Analysis and Design of L-strip Proximity Coupled Circular Microstrip Antenna

Transcription

1 192 Analysis and Design of L-stri Proximity Couled Circular Microstri Antenna Ganga Prasad Pandey 1*, Binod Kumar Kanaujia 2 1* Deartment of Electronics and Communication Engineering, Maharaja Agrasen Institute of Technology, Rohini, Delhi, INDIA , ganga.mait@gmail.com 2 Deartment of Electronics and Communication Engineering, Ambedkar Institute of Technology, Geeta Colony, Delhi, INDIA , bkkanaujia@yahoo.co.in Surendra. K. Guta 3 and A. K. Gautam 4 3 Deartment of Electronics Engineering, Ambedkar Institute of Integrated technology, Shakarur, Delhi, INDIA, surendrashubhi@gmail.com 4 Deartment of Electronics and Communication Engineering, G. B. P. E. C. Pauri Uttarakhand, INDIA, gautam1575@yahoo.co.in Abstract An L-stri roximity couled circular microstri antenna is roosed. The structure is investigated using circuit theoretic aroach and simulated using IE3D simulation software. The atch is designed on a thick substrate of thickness of 11 mm for a design frequency of 3.74 GHz and rovides ultra wide band oeration. The numerical results for inut imedance, VSWR, radiation attern, efficiency and gain are resented. Bandwidth is found to be deendent on length of horizontal art of L-stri. A bandwidth of 69.52% is achieved (for VSWR 2) for y 0 =0.112λ 0 and h 2 =0.097λ 0. The beam of antenna rotates with oerating frequency. Index Terms L-stri feed, Tunability, roximity couled, wideband, radiation attern, antenna gain, radiation efficiency, return loss, directivity. I. INTRODUCTION Microstri atch antennas have been attracting the antenna designers because they offer the features of low rofile, light weight, and comatibility with integrated-circuit technology. Recently, atch antennas have been receiving a great interest in various wireless communication systems since they can rovide advantages over traditional antennas in terms of efficiency and electromagnetic couling to the human head. In many alications, the requirements of bandwidth, tunability and hysical size are quite imortant. Moreover, many efforts have been devoted to bandwidth widening techniques of microstri antennas, including short-circuited termination for microstri-fed slot antennas by Ching- Lieh Li, Pei-Ying Lin, and Chun-Kai Huang [1], dual slot loading by Amit A. Deshmukh and Girish

2 193 Kumar [2], use of notch by Y. H. Ge, K. P. Esselle, and T. S. Bird [3], U-shaed ground lane by W. H. Hsu, and K. L. Wong [4]. For thick atch antenna, the coaxial feed is tyically used. However, the robe inductance limits its imedance bandwidth to less than 10%. K.M. Luk, Y.X. Guo and K.F. Lee [5] used U-slot and L-robe feed, P.S. Hall [6] used robe comensation, C.L. Mak, K.F. Lee, and K.M. Luk [7] used T-shae robe for feed and T. Huynh and K.F. Lee [8] used L-shae robe to overcome band limitation roblem. Many researchers have used L-shaed microstri line as a feed line. M.K. Meshram used L-stri to to achieve a bandwidth of 56.67% [9] in rectangular microstri antenna. Zhongbao Wang, Shaojun Fang, and Shiqiang Fu used modifies L-stri to 22% bandwidth with imroved gain of 9 dbi [10]. In this aer, ultra-wideband roximity couled L-stri fed Circular Microstri Antenna (CSMA) has been resented. Using a foam layer of thickness 11 mm as a substrate, an imedance bandwidth of 69.52% and gain of uto 8 dbi has been achieved which is better than earlier reorted results by T. Huynh and K.F. Lee [8]. No otimization was adoted in the design. The antenna is simulated using IE3D software. The comuted results using circuit theoretic aroach agree well with simulated data. The simulation for antenna efficiency, radiation efficiency, radiation attern, gain and directivity has also been carried out. II. THEORETICAL INVESTIGATION The L-stri roximity couled circular microstri antenna is analyzed using circuit theoretic aroach and cavity model. Broad banding is achieved by using thick substrate. But this reduces couling between atch and microstri feed. Various techniques have been used to counter this roblem. The antenna given Y. X. Guo, K. M. Luk and K. F. Lee [11] is taken as reference for comarison. In the resent analysis L-shaed micro-stri feed is used. The roosed structure is shown in fig. 1. The antenna structure contains a thick substrate of thickness H. An L-shaed stri line is designed to coule the ower to atch electromagnetically. This L-shaed feed is connected to a standard microstri feed which in turn is connected to source. The fig. 2 shows equivalent circuit of roosed antenna. The length of horizontal art of L-stri under atch is ket less than quarter wavelength because u to λ/4 length of an oen circuited stub, the nature of imedance is caacitive. The caacitance thus introduced is suressed by the inductance arising from vertical art of L-stri. Aart from these, a series resistance arises due to finite conductivity of coer used. The exressions of series resistance (R s ) and series inductance (L s ) as given by R. K. Huffman (1987) [12] are Ls = 0.2h2[ln{2h 2 /( ws + ts)} + (nh) (1) {( ws + ts) / h2} + 0.5]

