Broadband aperture-coupled equilateral triangular microstrip array antenna

Transcription

1 Indian Journal of Radio & Space Physics Vol. 38, June 2009, pp Broadband aperture-coupled equilateral triangular microstrip array antenna S N Mulgi $,*, G M Pushpanjali, R B Konda, S K Satnoor & P V Hunagund Department of PG Studies and Research in Applied Electronics, Gulbarga University, Gulbarga , Karnataka, India $ s.mulgi@rediffmail.com Received 15 March 2007; revised 30 July 2007; re-revised received 17 December 2008; accepted 26 December 2008 The aperture-coupled equilateral triangular microstrip array antenna comprising parasitic element and slot for broadband operation is designed and presented. An experimental study is carried out to see the effect of slot placed in the parasitic and radiating elements for enhancing the impedance bandwidth. The four-element aperture-coupled equilateral triangular microstrip array antenna (FAEMA) using a common parasitic element in the form of gap-coupled between the two radiating element gives an impedance bandwidth of 18.46%. If the slot is placed at the center of all elements of FAEMA, the impedance bandwidth is found to be 18.57%. However, this bandwidth is enhanced to 19.04% by placing the slot at the center of radiating elements of FAEMA. Further, the bandwidth is increased from to 19.07% by placing a slot at the center of parasitic element of FAEMA. This bandwidth is 38% more when compared to four-element array antenna without using parasitic elements. The gain, azimuthal radiation patterns and input impedance of proposed antennas are studied and reported. Details of the antenna design are given and experimental results are discussed. Keywords: Equilateral triangular array antenna, Aperture coupled antenna, Microstrip array antenna PACS No.: Ba 1 Introduction One of the major drawbacks of microstrip antenna (MSA) is its narrow impedance bandwidth characteristics. Various methods have been reported for improving the impedance bandwidth of MSAs. Examples are: addition of parasitic element with radiating patch 1, aperture-coupled technique 2, stacked technique 3, proximity coupling to the patch antenna 4, etc. Among various shapes of microstrip patch antennas, the rectangular patch is widely used because of its simplicity in design and analysis. But triangular microstrip patch has advantage of being physically smaller for operating at a given frequency as compared to rectangular patch 5. However, bandwidth of conventional triangular microstrip patch and their array is relatively small when compared to rectangular microstrip patch. Hence, an effort is made to enhance the impedance bandwidth of equilateral triangular microstrip array antenna using parasitic element and slot technique 6. The use of parasitic element, in the form of gap coupled to the radiating elements, fed through aperture coupling for enhancing the impedance bandwidth of two-element slot equilateral triangular array antenna is already reported 7. In the present study, number of array elements is increased to four and impedance bandwidth is measured separately by placing the slot in radiating and parasitic elements. The similar study was conducted by Chakraborthy et al. 2 wherein antenna configuration consisted of 16 array elements. They used aperture-coupled feeding for exciting array elements and obtained nearly 11% of impedance bandwidth. In the present study, antenna configuration consists of only two parasitic elements in the form of gap-coupled to four radiating elements. The maximum bandwidth of 19.07% is achieved when slot is used at the center of parasitic element which is 73.36% more than found earlier 2. The length (L P ) and width (W P ) of parasitic element is calculated by using the basic equations available for designing of rectangular patch 8. The design frequency of parasitic element and equilateral triangular element are taken same. The length (L s ) and width (W s ) of the slot is selected as λ o /5 and λ o /32, respectively, where λ o is the free space wavelength in cm. This slot is considered as a wide slot because its width is comparable to the length. The wide slot is selected as it is more effective in enhancing the impedance bandwidth as compared to narrow slot 8. The effect of slot in enhancing the impedance bandwidth is studied by inserting it at the center of radiating and parasitic elements. 2 Description of antenna geometry Figure 1 shows the geometry of FAEMA designed and fabricated using commonly available glass epoxy

