Compact Planar Dual Band Antenna for WLAN Application

Transcription

1 Progress In Electroagnetics Research Letters, Vol. 7, 89 97, 217 Copact Planar Dual Band Antenna for WLAN Application Riki Patel * and Trushit Upadhyaya Abstract A iniaturized dual-band icrostrip antenna has been designed and analyzed for Wireless LAN application. The proposed antenna coprises a radiating patch, fed by a icrostrip line on a 1.6 thick FR4 dielectric aterial substrate. The antenna easureent illustrates ipedance bandwidth of around 1% at 2.4 GHz resonance and 6% at 5.5 GHz resonance. The easured stable return loss and radiation patterns are presented for the proposed dual-band electrically sall icrostrip antenna for wireless applications. 1. INTRODUCTION Electrically sall antennas are becoing very popular due to their copact diensions, and as a result, they are frequently used in any wireless applications. These antennas are also known as electrically sall antenna (ESA), physically sall antenna (PSA), physically constrained sall antenna (PCSA) and functionally sall antenna (FSA) [1]. The basic function of an antenna is to act as a transducer which converts alternating current to radio frequency waves and vice versa. Printed icrostrip patch antennas offer copetitive solution in day to day life, due to very wide spread wireless counication and various types of wireless technologies. They are popular due to low cost, low weight, less troublesoe fabrication, ease of integration in counication syste, very copact size and ulti-frequencies. However, iniaturized icrostrip patch antenna suffers fro very narrow bandwidth typically in the order of 1% 2%. Typically, antenna copactness is achieved by engineering the antenna diensions with antenna paraeters tradeoff. Techniques such as fractal geoetry [2], negative refraction [3 5] and defected ground plane [6, 7] are frequently utilized for size reduction. Available literature has addressed a few challenges of antenna gain, bandwidth and liiting Q-factor [8 12]. Many antennas are presented in literature to be echanically and electrically copact for both near-field and far-field applications [13 16]. Essentially, antenna perforance and size are significantly interconnected. To cope with deand of copact present and future wireless counication services, cost effective iniaturized icrostrip antennas possessing fair radiation characteristics are fundaentally required. Eleentary ethod to achieve the dual-band characteristic in a icrostrip patch antenna is to create slots in patch geoetry. In this article, a iniaturized, low profile icrostrip patch antenna is proposed for dual-band operation. The proposed antenna has an ipedance bandwidth in the range of 6% 1% with respect to the corresponding resonance frequencies. In the proposed ESA, radiation characteristics are optiized by height of substrate and engineered feed line. In addition, good ipedance bandwidth is achieved at dual frequencies. A note on antenna radiation efficiency (η) and Q-factor is presented for understanding the effect of antenna diensions on antenna bandwidth and Q-factor. Received 27 June 217, Accepted 14 Septeber 217, Scheduled 25 Septeber 217 * Corresponding author: Rikikuar Patel (rikipatel.ec@charusat.ac.in). The authors are with the Charotar University of Science and Technology, India.

2 9 Patel and Upadhyaya W r 1 L Wp Top View Lp E " (a) H (b) Side View Figure 1. Model of the proposed antenna. (a) Geoetry. (b) Fabrication of the prototype antenna. 2. ANTENNA DESIGN Geoetry and design paraeters of the proposed dual-band electrically sall icrostrip patch antenna are shown in Figure 1. The designed antenna is fabricated on an FR4 substrate having thickness of 1.6 with relative dielectric constant ε r =4.4 andtanδ =.2. Finite Eleent Method (FEM) based full wave electroagnetic High Frequency Structure Siulator (HFSS) is utilized for designing the proposed dual-band electrically sall icrostrip patch antenna. A rectangular patch antenna is ade electrically sall by integration of square and circular shape cuts in the design. Use of square and circular shapes along the width and length of the upper side patch significantly helps to achieve the desired resonance, and unifor circular and square cuts ake fabrication easier. The size of the icrostrip line is optiized to get a good deal on ipedance atching at target frequency bands. The size of the patch is , circular ring radius (r 1 ) 2.9, and length and width of squares 5.6. Typical design equations for width and length diensions of a icrostrip patch antenna are as follows. Patch width can be given as [17] W = v 2 (1) 2f r ε r +1 where v =Freespacevelocity; f r = Resonant frequency ε r = Relative perittivity ε reff = The effective perittivity, 2ΔL = Extension in length due to fringing effects ε reff = ε r +1 + ε [ r h ] 1 2 (2) 2 2 W

