared for boththe structures. Before the prototypes fabrication, full wave EM simulation results have beenobtained with the aid of Agilent advanced design system software. The S-parameter measurementswere performed by using Agilent N5230 PNA series vector network analyzer.Figure 7 presents the measured and simulated S11 parameters for a compact hybrid coupler with, aswell as without DGSs. A passband seen in the operating resonance frequency 2.44GHz andstopband is centred around 7.32GHz. It is observed that the harmonic rejection in the stopbandextends up to 10GHz for the proposed couplers with DGSs over the conventional 180° hybrid ringcoupler. Also it can be clearly seen in Figure.7, the S11 happens to be much better for arrow-headDGSs. For fo and 3fo, all the designs return loss is below -20dB and above -1.5dB respectively. Itcan be noticed that the optimal harmonic rejection happens to be much better than which one is nothaving DGSs. It is observed that the compact hybrid coupler with triangular DGS has slightly betterharmonic rejection ability than other designs. The experimental S13 comparison response is shownin Figure.8. It was found to be isolation is better than -18dB for the whole band for all the designswhich are having DGS. Figure 7. Return loss (S11) 102
  10. 10. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME Figure 8. Isolation loss (S13) Figure 9. Coupling loss (S12) Figure 10. Coupling loss (S14)Figure 9 and 10 shows the coupling responses of S12 and S14 respectively. It is clearly observed that,the proposed couplers have the coupling ratio of -3.91dB at 2.44GHz and below -10 dB up to10GHz for all the designs except conventional coupler. It is clear from the results that the thirdharmonics of the proposed coupler were perfectly suppressed. Therefore the harmonic rejectionability of the proposed coupler have improved lot and better than the conventional hybrid coupler,while maintaining 3dB power dividing performance. Figure 11 and 12 shows the simulated andmeasured phase differences between its output ports of the compact hybrid couplers with DGSs andconventional hybrid coupler. It is worth noting that the good out-of-phase (180º ± 6º) characteristicsbetween the output ports are obtained at the desired frequency 2.44GHz (in the passband), whilerejecting harmonic elements effectively only for the designs which are having DGSs. Finally it isobserved that the proposed compact hybrid coupler with triangular DGS has better harmonicrejection ability compared to other designs because it has only minimum defected area. 103
  11. 11. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME Figure 11. Phase response (S12) Figure 12. Phase response (S14)6. CONCLUSION It can be concluded that a proposed couplers can be implemented in a few mm2 area withoutany lumped element and will be easy to integrate with other devices. Usually low pass filter andband reject filters were used in the microwave and millimeter wave front-end to reject the unwantedharmonics. However, this approach increases the overall RF-front-end size and insertion loss. Toovercome the above limitations the proposed coupler can be used because it has the ability to mixthe RF and LO signal as well as rejects the unwanted harmonics. Hence there will be no need ofseparate filters for harmonic rejection. Thus the size of the microwave and millimeter wave-front-end size becomes smaller by using the proposed couplers. So it is well suited for designing anycompact and low cost MMICs circuits which can be used in microwave communication systems. Itis also observed that variation in the measured performance is mainly due to imprecise fabrication,simulation mesh density and also by the junction discontinuities. It is believed that better quality(harmonic rejection) results can be obtained by optimizing the DGS section and by using otherfractal curves. This task is left for further investigations, which can be used for further sizereduction and harmonic rejection. 104
  12. 12. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEMEACKNOWLEDGEMENTThe author wish to acknowledge the fabrication and testing support of the TIFAC-CORE inWireless Technologies, Thiagarajar Advanced Research Center, RF Systems Lab, ThiagarajarCollege of Engineering, Madurai, Tamilnadu, India.REFERENCES[1] T.Wang, and Ke Wu, “Size-reduction and band-broadening design technique of uniplanar hybrid ring coupler using phase inverter for M(H)MIC’s,” IEEE Trans. Microwave Theory Techniques, vol.47, no.2, pp. 198-206, February 1999.[2] Y.J.Sung, C.S.Ahn, and Y.S.Kim, “Size reduction and harmonic suppression of rat-race hybrid coupler using defected ground Structure,” IEEE Microwave Wireless Components letters, vol.14, no.1, pp. 7-9, January 2004.