3 194 w S CMSA To view 2a Microstri feed h 2 h 3 y 0 H Side view GND h 1 Fig 1. Structure of roosed antenna. C s1 C f2 Z in R s L s C f1 C 1 C f2 L C R Fig 2. Equivalent circuit of L-stri roximity couled CSMA. R s = 4.13h2 ( ws + ts) f. ρ / ρ 0 (2) Where w s is width and t s is thickness of stri in mm, h 2 is height of L-stri, f is oerating frequency in GHz, ρ is secific resistance of the stri (Ω cm) and ρ 0 is secific resistance of coer. All antenna metallization is taken as erfect excet vertical ortion. There is a caacitance (C s1 ) arising due to vertical electric fields between horizontal art of L-stri and ground lane in series with above L s and R s and is calculated as C = ε rε 0wsy0 /( h1 2) (3) s 1 + h Where y 0 is enetration of L-stri into atch ε r is relative dielectric constant and ε 0 is dielectric constant of vacuum. There is a fringing caacitance between oen end of L-stri and ground lane (C f1 ), between oen end of L-stri and atch (C f2 ) and between radiating edge of atch and horizontal art of L-stri (C f2 ). These caacitances are calculated by evaluating extended effective length of L-

4 195 stri. The exression of extension in the length of an oen ended microstri line is given by T. C. Edward [13] and is given as l e = 0.412h( εe + 0.3)( ws / h ) ( εe 0.258)( ws / h + 0.8) (4) Where ε e is effective dielectric constant of material buried under the microstri line and ground lane. From T. C. Edward [13] the associated fringing caacitance is calculated as Cf = le εreff / cz 0 (5) Where l e is extension in length of L-stri feed, c is velocity of light in vacuum, Z 0 is characteristic imedance of feed and ε reff is effective dielectric constant. The fringing caacitance between horizontal art of L-stri and ground lane (C f1 ) is calculated by utting h=h 1 +h 2 and the two caacitances between atch and horizontal art of L-stri (both C f2 ) is calculated by utting h=h 3. Fringing caacitance between atch and L-stri is calculated using equations (4) and (5), ignoring curvature of atch. The caacitance due to vertical electric field between horizontal art of L-stri and atch is calculated as C1 εrε / h = 0 y0ws 3 (6) The equivalent circuit of L-stri fed circular microstri antenna is shown in fig. 2. The structure contains a series RLC resonant circuit in series with a arallel RLC resonant circuit. The arallel RLC circuit is equivalent of circular microstri antenna. The resonance resistance R of atch, antenna caacitance C and inductance L are calculated by Stuart A. Long, Liang C. Shen, Mark D. Walton and Martin R. Allerding [14] and is given as R 2 2 = J ( k( a y0)) /[ G J { ka}] (7) n T n C = QT /{ 2πf resr } (8) And L = R /{ 2πf resqt } (9) Where Q T is total quality factor, G T is total conductance of atch of radius a incororating radiation loss, conduction loss and dielectric loss [15] and f res is resonant frequency of atch [16]. Thus total inut imedance of the circuit is given as

5 196 Z in = R + (1/ R s + jωl ) + s 1 jωc 1 + jωc total + (1/ jωl ) (10) where C total is total caacitance arising due to L-stri (i. e. C 1, C s1, C f1, and C f2 ) and is calculated as C total ( C1 + 2C f 2 )( Cs1 + C f 1) = (11) ( C + 2C + C + C ) 1 f 2 s1 f 1 The reflection coefficient of the antenna is given as Zin Z 0 Γ = (12) Zin + Z 0 and the VSWR is calculated as 1+ Γ VSWR = (13) 1 Γ III. DESIGN PARAMETERS The basic design arameters of the roosed antenna are same as taken by Y. X. Guo, K. M. Luk and K. F. Lee [11] for comarison urose. The radius of atch (a) is 17 mm, total height (H) of substrate is 11mm, and dielectric constant is 1.07 (foam layer). The arameters which are new for the design are - height of microstri feed (h 1 = 1.6 mm or 0.02λ 0 ), height of L-stri (h 2 = 7.8 mm or 0.097λ 0 ) and ga between circular atch and horizontal art of L-stri (h 3 = 1.6 mm or 0.02λ 0 ). The width and length of L-stri are 5mm and 9.5 mm (0.097λ 0 ) resectively. The design frequency of the antenna is 3.74 GHz (λ 0 = 80.2 mm). A 50 ohms microstri line on 1.6 mm thick substrate was taken to feed the ower to L-stri (w s = 5 mm). IV. RESULTS AND DISCUSSIONS The L-stri roximity couled microstri CMSA is analyzed and the results are comared with the ones obtained by Y. X. Guo, K. M. Luk and K. F. Lee [11]. The variation of inut imedance with frequency for different horizontal length of L-stri of roosed structure is shown in fig. 3. The caacitive nature of antenna increases with horizontal length of L-stri. The resonance resistance decreases as oen end of L-stri moves towards center of atch. This indicates that oen end is working as feed oint. The variation of VSWR with frequency for different horizontal length of L-