2 MULGI et al.: BROADBAND APERTURE-COUPLED EQUILATERAL MICROSTRIP ARRAY ANTENNA 175 substrate material S 1 having 1.66 mm thickness (h) and 4.2 dielectric constant (ε r ). The radiating and parasitic elements are etched on the top surface of the substrate S 1. The distance between two equilateral triangular elements from their center (D 1 ) is taken as 3λ o /4. Usually in the array configuration, spacing between the two radiating elements is kept at a distance of λ o /2 for minimum sidelobes 9. But in the design of FAEMA, spacing between the two radiating elements (D 1 ) is taken as 3λ o /4 from their center. This is to accommodate the corporate feed arrangement between the elements. In this case, the feed line cannot be accommodated if spacing between two radiating elements is λ o /2. However, spacing between the radiating elements can be kept to any value. But the feed line has to be extended accordingly which increases the radiation loss in the feed line. Hence, the corporate feed arrangement is kept as compact as possible for minimum losses by selecting the spacing between two radiating elements to be 3λ o /4. This Fig. 1 Designed geometry of FAEMA: (a) Array elements; (b) Slot array on ground plane; and (c) Feed network spacing may cause few sidelobes in the radiation pattern. The gap between the radiating and parasitic element S is optimized 10. The gap S between one radiating and one parasitic element was varied from to 0.2λ g at an equal interval of 0.025λ g, where λ g, is the operating wavelength in cm. At each variation, the voltage standing wave ratio (VSWR) was measured. It was found that when S is 0.025λ g, the antenna shows minimum VSWR. By using this value, two elements gap-coupled rectangular microstrip antenna was constructed for wideband operation. Hence, it is seen that when parasitic element is dept closed to the radiating element there is more coupling effect which causes enhancement in the impedance bandwidth. There is no coupling effect when parasitic element is kept far away from the radiating element and the antenna gives the bandwidth same as that of conventional antenna. In this case also, the value of S is taken as 0.025λ g between radiating and parasitic element as antenna is designed for the same frequency 9.4 GHz. The calculated value of S is found to be 0.04 cm, which is possible to fabricate practically. The parasitic element is rectangular in shape. The radiation takes place from the broader side of rectangular element due to fringing fields 8. The mutual coupling between two radiating elements is maximum when these are placed along the width of rectangular parasitic patch coupled by a gap of S, which is optimized 7 for wider impedance bandwidth. The length of parasitic element (L p ) in FAEMA is extended in order to maintain the distance 3λ o /4 between the two equilateral triangular radiating elements from their center. The corporate feed arrangement is etched below the substrate S 2 as shown in Fig. 1(c) having dielectric constant ε r and thickness h as that of S 1. The corporate feed arrangement consists of matching transformers, quarter wave transformers and microstrip bends used for better impedance matching to the coupling slots. The coupling slots are placed on the top surface, as shown in Fig. 1(b), which is the ground plane of the substrate S 2 exactly at the tip of the 50 Ω microstrip feed line of corporate feed arrangement. The substrate S 2 is placed below the substrate S 1 that forms aperture coupled feeding. The elements placed on the top surface of substrate S 1 as shown in Fig. 1(a) energizes through coupling slots. The SMA connector is used at the tip of 50 Ω feed line for feeding the microwave power. Figure 2(a) shows the geometry of conventional four-element aperture coupled equilateral triangular

3 176 INDIAN J RADIO & SPACE PHYS, JUNE 2009 microstrip array antenna (CFAEMA) without parasitic element. Figure 2(b) shows the geometry of slotted four-element aperture-coupled equilateral triangular microstrip array antenna (SFAEMA). In this, slots are placed exactly at the center of the radiating and parasitic elements. The geometry of four-element slot in radiating elements of aperture-coupled equilateral triangular microstrip array antenna (FRAEMA) is shown in Fig. 2(c). Four-element slot at the center of parasitic elements of aperture-coupled equilateral triangular microstrip array antenna (FPAEMA) is as shown in Fig. 2(d). The elements of CFAEMA, SFAEMA, FRAEMA and FPAEMA are excited using the same aperture coupled feed arrangement as in Fig. 1. The computer software Auto CAD-2006 is used to achieve better accuracy in designing of these antennas. The antennas are fabricated using photolithography process. The dimensions of patch geometry, feed line specifications, slot dimensions, etc. of Figs 1 and 2 are given in Table 1. 3 Experimental results The impedance bandwidth over return loss less than 10 db for the proposed antennas is measured for X-band frequencies. The measurement is taken on vector network analyzer E8362B. The variation of return loss versus frequency of FAEMA is shown in Fig. 3. From this graph, the impedance bandwidth is calculated using the formula: BW = [(1/f c ) (f H f L )] 100 % (1) where, f H and f L, are higher and lower cut-off frequency of the band, respectively, when its return loss reaches 10 db; and f c, is the centre frequency of this band. The measured impedance bandwidth of FAEMA is found to be 18.46%. The variation of return loss versus frequency of CFAEMA is also shown in Fig. 3 for comparison. This antenna operates at three bands of frequencies B 1, B 2 and B 3. The overall impedance bandwidth of this antenna is found to be 13.75%. Hence, it is clear from Fig. 3 that the use of parasitic element in FAEMA makes the Table 1 Various dimensions of patch geometry Fig. 2 Geometry of: (a) CFAEMA; (b) SFAEMA; (c) FRAEMA; and (d) FPAEMA Side length of the patch (A) 1.21 cm Length of 50 Ω line (L 1 ) 0.41 cm Width of 50 Ω line (W 1 ) 0.30 cm Length of 70 Ω line matching transformer (L 2 ) 0.41 cm Width of 70 Ω line matching transformer (W 2 ) 0.16 cm Length of 100 Ω line (L 3 ) 1.85 cm Width of 100 Ω line (W 3 ) 0.07 cm Length of 100 Ω line (L 4 ) 0.83 cm Length of slot (L s ) 0.64 cm Width of slot (W s ) 0.10 cm Length of the parasitic patch (L P ) 0.82 cm Width of the parasitic patch (Wp) 0.99 cm Distance between the driven and parasitic element (S) 0.04 cm Radius of SMA connector (R C ) 0.15 cm Distance between two driven elements (D 1 ) 2.39 cm Operating wave length (λ g ) 1.64 cm Free space wavelength (λ 0 ) 3.19 cm