[3] S. Dwari, and S.Sanyal, “Size reduction and harmonic suppression of microstrip branch-line coupler using defected ground structure,” Microwave and optical letters, vol. 48, no.10, pp. 1966-1969, October 2006.[4] M.L. Chuang, “Size reduced ring coupler using multiple open stubs with restricted minimum line width,” Microwave and optical letters, vol.48, no.1, pp. 172-174, January 2006.[5] T.Hirota, A.Minakawa, and M.Muraguchi, “Reduced-size branch-line rat-race hybrids for uniplanar MMICs,” IEEE Trans. Microwave Theory Techniques, vol.38, no.3, pp. 270-275, March1990.[6] D.I.Kim, and G.S.Yang, “Design of new hybrid-ring directional coupler using λ/6 or λ/8 section,” IEEE Trans. Microwave Theory Techniques, vol.39, no.10, pp. 1779-1784, October 1991.[7] T.Wang, and K.Wu, “Size reduction and band-broadening design technique of uniplanar hybrid ring coupler using phase-inverter for M (H) MICs,” IEEE Trans. Microwave Theory Techniques, vol.47, no.2, pp. 198-206, February 1999.[8] M.L.Chuang, “Miniaturized ring coupler of arbitrary reduced size,” IEEE Microwave Wireless Components letters, vol.15, no.1, pp. 16-18, January 2005.[9] H.Tanaka, N.Banba, S.Ari, and T.Nishikawa, “2GHz one octave-band 90º hybrid coupler using meander line optimized by 3D FEM” in IEEE Microwave Theory Techniques Symp. Digest, pp. 903-906, 1994.[10] S.M.Wang, C.H.Chen, and C.Y.Chang, “A study of meander microstrip coupler with high directivity” IEEE Microwave Theory Techniques Symp. Digest, pp. 63-66, 2003.[11] K. W. Eccleston, and Sebastian H.M. Ong, “Miniaturized planar microstrip line branch-line and rat-race coupler,” IEEE Trans. Microwave Theory Techniques, vol.51, no.10, pp. 2119-2125, October 2003.[12] Joanna Qiszewska, S.Vaccaro, J.F. Zurcher, and A.K. Skrivervik, “A new hybrid ring geometry well suited for CAD implementation,” Microwave optical technology letters, vol.40, no.4, pp.285-287, April 2004.[13] K.M.Shum, Q.Xue, and C.H.Chan, “A novel microstrip ring hybrid incorporating a PBG cell,” IEEE Microwave Wireless Component letters, vol.11, no .6, pp. 258-260, June 2001.[14] Y.J.Sung, C.S.Ahn, and Y.S.Kim, “Size reduction and Harmonic suppression of Rat- Race Hybrid Coupler using Defected Ground Structure,” IEEE Microwave and Wireless Components Letters, vol.14, no.1, pp. 7-9, January 2004. 105
  13. 13. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 2, July-September (2012), © IAEME[15] Su-Yeol Lee, Y.Chung, T.Itoh, and Dal Ahn, “Design of 90º hybrid coupler with harmonic rejection characteristics,” in IEEE Microwave Theory Techniques Symp. Digest, pp. 335-338, 2004.[16] J. D. Beana, J. Bonache, F.Martin, R. M. Sillero, “Equivalent circuit models for split ring resonators, and complementary split Ring Resonators coupled to planar transmission lines,” IEEE Trans. Microwave Theory Techniques, vol.53, no.4, pp.1451- 1461, April 2005.[17] H. Ghali, T. A.Moselhy, “Miniaturized fractal rat-race, branch-line and coupled-line hybrids,” IEEE Trans. Microwave Theory Techniques, vol.52, no.11, pp. 2513-2519, November 2004.[18] H. Ghali, T.A.Moselhy, “Design of fractal rat-race coupler” in IEEE Microwave Theory Techniques Symp. Digest, pp. 323-326, 2004.[19] K.Kim, N.Kingsley, M.A.Morton, S. Pinel, J.Papaolymeru, Manos M.Tentzeris, J. Laskar, and J.G. Yook, “Koch fractal shape Microstrip band pass filters on high resistivity silicon for the suppression of the 2nd harmonic,” Journal of the Korea Electromagnetic Engineering Society, vol .6, no.4, pp. 1-6, April 2006.[20] Ze-Hai Wu, “Characteristic Investigation of Koch Island Fractal patch”, IEEE APMC 2005 proceedings.[21] J.L.Park, C.S.Kim, J.S.Park, Y.Qian, D.Ahn, and T.Itoh, “Modeling of a photonic band gap and its application for the low-pass filter design,” IEEE Proceedings of APMC’99, pp.331-334, 1999.[22] Adel B. Abdel-Rahman, A.K.Verma, A. Boutejdar, and A.S.Omar, “Control of band stop response of Hi-Lo microstrip low pass filter using slot in ground plane,” IEEE Trans. Microwave Theory Techniques, vol.52, no.3, pp. 1008-1013,March 2004. K. Annaram was born in India in the year 1977. She completed B.E and M.E degree in Electronics and communication engineering and Communication systems from Thiagarajar College of Engineering, Madurai Kamaraj University, India in 1999 and 2001 respectively. She received her Ph.D degree from Anna University Chennai in 2010. She is now working as a Professor in Kamaraj College of Engineering and Technology, Departmentof electronics and communication engineering. She is a life member in ISTE, ATMS andIEICE. Prior to coming to Kamaraj College of Engineering and Technology, she was aresearch associate for the Thiagarajar advanced research center, Madurai and RF designengineer for Quasar Innovations limited, Bangalore. Her research interest includes Metamaterials, defected ground structures and miniaturized RF and microwave circuits andsystems. 106

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