Compact and Small Sized Single, Double and Multi- Folded Hairpin Line Microstrip Bandpass Filters for RF/ Wireless Communications

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1 International Invention Journal of Engineering Science and Technology Vol. 2(1) pp , February, 2015 Available online Copyright 2015 International Invention Journals Full Length Research Paper Compact and Small Sized Single, Double and Multi- Folded Hairpin Line Microstrip Bandpass Filters for RF/ Wireless Communications Jagdish Shivhare 1, B V R Reddy 2 1. Department of Electrical, Electronics and Communication Engineering, ITM University, Sector-23A, Gurgaon , India 2.University School of Engineering & Technology, Guru Govind Singh Indraprashth University, Sector -16C, Dwarka, Delhi, India Abstract A conventional hairpin line resonator size is normally very large. The folded hairpin line resonator filters are smaller in size and easy to design, simulate/optimize and fabricate. The contents of this technical paper is to presented a new class of folded hairpin line microstrip resonator filters with great reduction (60-65%) in size compared to the conventional hairpin line filters. The proposed single, double and multi-folded hairpin line microstrip filters are narrow band, high selectivity, small sized and low cost band pass filters for RF/wireless trans/receive communication systems for ground and space applications. The expected performance and frequency responses have been simulated/optimized by using The Agilent-make ADS/IE3D-Zealand softwares. The measured results are very close to the simulated/optimized results with great reduction in size compared to the conventional hairpin line filters. Keywords: Substrate, folded-hairpin line resonator, miniaturized microstrip filters, coupling coefficients, narrow band, selectivity, slow wave, open-loop resonator, computed response, ADS and IE3D Softwares. INTRODUCTION Wireless and mobile communication systems have presented new challenges to the design of high quality miniature RF/microwave filters. Planar filters are the alternatives as they can be fabricated using printed circuit technology with low cost and size. The basic microstrip circuits consist of a single dielectric substrate, coated with a conductive material transmission line on one side and a metal ground plate on the other side. The folded-hairpin line topology has the advantage of desirable narrowband, good return loss, compact structure and low cost microstrip filters for trans/receive communication systems. The small size and compactness of the filter makes the design attractive for *Corresponding Author jpshivhare@itmindia.edu, jshivhare.isro@gmail.com further development and applications in the modern mobile radio communication systems. The design of a filter having folded resonators has been accomplished in the given steps: the width of the microstrip is determined for 50 ohms, the peripheral of each resonator is made a square, length of the U-shaped coupled lines is extended to its maximum, the width of the lines is made large and the space (gap) between coupled lines is made as small as possible (Pozar, 2005; Wang et al., 2007). We have chosen the RT-Duroid-Alumina substrate of dielectric constant 10.2 and thickness of 1.27 mm to design the folded hairpin line resonators for the desired microstrip filters. By using the imperical equations, Graphs and simulation/optimization, we have obtained the total size of a fourth-order parallel coupled bandpass filter at 1325 is 95 mm x 15 mm (1425 mm 2 ). At the same center frequency, the total size of the fourth-order