4 MULGI et al.: BROADBAND APERTURE-COUPLED EQUILATERAL MICROSTRIP ARRAY ANTENNA 177 antenna to operate at only one band of frequency B 4. This impedance bandwidth is 18.46%, which is 34.3% more than that of CFAEMA. The measured impedance bandwidth of SFAEMA, FRAEMA and FPAEMA are shown in Fig. 4 and are found to be 18.57, and 19.07%, respectively. The enhancement of impedance bandwidth is due to the fact that the radiating element (equilateral triangular), parasitic element and slot resonate nearer to the fundamental resonance of equilateral triangular elements which causes enhancement in the impedance bandwidth 3. From Fig. 4, it is also seen that when slot is inserted at the center of parasitic element (FPAEMA), the antenna gives wider impedance bandwidth (19.07%) and with a greater return loss of 34 db when compared to other antennas. For calculating the gain, the power received (P t ) by antenna under test (AUT), and power received (P s ) by pyramidal horn antenna are measured separately. With the help of these experimental data, the gain (G T ) in db is calculated using the formula 9 : (G T ) db = (G s ) db + 10 log (P t /P s ) (2) where, G s, is the gain of pyramidal horn antenna. From this calculation, the gain of proposed antennas FAEMA, SFAEMA, FRAEMA and FPAEMA is found to be 8.47, 9.36, 8.51 and 7.37 db, respectively. It is evident that when slots are used at the centre of parasitic elements (FPAEMA) the gain increases considerably as compared to other antennas. However, the gain of CFAEMA is 7.26 db, which is slightly higher than that of FPAEMA ( 7.36 db). But the impedance bandwidth of CFAEMA is quite low than that of FPAEMA. The co-polar and cross-polar H-plane radiation patterns of FAEMA and CFAEMA are measured in their operating bands. Typical radiation patterns are as shown in Figs 5(a) and (b), respectively. It is clear from the comparison of these figures that nature of radiation pattern remains almost the same when parasitic elements is used in FAEMA. The cross-polar power level of CFAEMA is less than or equal to 10 db in the range ( 45 o ) and (+45 o ). But the crosspolar power level is more than 10 db for the same range in case of FAEMA. The radiation patterns of SFAEMA, FRAEMA, and FPAEMA are measured in their operating bands and are shown in Figs 5(c), (d) and (e), respectively. From these figures, it is seen that patterns are broad sided and linearly polarized 11. It is seen from Fig. 5(c) that the cross-polar power level of SFAEMA is minimum when compared to cross-polar power levels of other antennas (FAEMA, FRAEMA and FPAEMA) and the pattern is uniform from the center axis. This is mainly due to use of slots in radiating and parasitic elements in SFAEMA. But the impedance bandwidth of this antenna (18.57%) is slightly less than that of FPAEMA (19.07%) in which the slots are placed at the centre of parasitic element. The increase in the sidelobe levels seen in radiation pattern graphs may be due to the placement of radiating elements having distance 3λ o /4 from their center and reflections from nearby objects. It is clear from this study that the use of slots is quite effective in enhancing the impedance bandwidth and gain when compared to FAEMA without much changing the radiation characteristics. As FPAEMA gives the highest impedance bandwidth among the proposed antennas, its input impedance is measured on vector network analyzer. The variation of input impedance Fig. 3 Variation of return loss versus frequency Fig. 4 Variation of return loss versus frequency