3 Progress In Electroagnetics Research Letters, Vol. 7, Length of patch can be given as [17] v L = 2ΔL (3) 2f r εreff ( ) W (ε reff +.3) h ΔL =.412h ( ) (4) W (ε reff.3) h +.8 where hs is height of the substrate in. The size of ground plane is the sae as that of the substrate diensions. Sandwich structure is ade with two substrates of height 1.6, each separated by air gap height of 1.6. The substrates are stacked and attached with icrowave adhesive to get the overall height of antenna as 4.8 fro the ground plane as illustrated in Figure 2. Due to the presence of air gap in the substrate, effective refractive index gets significantly reduced. In addition, with increase in substrate thickness, bandwidth also gets increased. Dual-frequency operation is achieved due to the optial use of square and circular slots. Initially the antenna resonates at 2.4 GHz, but due to addition of slots, the antenna also resonates at 5.5 GHz. The echanical paraeters of the antenna are shown in Table 1. Figure 2. Layered antenna structure. Table 1. Paraeters of proposed iniaturized icrostrip antenna. Paraeters Diension () Substrate Height (hs) Air gap Height (ahs) Superstrate Height (shs) Patch Width (P width ) Patch Length (P length ) Substrate (L) Substrate (W ) Square Width (W p) Square Length (L p) RADIATION EFFICIENCY (η) & QUALITY FACTOR (Q) OF ELECTRICALLY SMALL ANTENNA In 214, ultiple electrically sall antenna design ethodologies were suarized by Fujioto and Morishita [1]. They have adequately addressed the theoretical liits of electrically sall antennas. It has been entrenched that for an electrically sall antenna, confined in given diensions, the antenna possesses certain iniu quality factor (Q-factor). The constraint of Q-factor presents challenge in achieving high ipedance bandwidth for the copact antennas. The higher the Q-factor is, the saller is the ipedance bandwidth of an antenna [12, 18, 19] Radiation Efficiency Ipedance atching is a critical proble in sall antennas. antenna can be given as [1, 18, 19]. R r η a = (R r + R ) The efficiency of an electrically sall (5)

4 92 Patel and Upadhyaya η a = ESA Efficiency R r = Radiation Loss (Ω) R = Material Loss (Ω) The ipedance of sall antenna has ore capacitive coponent. The overall syste efficiency, i.e., atching network, antenna and feed line, can be given as [1]. η s = η a η (6) η s =systeefficiency η = atching network efficiency η η a ( 1+ Q ) (7) a Q Q a = Q of sall antenna Q = Q of atching network The atching network efficiency as shown in Equation (7) is dependent on Q of sall antenna and Q of the atching network utilized in the antenna. With antenna being sall in electrical diensions, it is apparent that there shall be fundaental liitation on iniu achievable Q-factor. This sharpens the antenna selectivity. However, the presented antenna has partially increased the selectivity by introducing an air gap in the substrate. Antenna efficiency can be obtained by wheeler cap ethod. Near-field region λ of ESA is considered within the radius of 2π. In 1996, McLean developed further work on Q-factor for a copact antenna. The Q-factor for such antennas can be given as [18]: Q = 1 (ka) (8) ka Figure 3 shows a graph of the iniu Q associated with the TE1 or TM1 ode of linearly polarized antenna in free space [18]. The exact curve is derived by McLean and can be used in any estiations of iniu Q. Figure 3. ESA Q factor vs ka curve for electrically sall antenna Bandwidth The approxiate bandwidth for an RLC type circuit in ters of Q-factor is: BW = S 1 Q S (9)