2 Shivhare and Reddy 11 conventional hairpin line bandpass filter is 25 mm x 25 mm (625 mm 2 =A). Also at the same center frequency, the total size of the fourth-order single fold, double fold and multifold hairpin line bandpass filters are 400 mm 2 (64 % of A), 289 mm 2 (47 % of A) and 225 mm 2 (36 % of A) respectively. The Concept of Folded Hairpin Line Resonators The length of parallel coupled filter is too long and the size increases with the order of filter. To solve this problem, hairpin line filter, using folded λ/2 resonator (U-shaped) structures were developed (Figure 1). Further reduction in size is made by folding again the two arms of the conventional hairpin line resonator (single-fold) to form a pair of closely coupled lines to enhance the capacitive nature of open end arms (Iliev and Nedelchev, 2002; Jovanovic and Nesic, 2005; Ruan and Chun, 2006). This structure helps reduce the size of filter upto 35-40% of the size of the conventional hairpin line bandpass filter. Even more reduction in size i.e 45-50% and 60-65% is employed by further folding the two arms of the single-folded hairpin line resonators i.e. double-folded and multi-folded hairpin line resonators (Fan et al., 2007; Alfano et al., 2005; Wu et al., 2003; Pozar, 2005; Wang et al., 2007). The filters consisting the folded resonators are of moderate quality factor and high stop band attenuation compare to the filters with conventional hairpin line resonators (Jovanovic and Nesic, 2005; Shi and Zhu, 2004; Kuo and Shih, 2003; Jantaree and Akkaraekthalin, 2003; Lai et al., 2010). Design Procedure of Folded Hairpin Line Resonator Filters Filters with Hairpin-line resonators are relatively simple to design and build. The design methodology and supporting softwares are available to design, simulate/optimize the single, double and multi-folded hairpin line filters (Hong and Lancaster, 2006; Akkaraethalin and Jaruek, 2006; Zhang et al., 2007; Zhu and Menzel, 2005; Kazerooni and Cheldavi, 2006). Design calculations of folded hairpin line microstrip filters can be done in the following steps: 1. Finding the element values of LPF prototype by using the approximate synthesis method. The relations between the bandpass design parameters and the lowpass elements are (Deng et al., 2007; Zhao et al., 2007; Jen-Tasi et al., 2005; Tsai et al., 2003). Where : fractional bandwidth of the bandpass filter, C : Capacitance of the lumped capacitor J : Characteristic admittance of the inverter, N : degree of the filter 2. To calculate the resonator parameters: The length of the coupled lines can be calculated by: where : Electric length of the resonator : Characteristic impedance :Even mode impedance : Odd mode impedance 3. Calculations for the coupling parameters: The values of coefficient of coupling between resonators can be calculated against the distances between the resonators. The design technique uses an approximation polynomial and a low filter prototype (Zhu et al., 2007; Xiao et al., 2007; Moon-Seok et al., 2005; Wang and Zhu, 2004). The loaded Q factor and the mixed coupling coefficients between different resonators can be calculated by using the equations, graphs and commercial softwares. 4. Calculation of the input tapped electrical length (Figure 2): 5. Calculation of the geometric parameters of the filter for an exact substrate. 6. Optimization of filter parameters by varying the geometric dimensions. In designing a resonator the width of microstrip is determined by setting its characteristic impedance to 50 ohms (Hong and Lancaster, 2004; Singh et al., 2008; Chang et al., 2003). In the design topology the source power transfer to the load is obtained by coupling to the resonator-transmission line (Tsai and Wu, 2011; Fei et al., 2012; Singh and Singhal, 2013). The end resonators may be externally coupled by tapping instead of using a coupled section. They provide circuits of high power handling capacity and economical to produce, hybrid and monolithic Integrated Circuit (IC). The Single-fold,