5 178 INDIAN J RADIO & SPACE PHYS, JUNE 2009 Fig. 5 Variation of relative power versus azimuth angle of: (a) FAEMA; (b) CFAEMA; (c) SFAEMA; (d) FRAEMA; and (e) FPAEMA Fig. 6 Variation of input impedance of FPAEMA of this antenna is shown in Fig. 6. The impedance plot shows the central loops that account for its wideband operation. Further, from Fig. 4 it is clear that the antenna is resonating for a single wideband of frequency in X-band. This antenna operates in the frequency range GHz where its return loss is below 10 db, which indicates better impedance matching within the operating band of frequencies. 4 Conclusions The present study shows that the impedance bandwidth of equilateral triangular microstrip array antennas can be enhanced considerably by using parasitic elements in the form of gap-coupled to the radiating elements and embedding slots in both parasitic and radiating elements with aperture-coupled feeding. The maximum impedance bandwidth is

6 MULGI et al.: BROADBAND APERTURE-COUPLED EQUILATERAL MICROSTRIP ARRAY ANTENNA 179 achieved in the case of FPAEMA (19.07%). This impedance bandwidth is 38% more when compared to four-element array without parasitic elements (CFAEMA) and 0.16%, 2.69% and 3.30% more when compared to FRAEMA, SFAEMA and FAEMA, respectively. It is clear that the use of slot at the proper locations only enhances the impedance bandwidth considerably. However, the cross-polar power level is minimum (i.e. less than or equal to 10 db from 60 o to +60 o ) in case of SFAEMA when compared to other antennas. These antennas are superior as they are physically compact in size as compared to rectangular, square or circular microstrip antennas designed for the same resonant frequency. Hence, these compact, wideband antennas are attractive for present day scientific and industrial applications in fields like mobile computing and communications 2. Acknowledgements The authors would like to thank Department of Science and Technology (DST), Govt. of India, New Delhi, for sanctioning Network Analyzer under FIST project. The authors also acknowledge their thanks to Shri D Govind Rao, LRDE, Bangalore, for providing measurement facilities and useful discussion. References 1 Au T M & Luk K M, Effect of parasitic element on the characteristics of microstrip antenna, IEEE Trans Antennas Propag (USA), 32 (1991) Chakraborty S, Gupta B & Poddar D R, Development of closed form design formulae for aperture coupled microstrip antenna, J Sci Ind Res (India), 64 (2005) Bhatnagar P S, Edimo M, Mohdjoube K & Terret C, Experimental study on stacked aperture fed triangular microstrip antenna, Proc. APSYM-CUSAT 92, (Center for Research in Electromagnetics and Antennas (CREMA), Dept. of Electronics, Kochi, (India)), 1992, pp Pozar D M & Jackson R W, An aperture-coupled microstrip antenna with a proximity feed on a perpendicular substrate, IEEE Trans Antennas Propag (USA), 35 (1987) Lee K F, Luk K M & Dahele J S, Characteristics of the equilateral triangular patch antenna, IEEE Trans Antennas Propag (USA), 36 (1988) Lu J H, Tang C L & Wong K L, Novel dual-frequency and broad-band designs of slot-loaded equilateral triangular microstrip antennas, IEEE Trans Antennas Propag (USA), 48 (2000) Pushpanjali G M, Konda R B, Mulgi S N, Satnoor S K, Hadalgi P M & Hunagund P V, Design of wideband equilateral triangular microstrip antennas, Indian J Radio Space Phys, 35 (2006) Bahl I J & Bhartia P, Microstrip antennas (Artech House, New Delhi), Balanis C A, Antenna theory analysis and design (John Willey, New York), Mulgi S N, Vani R M, Hunagund P V & Hadalgi P M, A compact broadband gap-coupled microtrip antenna, Indian J Radio Space Phys, 33 (2004) Praveen Kumar A V & Mathew K T, Cylindrical dielectric resonator antenna with a coplanar parasitic conducting strip, Proc. APSYM-CUSAT 06, (Center for Research in Electromagnetics and Antennas (CREMA), Dept. of Electronics, Kochi, (India)), 2006, 173.