5 Progress In Electroagnetics Research Letters, Vol. 7, S = S :1VSWR BW = noralized bandwidth. Figure 4 displays the relation between radius enclosing the antenna ground plane and bandwidth of the antenna [1, 18]. When the horizontal current is flowing in the ground plane, the radiation efficiency is reduced. The storage of near-field energy increases as antenna Q-factor becoes larger, and the bandwidth becoes saller. In any practical cases, the ground plane is closed which will reduce the achievable bandwidth of the ESA. % Bandwidth of antenna (2:1 VSWR) % Bandwidth (2:1 VSWR) % Bandwidth (2:1 VSWR) Radius Enclosing antenna fro Ground plane (wavelength) Figure 4. Fundaental liit of ipedance bandwidth of sall antenna. 4. RESULTS AND DISCUSSIONS Siulated and easured reflection coefficients (S 11 ) of the antenna are illustrated in Figure 5. The antenna resonates at 2.4 GHz and 5.5 GHz frequencies. This covers the requireent of WLAN applications. The siulated Voltage Standing Wave Ratio (VSWR) for the proposed antenna is 1.29 and 1.18, respectively. The proposed antenna is fabricated, and antenna paraeters are easured using Agilent Vector Network Analyzer N9912A in an unshielded anechoic chaber environent. The calibration of VNA is carried out with and without load for antenna easureent. The easured VSWRs for the target frequencies are 1.1 and 1.13, respectively. Typically, such applications require VSWR less than 1.5; therefore, the proposed prototype satisfies the VSWR requireent of the antenna. Return Loss S11 (db) Siulation Measured Figure 5. Return loss of proposed electrically sall antenna. Figure 6 shows the siulated refection coefficients for a paraeter study of the square length and width (W p )and(l p ) of cut on superstrate. Here, it can be seen that target frequencies are effectively optiized for W p and L p values of 5.6.

6 94 Patel and Upadhyaya Wp=Lp=4 Retrun Loss (db) -1 Wp=Lp=4.4 Wp=Lp=4.6 Wp=Lp=4.8 Wp=Lp= Wp=Lp=5.4 Wp=Lp= Figure 6. Return loss different values of Wp and Lp. -1 Return Loss (db) r1=1.5-2 r1=1.9-3 r1=2.1 r1=2.5-4 r1=2.7-5 r1=2.9-6 r1=2.93 r1= Figure 7. Return loss different values of r1. As illustrated in Figure 7, variation of circle on cut at superstrate value of (r1 ) significantly affects the atching characteristic of the lower and higher frequency bands. It is observed that as the value of r1 increases, the frequency bands shift towards the lower frequencies side. Also the value of r1 decreases, and the frequency bands shift towards the higher side of resonant range. The achieved optial value of circle (r1 ) is 2.9. The value of air gap height is varied fro 1.8 to 2.2 while keeping overall antenna electrical diensions consistent. The outcoe is illustrated in Figure 8. Due to reduction in effective dielectric constant, the antenna gain is iproved arginally up to 1 dbi. The optial value of ahs is found to be 1.8. The surface current density of the proposed antenna is depicted in Figure 9 which illustrates axiu current density on top copper of the design. Dielectric substrate used in the proposed design has high dielectric loss which leads to low antenna gain; however, FR4 aterial is less expensive, and it is readily available. Dual-band antenna resonates at 2.4 GHz and 5.5 GHz with bandwidths of 1% and 6%, respectively. The siulated and easured return losses have very good agreeent. The current is uniforly distributed through uch of the radiating eleent. The antenna easureent is carried out in a sized anechoic chaber as illustrated in Figure 1. The E-plane and H-plane radiation patterns for target frequencies are illustrated in Figure 11. E-plane and H-plane radiation patterns for 2.4 GHz are directive whereas at second resonance it is nonunifor. The antenna has radiation efficiency of 78% at 2.4 GHz and 75% at 5.5 GHz. The isatch

7 Progress In Electroagnetics Research Letters, Vol. 7, Return Loss (db) ahs='1.5' -1 ahs='1.7' -2 ahs='1.8' ahs='1.9' -3 ahs='2.1' ahs='2.3' -4 ahs='2.5' -5 ahs='2.7' ahs='2.9' Figure 8. Effect of air gap variation. (a) (b) Figure 9. An electrically sall antenna surface current distribution at (a) 2.4 GHz, (b) 5.5 GHz. (a) (b) Figure 1. Anechoic chaber for radiation easureent. (a) E-plane. (b) H-plane.