3 12 Int. Inv. J. Eng. Sci Tech. Figure 1. Actual, capacitor loaded, single-fold, double-fold and multi-fold hairpin line microstrip resonators Figure 2. Input/output tapped electrical length. Fig.2 Input/output tapped electric designed and developed at the same common center frequency of 1325 to compare their performances with reduction in size by 35-40%, 45-50% and 60-65% respectively. The external quality factor, coupling coefficients and the coupling between adjacent microstrip resonators and other elements of the filter can then be determined by The filter has fourth-order cross coupled structure. Dimensions of resonators of the filter L 1 : 8.71 mm, W: 1.14 mm, G : 0.31 mm, D 12= D 34 :1.61 mm and D 14 =D 23 :1.81 mm * Size of the filter: 20 mm x 20 mm (400 mm 2 i.e. 64% of A) The measured results are close to the simulated response. Design of a double-folded resonator, filter at 1325 Where f p1 and f p2 are the lower and higher split resonant frequencies of a pair of coupled resonators. We have used EM simulator to model the coupling coefficient and external Q. The hairpin transmission lines and bends are realized by using the MBEND90x and MLIN elements. (ADS Agilent-make Softwares for Design and Simulation, 2011; Ansoft-HFSS-3D software for Electromagnetic modeling, 2012; Zealand Software IE3D for Simulation and Optimization, 2013) The proposed double-fold hairpin line bandpass filter is shown in figure 7 to 9. This filter is a fourth-order crosscoupled structure and the couplings exist between adjacent and non-adjacent resonators. Dimensions of resonators of the filter L 1 : 7.11 mm, L 2 : 3.63, W: 1.16 mm, G : 0.31 mm, D 12= D 34 :1.69 mm and D 14 =D 23 :1.87 mm *Size of the filter: 17 mm x 17 mm (289 mm 2 i.e. 47% of A) The measured results are close to the simulated response Desired specifications of the filter Centre Frequency (CF) : 1325 Insertion loss : < 3 db 3 db width (Lower side) : 24 w. r. t. c. f. 3 db width (Upper side) : 24 w. r. t. c. f Lower side attenuation : 32 dbc at 1265 Upper side attenuation : 32 dbc at 1385 Input/output Impedance : 50 Ohms Design of a single-folded resonator-filter at 1325 The proposed single-fold hairpin line bandpass filter is shown in figure 3 to 6. Design of a multi-folded resonator- filter at 1325 L 1 : 6.57 mm, L 2 : 4.36 mm, L 3 : 3.01 mm W 1 : 1.31 mm W 2 : 0.51 mm, G 1 : 0.45 mm G 2 : 0.39 mm G 3 : 0.44 mm, G 4 : 0.82 mm D 12 =D mm, D 23 =D 14 : 1.21 mm * Size of the filter:15mm x 15 mm(225 mm 2 i.e 36 % of A) The measured results are close to the simulated response. Figure 10 to 12 and Table 1. RESULTS AND DISCUSSIONS At the desired center frequency of 1325, the simulated and measured 3dB bandwidths of fourth-order

4 Shivhare and Reddy 13 Figure 3. Structure of a Single- folded resonator Figure 4. Coupling Coefficients for end-coupled resonators(1) Figure 5. A fourth-order filter having single-folded Figure 6. Simulated response and measured result of resonators. the fourth order filter having single-folded resonators Figure 7. Structure of a double-folded resonator Figure 8. A fourth-order filter consisting of double-folded resonators

5 14 Int. Inv. J. Eng. Sci Tech. Figure 9. Simulated response and measured results Figure 10. Structure of a fourth-order multi-fold Figure 11. A fourth-order multi-folded filter resonator Figure12. Simulated response and measured results