6 197 stri is shown in fig. 4. The fig. shows that matching imroves with the horizontal length of L-stri. At the same time, bandwidth decreases due to increased quality factor of the structure. The bandwidth for different y 0 is given in Table I. it is clear that bandwidth decreases with increase in y 0 at constant value of h 2. The simulated result is also given in the table which shows a close resemblance with calculated bandwidth. Fig. 5 shows variation of inut imedance at various heights of L- stri for fixed horizontal length of L-stri (y 0 =0.112λ 0 ). With the height of L-stri the inductive Re[Zin], Im[Zin] Frequency(GHz) [R real art, X---Imaginary art] Fig 3. Variation of inut imedance with frequency for different L-stri lengths h 2 =0.097λ 0. TABLE I. BANDWIDTH FOR DIFFERENT y 0 AT h 2 =0.097 Λ 0. Result y 0 f H (GHz) f L (GHz) f(ghz) %BW 0.106λ Calculated 0.112λ λ λ Simulated 0.112λ

7 198 Frequency(GHz) Fig 4. Variation of VSWR with frequency for different horizontal length of L-stri at h 2 =0.097λ 0. Re[Zin], Im[Zin] VSWR Frequency(GHz) Fig 5. Variation of inut imedance with frequency for different height of L-stri at y 0 =0.112 λ 0.

8 199 nature increases which is obvious. The variation of VSWR with frequency at different height of L- stri is shown in fig. 6. The bandwidth for different height of L-stri is given in Table II. The bandwidth decreases with height of L-stri. It is very similar to bandwidth variation with length of horizontal art of L-stri (y 0 ). Again the simulated and calculated results are in good agreement. The antenna was simulated on Zealand IE3D v 14.0 software [17]. The variation of VSWR and inut imedance at y 0 =0.112λ 0 and h 2 =0.097λ 0 are shown in figs. 3, 4, 5 and 6. The calculated results using circuit theoretic aroach and simulated results were in good agreement. TABLE II. BANDWIDTH FOR DIFFERENT h 2 AT y 0 =0.112 λ 0. Result H 2 f H (GHz) f L (GHz) f(ghz) %BW Calulated 0.091λ λ λ Simulated 0.097λ VSWR Frequency(GHz) Fig 6. Variation of VSWR with frequency for different heights of L-stri y 0 =0.112 λ 0.

9 200 The return loss of an antenna shows how well antenna ort is matched with source. A good return loss or VSWR alone is not measure of a good antenna as it does not tell how well the radiation is taking lace. Hence investigation of Directivity, radiation efficiency, antenna efficiency and antenna gain is required. Radiation efficiency of an antenna is defined as ratio of ower radiated to ower given to antenna excluding return loss at antenna ort. However, the antenna efficiency is defined as ratio of radiated ower to actual ower fed to antenna (includes return loss at ort). For a good antenna high gain, high efficiency is desirable. The variation of directivity at different oerating frequency is shown in fig. 7. It is clear that directivity is maximum (8.4 dbi) at the design frequency and it remains above 6 dbi for the entire range. The fig. 8 shows variation of maximum antenna gain Frequency (GHz) Fig 7. Variation of maximum field directivity with frequency. Gain (dbi) Directivity (db) Frequency (GHz) Fig 8. Variation of Gain of the antenna with frequency.

