CIRCULARLY POLARIZED APERTURE COUPLED MICROSTRIP SHORT BACKFIRE ANTENNA WITH TWO RINGS

International Journal of Electronics and Communication Engineering and Technology (IJECET) Volume 8, Issue 2, March - April 2017, pp. 13 25, Article ID: IJECET_08_02_003 Available online at http://www.iaeme.com/ijecet/issues.asp?jtype=ijecet&vtype=8&itype=2 ISSN Print: 0976-6464 and ISSN Online: 0976-6472 IAEME Publication CIRCULARLY POLARIZED APERTURE COUPLED MICROSTRIP SHORT BACKFIRE ANTENNA WITH TWO RINGS Kawa ABDOULA Dept. of Communication Engineering, Technical University of Varna, Student SKA Str. 1, 9010 Varna, Bulgaria ABSTRACT A circularly polarized microstrip short back fire antenna (CPMSBA) with two ring corrugated rim using aperture coupling feed method is proposed in this paper. The antenna is designed to operate in KU-band. The simulation results verify the circular polarization. The axial ratio bandwidth bw AR is 3.74%, gain is 10.43 dbi and radiation efficiency is 89.7%. The antenna has a compact structure and high electrical and mechanical characteristics. Key words: Aperture Coupled Microstrip Antenna, Back Radiation, Circularly Polarized Microstrip Antenna with Two Rings, Microstrip Short Backfire Antenna (MSBA) Cite this Article: Kawa ABDOULA, Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings, International Journal of Electronics and Communication Engineering and Technology, 8(2), 2017, pp. 13 25. http://www.iaeme.com/ijecet/issues.asp?jtype=ijecet&vtype=8&itype=2 1. INTRODUCTION The microstrip patch antennas have been used in many commercial and military applications including radar, mobile communications, satellite communications and Wireless Local Area networks (WLANs). The origin of microstrip antennas apparently dates back to 1953, when Deschamps proposed the use of microstrip feed lines to feed an array of printed antenna elements [1] - [ 2]. The microstrip patch antenna was first introduced by Munson in a symposium paper in 1972 [3]. Microstrip antennas are fed by one of four methods microstrip line coaxial probe aperture coupling and (d) proximity coupling. Microstrip patch antennas have several well-known advantages over other antenna structures, including their low profile and hence conformal nature, lightweight, low cost of production, robust nature, and compatibility with microwave monolithic integrated circuits and optoelectronic integrated circuits technologies [4]. Microstrip patch antennas suffer from several inherent disadvantage of this technology in its pure form, namely, they have small bandwidth and relatively poor radiations http://www.iaeme.com/ijecet/index.asp 13 editor@iaeme.com

Kawa ABDOULA efficiency resulting from surface wave excitations and conductor and dielectric losses. Many specialized techniques have been developed to increase the bandwidth of a microstrip antenna. These include either using thick foam substrates along with aperture coupled feeds to avoid the probe reactance limitation, or using capacitive elements to compensate for the probe inductance. Even further increases may be achieved by using configurations that exhibit dual or multiple resonances, including stacked resonators or antennas surrounded by parasitically coupled elements. [5] The microstrip antennas (MSAs) may be designed for circular polarizations by adjusting their physical dimensions so as to produce two degenerate orthogonal modes with in the cavity region. This in turn results in the radiation of two orthogonally polarized waves near the broadside direction. Thus circularly polarized radiation is obtained when two orthogonal modes are excited with equal amplitude and in-phase quadrature [6]. 2. RADIATION MECHANISM The basic microstrip antenna is consisting of a radiating patch on one side of a dielectric substrate, which has a ground plane of the other side. The microstrip patch and the ground plane together form a resonant cavity (filled with the substrate material). The cavity is lossy, due not only to the material (conductor and dielectric) loss, but also to the (desirable) radiation into space [7]. In Figure 1. The electromagnetic energy provides by the feed microstrip line passes though rectangular slot into the first resonator formed by patch element and the ground plane. After multiple reflections between the inner walls of the first resonator, a part of this energy radiated directly into the space and other part of this Figure 1 Geometry of the antenna Cross section Front view http://www.iaeme.com/ijecet/index.asp 14 editor@iaeme.com

Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings Electromagnetic energy penetrates via the short sides of the patch element to the second resonator, formed by the small reflector and ground plane. After multiple reflections between the inner walls of the second resonator, the electromagnetic energy radiated between the short sides of the small reflector and the rim in the broadside of the antenna. The rims reduce the back and side radiations of the proposed antenna. 3. DESCRIPTION OF THE ANTENNA Figure 1. shows the geometry of the aperture coupled microstrip short backfire antenna (ACMSSBFA) The antenna consists of the following elements: 1. Screen 2. Screen Substrate 3. Feed line 4. Feed substrate 5. Ground (D2) 6. Rim 7. Rectangle Cross-Slot 8. Patch 9. Patch substrate 10. Small Reflector (D1) 11. Additional substrate. 12.Small reflector Substrate 13. Additional ring and five substrates as follows: additional substrate AS (Taconic TLX-7: "εrt = 2.6, tan _t = 0.0019); small reflector substrate SRS (Arlon AD 410: "εrq = 4.1, tan _q = 0.0030, the small reflector substrate is realized by two layers with standard thickness of 3.175 mm); patch substrate PS (Arlon AD 600: "εrp = 6.15, tan _P =0.0030); feed substrate FS (Arlon AD 600: "εrf = 6.15, tan _f = 0.0030) and screen substrate SS (Taconic TLX-7: "εrs=2.6, tan_s=0.0019). 4. EFFECT OF THE DIMENSION OF THE ANTENNA ON ITS ELECTRICAL CHARACTERISTICS The simulation of the CP antenna model is done by the software package CST Microwave Studio 2010. The final results are verified with several others software packages. Seven antenna electrical characteristics such as module of reflections coefficient S 11 (return loss), axial ratio AR, gain G, back radiations level BRL, radiations efficiency η eff and radiations pattern are investigated in this paper. Nine CP antenna dimension and constructive parameters such as length of patch L P, patch size coefficient K P = L P/ W P, average length of the cross slot L a. Cross size coefficient K S =L a1 /L a2, length of the small reflector L sr, small reflector size coefficient K1=L sr /W sr, width of the first ring W 1, width of the second ring W 2 and length of the matching stub L S are chosen as independent variables in the optimization. Their influence of the return loss, axial ratio and gain behavior are shown in figures 2-10. figures 2-10 shows strong influences of the all 9 dimensions and constructive parameters on the return loss and axial ratio, while the same parameters and dimensions have low influence on the antenna gain behavior. Co- and cross polar radiation patterns in φ = 45º-plane at three frequencies (f min =11.175 GHz, f 0 = 0.5(f min +f max ) =11.388 GHz and f max = 11.602 GHz) are shown in figure 11. Figure 11 shows small changes of the radiations characteristics over the whole operating frequency range. Figure 12 shows the return loss, axial ratio and gain of the proposed CP antenna calculated by two different software packages- CST Microwave Studio 2010 and HFSS version 11.1. The both curves in the figure 12. have similar behavior and their values close to each other. CST Microwave gives the following results: central frequency f 0 =11.709 GHz and impedance bandwidth bw S11 = 10.83%, while the HFSS Product is f 0 =11.560 GHz and bw S11 = 9.69% respectively, i.e. the above results differ only by 1.3% with respect to central frequency and by 10,5% regarding the impedance bandwidth, which indicate a good agreement of the obtained simulated results. Figure 12. shows the axial ratio. CST MWS gives the following results: central frequency f 0 =11.388 GHz and axial ratio bandwidth bw AR =3.74%, while the HFSS Product is http://www.iaeme.com/ijecet/index.asp 15 editor@iaeme.com

Kawa ABDOULA f 0 =11.42 GHz axial ratio bandwidth bw AR = 3.39% the results differ by 0.3% with respect to central frequencies and by 9,35% regarding the axial ratio bandwidths bw AR. Figure 12. shows the antenna gain. CST MWS gives the G max =10.43 dbic while the HFSS gives the G max =10.904 dbic, the results differ by 4.54%. The dimensions of the proposed CP antenna are listed in Table 1, the optimized parameters of the antenna structure are listed in Table 2, and the e electrical parameters of the antenna are shown in the Table 3. Dimensions [ mm] Table 1 Dimension of the antenna Description D 2 24 Big Reflectors inner diameter tg 0.0175 Big reflector& ground thickness L 13.155 Antenna length Wsr 3.30 Small reflector width W 7.6825 Rim width tt 0.035 Small reflector thickness tw 0.5 Rim thickness Wp 2.38 Patch width tp 0.035 Patch thinness Wa1 0.45 Slot1 width= 0.1La1 Wa2 0.40 Slot2 width = 0.1 La2 l 11.9 Microstrip feed line length Ls 1.0 Stub length Wf 0.98 Microstrip feed line width tf 0.0175 Microstrip feed line thickness Ds 24 Screen diameter ts 0.035 Screen thickness ht 1.58 Additional substrate thickness hq 6.35 Small reflector substrate thickness hp 1.27 Patch substrate thickness hf 0.635 Feed substrate thickness hs 3.175 Screen substrate thickness t1 0.5 Ring thickness h 1, h 2 0.4 Distance between the upper edge of the peripheral screen and the upper edge of the first and second ring http://www.iaeme.com/ijecet/index.asp 16 editor@iaeme.com

Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings Table 2 Optimized parameters of the antenna Name Value [ mm] Description LP 2.7 Patch length K1 1.12 Small reflector ratio Lsw/Wsr KP 1.13 Patch Ratio LP/WP KS 1.10 Slot Ratio La1/La2 La 4.3 Slot Length La1 (2*La*Ks)/(Ks+1) Aperture 1 Length La2 2*La/(Ks+1) Aperture 2 Length Ls 1.0 Stub Length Lsr 3.7 small reflector Length Wa1 La1/10 Aperture 1 Width Wa2 La2/10 Aperture 2 Width WP LP/KP Patch Width Wsr Lsr/K1 (W1) Small Reflector Width W1 3.0 First ring Width W2 3.0 Second ring Width Table 3 Electrical parameters of the antenna Impedance Bandwidth Minimum frequency f min, GHz 11.075 Maximum frequency f max, GHz 12.343 Central frequency f 0, GHz 11.709 Relative bandwidth bw,% 10.83% Frequency bandwidth BW, GHz 1.27 Axial Ratio bandwidth, Back Radiation, Gain and efficiency Minimum frequency f min AR, GHz 11.175 Maximum frequency f max AR, GHz 11.602 Central frequency f 0 AR, GHz 11.388 Frequency bandwidth BWAR, GHz 0.427 Relative bandwidth bw AR, % 3.74 G min, dbi 10.17 G max, dbi 10.43 BR min, db -17.84 BR max, db -16.091 Efficiency η max, % 89.74% http://www.iaeme.com/ijecet/index.asp 17 editor@iaeme.com

Kawa ABDOULA Fig 2. Effect of the patch length L P (L P = 2.4 mm blue dashed line, L P = 2.7 mm red solid line and L P = 3.0 mm green dotted line) on the electrical characteristics of the antenna: return loss, axial ratio, gain Fig. 3. Effect of the coefficient K P = L P /W P (K P = 1.03 blue dashed line, K P = 1.13 red solid line and K P = 1.23 green dotted line) on the electrical characteristics of the antenna: return loss, axial ratio, gain. http://www.iaeme.com/ijecet/index.asp 18 editor@iaeme.com

Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings Fig 4. Effect of the average length of the cross slot L a (L a = 4.0 mm Blue dashed line, L a = 4.3 mm red solid line and L a = 4.6 mm green dotted line) on the electrical ratio, characteristics of the antenna: return loss, axial ratio, gain Fig 5. Effect of the coefficient K S = L a1 /L a2 (K S = 1.05 blue dashed line, K S = 1.10 red solid line and K S = 1.15 green dotted line) on the electrical characteristics of the antenna: return loss, axial ratio, gain http://www.iaeme.com/ijecet/index.asp 19 editor@iaeme.com

Kawa ABDOULA Fig 6. Effect of the small reflector length Lsr (Lsr = 3.1 mm blue dashed line, Lsr = 3.7 mm red solid line and Lsr = 4.3 mm green dotted line) on the electrical characteristics of the antenna: return loss, axial ratio, gain. Fig 7. Effect of the coefficient K 1 = Lsr/Wsr (K 1 = 1.02 blue dashed line, K 1 = 1.12 red solid line and K 1 = 1.22 green dotted line) on the electrical characteristics of the antenna: return loss, axial ratio, gain. http://www.iaeme.com/ijecet/index.asp 20 editor@iaeme.com

Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings Fig 8. Effect of the first ring width W1 (W 1= 2.0 Fig 9. Effect of the second ring width W2 (W 2=1.0 mm blue dashed line, W1 = 3.0 mm red solid mm blue dashed line, W2 = 3.0 mm red solid line and W1 = 4.0 mm green dotted line) on the line and W2 = 5.0 mm green dotted line) on the electrical characteristics of the antenna: electrical characteristics of the antenna: return loss, axial ratio, gain. return loss, axial ratio, gain. http://www.iaeme.com/ijecet/index.asp 21 editor@iaeme.com

Kawa ABDOULA Fig 10. Effect of the stub length LS (L S = 0.5mm blue dashed line, L S = 1.0 mm red solid line and L S = 1.5 mm green dotted line) on the electrical characteristics of the antenna: return loss, axial ratio, gain. Fig 11. Radiation patterns of the antenna (copolar, RHCP red solid line and cross-polar, LHCP green dashed line), plane φ = 45º: f = f min = 11.175 GHz, f = f 0 = 11.388 GHz, f = f max = 11.602 GHz http://www.iaeme.com/ijecet/index.asp 22 editor@iaeme.com

Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings Fig 12. Return loss, AR and Gain of the antenna calculated by means of two different software s: CST MWS 2010 red solid line, HFSS v.11.1 blue dashed line. 5. DISCUSSION AND NUMERICAL RESULTS In this study an aperture coupled microstrip short backfire antenna was designed and investigated. The following features of the proposed antenna design need discussion. Advantages of the antenna The impedance bandwidth bw S11 of the CP antenna is 10.83% while its polarization bandwidth bw AR is equal to 3.74% this is good bandwidth compared with the conventional microstrip antenna. The bandwidth enhancement in this case is due basically to the type of chosen feed, suitable choice of the values of substrate dielectric constants and thicknesses, optimization of antenna dimensions and insertion of two resonances in the antenna impedance characteristic. The first of these resonances (the patch resonance) is at lower frequency while the second resonance (the backfire resonance) is at higher frequency. The CP antenna gain ranges from 10.17 dbic to 10.43 dbic within the antenna bandwidth, at the central frequency f 0 = 11.388 GHz the antenna has gain G 0 =10.40 dbic, radiation efficiency η eff0 = 89.74%. The presence of corrugated rim improves the antenna gain by approximately 0.5 dbic compared to the antenna with conventional rim. The back radiation level of the CP antenna varies between -16 db and -17.84 db across the antenna bandwidth. The basic contribution to this good result is due to the presence of the screen and rim. http://www.iaeme.com/ijecet/index.asp 23 editor@iaeme.com

Kawa ABDOULA The antenna construction is compact, robust and with a low volume. The volume and the aperture area of the antenna, for example are 3.4 times less than the corresponding dimensions of the CP SBFA with an air cavity. Disadvantages of the antenna The antenna gain is lower compared to the conventional CP SBFA with an air cavity due to the reduced dimension. 6. CONCLUSION A broadband circularly polarized aperture coupled microstrip short backfire antenna with two ring corrugated rim has been designed and numerically examined. The bandwidth widening of the antenna is achieved by use of two resonances: a patch resonance and a backfire resonance, it has maximum gain 10.43 dbi, maximum back radiation -16.091 db, efficiency 89,74% and axial ratio bandwidth 3.74%, this is good bandwidth compared with conventional microstrip antenna. The antenna is designed to operate within KU-band. The antenna has a compact construction and high electrical and mechanical characteristics. It can be used as a single antenna or as an element of microstrip antenna arrays with various applications in the various communication systems including Radar, Mobile communications, satellite communications and wireless local area network. 7. ACKNOWLEDGEMENTS The authors wish to acknowledge associate professor Georgi Kirov and associate professor Georgi Chervenkov at the Technical University of Varna, Bulgaria to their support during my study and investigation of the microstrip short backfire antenna (MSBA). REFERENCES [1] G. Deschamps, W. Sichak, Microstrip microwave antennas, Proc. of Third Symp. on USAF Antenna Research and Development Program. October 1953, 18 22, [2] J. T. Bernhard, E. Mayes, D. Schaubert, and R.J. Mailloux, A Commemoration of Deschamps and Sichak s Microstrip Microwave Antennas 50 Years of Development, Divergence, and New Directions, Proc. of the 2003 Antenna Applications Symp. (September 2003) 189 230. [3] R. E Munson, Microstrip phased array Antennas, Proc. of Twenty-Second Symp. on USAF Antenna Research and Development Program. October 1972. [4] K. F. Lee, Ed. Advances in Microstrip and Printed Antennas. John Wiley, 1997. [5] J. L. Volakis, Antenna Engineering Handbook, 4. Edition 2007 chapters 7.1. [6] D. M. Pozar, D., H. Schaubert, The Analysis and Design of Microstrip Antennas and Arrays, University of Massachusetts at Amherst, a selected Reprint Volume, IEEE Antennas and propagation Society, Sponsor The Institute of Electrical and Electronics Engineers. Inc., New York, page 17. [7] S.D. Tragonski, D. M. Pozar, Design of wideband circularly polarized aperture-coupled microstrip antennas. IEEE Transactions antennas and propagation. 1993, Vol 41, no 2. http://www.iaeme.com/ijecet/index.asp 24 editor@iaeme.com

Circularly Polarized Aperture Coupled Microstrip Short Backfire Antenna with Two Rings ABOUT AUTHOR Kawa ABDOULA was born in Sulaimaniyah, Iraq in 1962; he received his MSc degree in radio and television engineering from the Technical University of Varna, Bulgaria in 1988. He worked between 1988 and 1990 as engineer at the Technical University of Varna. Since 1990 he lives in Sweden. He worked at Ericsson Energy System 3 years, Emerson Energy System 2 years, Ericsson Mobile Communications 2 years, and 14 years as system engineer at Itron AB in Stockholm, Sweden, (current job). He is Ph.D. student by distance at Technical University of Varna. http://www.iaeme.com/ijecet/index.asp 25 editor@iaeme.com