6 Shivhare and Reddy 15 Table 1.Comparision of Simulated and Measured Results of Folded Hairpin Line Filters S. No. Parameters Unit Design Specs. Single-fold Simulated Single-fold Measured Double-fold Simulated Double-fold Measured Multi-fold Simulated Multi-fold Measured 1 Center frequency Insertion loss in db < band 3 3dB width (lower side) w.r.t.c.f db width (upper side) 5 Stopband attenuation (lower side) 6 Stopband attenuation (upper side) 7 Size of filter /(Reduction in size) w.r.t.c.f. dbc at dbc at at at 1385 mm 2 /% Minimum 400mm 2 400mm 2 i.e. 64% of A/ (36 % of A) mm 2 289mm 2 i.e. 47% of A/ (53 % of A) # Optimized dimension of a conventional hairpin line bandpass filter:25 mm x 25 mm : 625 mm 2 i.e. (A) for reference. 225mm 2 225mm 2 i.e. 36% of A/ (64 % of A) bandpsss filters consisting of single-fold, double-fold and multi-fold hairpin line resonators, are 3.7%/ 3.6%, 3.7%/ 3.8% and 4.3%/ 3.6% respectively. The simulated and measured results show minor variations in lower and upper stopband attenuation for single-fold and double fold structures but major variations for multi-fold structure due to undesired cross-coupling between folded arms of individual resonators and non-adjacent resonators in the filters. The optimized dimensions of these filters are 20 mm x 20 mm (400 mm 2 ), 17 mm x 17 mm (289 mm 2 ) and 15 x 15 mm(225 mm 2 ). CONCLUSION This paper presents single, double and multi-fold hairpin line bandpass filter design techniques. The reduction in size of the fourth-order single, double and multi-fold filters are 36%, 53% and 64% of the optimized size (A) of the conventional hairpin line bandpass filter at 1325 centre frequency. The measured results are close to the simulated/optimized results. There are limitations of the design in terms of inaccuracy of sharp folding and coupling between the adjacent and cross-coupled folded hairpin line resonators in the filters. The developed filters can be used for trans/receive RF/wireless/mobile communication systems. REFERENCES Akkaraethalin P, Jaruek J (2006). Microwave slow wave open-loop resonator filters with reduced size and improved stopband characteristics ETRI Journal,Vol 28, No. 5, October. Alfano I, Orazio AD, De Sario M, Petruzzelli V, Prudezano F (2005). A passband microstrip bandpass filter with continuous varying impedence Journal of ElectromagneticsWaves and applications,vol 19, No.9, Chang SF, Chen JL, Chang SC (2003). New bandpass filters with step-impedance resonators comb and hairpin structures, Proc. Asia Pacific Microw. Conf., pp Deng PH, Lin YS, Wang CH, Chen CH (2007). Compact Microstrip bandpass filters with good stopband rejection IEEE Transactions on Microwave Theory and Techniques,Vol.54, No.2, , February Fan JW, Liang CH, CHEN XW (2007). Design of Cross-coupled microstrip filter with Split-ring resonators PIER,75, Fei L, Gan H, Wang Z, Lu W (2012). Novel Compact triple-bandpass filter using λ/4 resonator pairs with common viaground proceeding of the Progress in the Electromagnetics Research Symposium, pp , Hong JS, Lancaster MJ (2004). Microstrip filters for RF/microwave application A Wiely Interscience publications, Canada. Hong JS, Lancaster MJ (2006). Coupling Microstrip Square Open- Loop Resonators for Cross-Coupled Planar Microstrip Filters IEEE Trans.Microw.Theory Tech. Vol. No. 5, October. Iliev IG, Nedelchev MV (2002). CAD of Cross Coup-lled Miniaturized Hairpin Bandpass Filters Microwave Review, pp-49-52, December. Jantaree JK, Akkaraekthalin P (2003) A microstrip bandpass filtersusing a symmetrical parallel coupled-line structure The 9th Asia Pacific Conference on Communications, Vol. 2, Jen-Tasi K, Ming-Jyh M, Ping-han L (2005). Microstrip filter with Compact miniaturized hairpin line resonators IEEE Microwave Theory and Guided Letters,Vol. 10,No.3, March 2005,pp Jovanovic S, Nesic A (2005). Microstrip bandpass filters with new type of capacitive coupled resonators, Electronics Letters 41,(1) page Jovanovic S, Nesic A (2005). Microstrip bandpass filters with capacitive coupled resonators Electronic Letters,Vol. 41, N Kazerooni M, Cheldavi A (2006). Simulation,analysis,design and applications of microstrip structure filters using multistrip method Progerss in Electromagnetic Research PIER, 63, , Kuo JT, Shih E (2003). Microstrip resonator bandpass filter with an extended optimal Rejection bandwidth IEEE Trans. Microw. Theory and Tech. Vol.51, Lai X, Wang N, Wu B (2010). Design of tri-band filter based on stub loaded resonator and DGS resonator IEEE Microwave Wireless

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