10 201 with frequency. The gain of the antenna remains more than 4 dbi for the entire range of oeration ( GHz).Total antenna efficiency and radiation efficiency is shown in fig. 9. The antenna efficiency remains above 80% for the entire range. Total antenna efficiency is above 70%. The radiation attern of the roosed antenna using IE3D at 3.1 GHz, 3.8 Ghz and 4.5 GHz is shown in Fig. 10. It is also observed that beam rotates with frequency of oeration. The radiation achieves its eak Radiation efficiency Antenna efficiency f=3.1ghz f=3.8ghz f=4.5ghz Fig 9. Variation of efficiency with frequency. E total (db) Efficiency (%) Thita(degree) Fig 10. Radiation attern of roosed antenna. at , , and at for 3.1GHz, 3.8 GHz and 4.5GHz frequencies resectively. The variation of beam width and its direction shift is shown in table III. The wave takes definite time to reach at the

11 202 feed end and get couled to the atch. This time delay causes hase difference which in turn affects the total field in the far field zone. Hence antenna beam rotates for different frequency of oeration. Table III. Beam Rotation at Different Oerating Frequency. Oerating frequency First half ower oint Maximum radiation oint Second half ower oint Beamwidth ( θ= θ 1 -θ 2 ) (θ 1 ) (θ 0 ) (θ 2 ) 3.1GHz GHz GHz V. CONCLUSION A novel L-stri fed circular microstri antenna has been resented for ultra wideband alication. An equivalent circuit was given for the structure and calculations were carried out for circular atch of 11 mm thickness. Various antenna roerties were investigated using circuit theoretic aroach and results were verified with simulation. The roosed antenna has an oerating frequency range from 2.85GHz to 5.45GHz (2.9GHz to 5.5GHz simulated) and bandwidth of 69.52% which is better than earlier reorted bandwidth (35%) and gain (8 dbi). It may also be concluded that the inut imedance is very sensitive to variation in horizontal length and height of L-stri feed. The beam of antenna is rotating with the frequency of oeration. REFERENCES [1] Ching-Lieh Li, Pei-Ying Lin, and Chun-Kai Huang, Imedance Bandwidth Imrovement for Microstri fed Slot Antennas Using Short Circuit Termination, Microwave and otical Tecnology Letters, Vol. 45, No. 1, , [2] Amit A. Deshmukh and Girish Kumar, Broadband Pairs of Slot Loaded Rectangular Microstri Antennas, Microwave and Otical Technology Letters, Vol. 47, No. 3, , [3] Y. H. Ge, K. P. Esselle, and T. S. Bird, A Comact E-shaed Patch Antenna with Corrugated Wings, IEEE Trans. Antennas and Proagation, vol. 54, no. 8, , Aug [4] W. H. Hsu, and K. L. Wong, Broad-band Probe-fed Patch Antenna with a U-shaed Ground Plane for Cross- Polarization Reduction, IEEE Trans. Antennas and Proagation, vol. 50, no. 3, , Mar [5] K.M. Luk, Y.X. Guo, and K.F. Lee, L-robe Proximity Fed U-Slot Patch Antenna, Electron Lett 34, , [6] P.S. Hall, Probe Comensation in Thick Microstri Patches, Electron Lett, 23, , [7] C.L. Mak, K.F. Lee, and K.M. Luk, Broadband Patch Antenna with a T-shaed Probe, Proc Inst Elect Eng 147, , [8] T. Huynh and K.F. Lee, Single Layer single Patch Wideband Microstri Antenna, Electron Lett 31, , 1995.

12 203 [9] M.K. Meshram, Analysis of L-stri Proximity Fed Rectangular Microstri Antenna for Mobile Base Station, Microwave and Otical Technology Letters, vol. 49, no. 8, , Aug [10] Zhongbao Wang, Shaojun Fang, and Shiqiang Fu, Wideband Dual-Layer Patch Antenna Fed by a Modified L- Stri, Journal of Microwaves, Otoelectronics and Electromagnetic Alications, Vol. 9, No. 2, , [11] Y. X. Guo, K. M. Luk and K. F. Lee, Regular Circular an Comact Semicircular Patch Antennas with a T-robe Feeding, Microwave and Otical Technology Letters, Vol. 31, No. 1, , [12] R. K. Huffman, Handbook of Microwave Integrated circuits, Artech House, Narwood, MA, [13] T. C. Edward, Foundation for Microstri Circuit Design, john Wiley, [14] Stuart A. Long, Liang C. Shen, Mark D. Walton and Martin R. Allerding, Imedance of a Circular Disc Printed Antenna, Electronics letters, Vol. 14, No. 21, [15] F. Abboud, J. P. Damiano and A. Paiernik, A New Model for Calculating the Inut Imedance of Coax-Fed Circular Microstri Antennas with and without Air Gas, IEEE Transaction on Antenna and Proagation, Vol. 38, No. 11, , [16] Debatosh Guha, Resonant Frequency of Circular Microstri Antennas with and without Air Gas, IEEE Transaction on Antenna and Proagation, Vol. 49, No.1, , [17] Zeland Software Co., IE3D v14.0, California, USA