8 96 Patel and Upadhyaya (a) (b) (c) (d) Figure 11. Siulated & easured radiation patterns. (a) E-plane at 2.4 GHz. (b) H-plane 2.4 GHz. (c) E-plane at 5.5 GHz. (d) H-plane 5.5 GHz. Realized Gain (dbi) GHz GHz Figure 12. Realized gain of fabricated prototype. in results is due to aterial tolerance, easureent error and environental effect. The proposed electrically sall antenna can effectively cover the WLAN frequency range. Figure 12 illustrates realized gain of the fabricated prototype at 2.4 GHz and 5.5 GHz frequencies. Maxiu gains of 1 dbi and.26 dbi are achieved for 2.4 and 5.5 GHz frequency bands, respectively. Further gain enhanceent techniques can be eployed to iprove the antenna gain.

9 Progress In Electroagnetics Research Letters, Vol. 7, CONCLUSION A cost effective dual-band copact antenna for WLAN is presented. The vital benefit of the proposed antenna is extree ease and siplicity in design and fabrication. By erely engineering the patches on top substrate, target frequencies in L-band and S-band can be achieved. The antenna design can be further enhanced by eploying gain enhanceent techniques. REFERENCES 1. Fujioto, K. and H. Morishita, Modern Sall Antennas, 214, dx.doi.org/1.117/ CBO Palandoken, M. and H. Henke, Fractal negative-epsilon etaaterial, IEEE International Workshop on Antenna Technology (iwat), Stuart, H. R. and A. Pidwerbetsky, Electrically sall antenna eleents using negative perittivity resonators, IEEE Transactions on Antennas Propagation, Vol. 54, , Ghosh, B., S. Ghosh, and A. B. Kakade, Investigation of gain enhanceent of electrically sall antennas using double-negative, single-negative, and double-positive aterials, Physical Review E, Vol. 78, 26611, Upadhyaya, T. K., S. P. Kosta, R. Jyoti, and M. Palandöken, Novel stacked μ-negative aterialloaded antenna for satellite applications, International Journal of Microwave and Wireless Technologies, Vol. 8, , 216, doi:1.117/s x. 6. Kiouche, H. and S. Oukil, Electrically sall antenna with defected ground structure, 212 9th European Radar Conference (EuRAD), IEEE, Geng, J.-P., J. Li, R.-H. Jin, S. Ye, X. Liang, and M. Li, The developent of curved icrostrip antenna with defected ground structure, Progress In Electroagnetics Research, Vol. 98, 53 73, Thal, H. L., New radiation Q liits for spherical wire antennas, IEEE Transactions on Antennas Propagation, Vol. 54, No. 1, Chu, L. J., Physical liitations in onidirectional antennas, J. Appl. Phys., Vol. 19, , Wheeler, H. A., Fundaental liitations of sall antennas, IRE Proc., Vol. 35, , Patel, R. H., A. Desai, and T. Upadhyaya, A discussion on electrically sall antenna property, Microwave Opt. Technol. Letter, Vol. 57, , 215, doi: 1.12/op Yaghjian, A. D. and S. R. Best, Ipedance, bandwidth, and Q of antennas, IEEE Transactions on Antennas Propagation, Vol. 53, , Zhang, Y. and H. Y. D. Yang, Bandwidth-enhanced electrically sall printed folded dipoles, IEEE Antennas Wireless Propagation Letter, Vol. 9, , Li, L.-W., C.-P. Li, and M.-S. Leong, Near fields of electrically sall thin square and rectangular loop antennas, Progress In Electroagnetics Research, Vol. 31, , Dang, L., Z. Y. Lei, Y. J. Xie, G. L. Ning, and J. Fan, A copact icrostrip slot triple-band antenna for WLAN/WiMAX applications, IEEE Antennas Wireless Propagation Letter, Vol.9, , Peng, C.-M., I.-F. Chen, and J.-W. Yeh, Printed broadband asyetric dual-loop antenna for WLAN/WiMAX applications, IEEE Antennas Wireless Propagation Letter, Vol. 12, , Balanis, C. A., Antenna Theory Analysis and Design, 3rd Edition, Wiley, Hoboken, NJ, USA, McLean, J. S., A re-exaination of the fundaental liits on the radiation Q of electrically sall antennas, IEEE Transactions on Antennas Propagation, Vol. 44, , Bancroft, R. and H. A. Wheeler, Fundaental diension liits of antennas ensuring proper antenna diensions in obile device designs, Centurion Wireless Technologies, 214.