64 L-strip Proximity Fed Broadband Circular Disk Patch Antenna 1 Prabhakar Singh* and 2 Dheeraj Kumar 1 Department of Applied Physics Delhi Technological University, New Delhi, India-110042 2 Babasaheb Bhimrao Ambedkar University, Lucknow *E-mail: prabhakarsingh3@gmail.com Abstract: In the present communication the design and analysis of L-strip proximity fed disk patch antenna is presented. The variation of horizontal and vertical length of L-strip has been investigated in circular disk patch antenna. The bandwidth of the antenna is found inversely proportional to the horizontal length of L-strip whereas it is almost invariable with vertical length of L-strip. Typically the bandwidth of this antenna is found to be 16.63 % with maximum gain of 8.515 dbi. Further L- strip proximity fed U-slot loaded disk patch is studied. In this geometry bandwidth of the antenna increases up to 37.01% when the vertical length of L-strip is 6.5 mm but there is marginal effect in the bandwidth with the variation of horizontal length of L-strip. Gain of this antenna varies from 8.195 dbi to 7.015 dbi for the entire operating frequency band. Radiation pattern of both the antennas is stable across the operating band. Index Terms: Proximity fed disk patch, L-strip patch, and broadband patch antenna I. INTRODUCTION During the last decade, much effort has been devoted to bandwidth enhancement of microstrip antennas. Several techniques such as applications of matching networks [1], multiple resonators [2], using different shapes of slots [3-4] and lossy materials [5] have been reported. Recently, work using L-shaped probes which allows single layered structure has been experimentally demonstrated [6-7] with improved bandwidth. An L-shaped strip/probe antenna has an excellent feeding structure suitable for wideband patch antennas with a thick substrate. Since this L-strip/probe feed introduces capacitance between the feed and the radiating patch and this capacitance cancels out the inductance due to a feed strip/probe itself. This effect makes it possible to broaden the impedance bandwidth of the antenna. In general, this type of feeding can be easily implemented by bending a straight strip or probe into an L- shape [8]. In this paper analysis of L-strip proximity fed disk patch antenna is carried out using IE3D antenna simulator [9]. Using this feeding technique a comparative study has been made between a simple disk and U-slot loaded disk patch antenna. Various antenna parameters such as return loss, antenna gain, radiation pattern have been obtained. This new kind of antenna exhibits excellent impedance bandwidth with considerable gain. In this configuration one can optimize the various antenna parameters by adjusting the dimensions of L-strip apart from the dimensions of U-slot in the patch. This is the advantage over the coaxially fed U-slot loaded patch antenna. The proposed antenna can be used for Wi-MAX (3400 MHz-3690MHz). II. ANTENNA GEOMETRY AND DESIGN SPECIFICATIONS Top view of the antenna is shown in Fig.1a, in which U- shaped slot is incorporated. First L- strip proximity fed simple disk with radius R is taken for study. The L-strip is added at the right end of the strip line (Fig. 1b) so that the total height between strip line and radiating patch can be reduced and also the amount of electromagnetic coupling between horizontal length of L-strip and disk patch can be increased (Fig. 2). Further when U-shaped slot is incorporated in radiating disk patch, two different currents flow in the patch which are responsible for the dual
65 resonance behavior (Fig.3). If the resonances are close they merge and give broadband nature. The resonance frequency of circular disk patch is calculated by k nm c f r = 2 π R ε e r R w s w Y 0 L s Fig.3. Current distribution for U-slot loaded disk patch at 3.766 GHz (a) Top view of the proposed antenna L b Radiating disk patch in which k nm = mth zero of the derivative of Bessel function of order n c= velocity of light R = effective radius of the circular disk [10] ε r e =dielectric constant of the substrate h 1 The design specifications for disk patch antenna is given in Table-1 h 2 Y o H Table 1 h 3 Dielectric constant of the substrate (ε r ) 1.05 (Foam) (b) Side view of the proposed antenna Fig.1. Geometry of the antenna Thickness of the substrate (H) Width of strip line (w) Thickness from ground plane to strip line(h 3 ) Thickness from strip line to horizontal L-strip(h 2 ) Thickness from horizontal L-strip to radiating patch (h 1 ) Horizontal length of L strip (Y o ) Radius of disk (R) 8.0 mm 6.0 mm 1.5mm 4.5mm 2.0 mm 16 mm 15 mm Fig.2. Current distribution for disk patch antenna at 5.02 GHz The design specifications for U-slot loaded disk patch antenna is given in Table-2
66 Table 2 Dielectric constant of the substrate (ε r ) Thickness of the substrate (H) Width of strip line (w) 1.05 (Foam) 11.50 mm 6.0 mm Thickness from ground plane to strip line(h 3 ) Thickness from strip line to horizontal L-strip(h 2 ) Thickness from horizontal L- strip to radiating patch (h 1 ) Horizontal length of L strip (Y o ) Radius of disk (R) Width of U slot (w s ) Length of U slot (Ls) U slot base length (L b ) 1.5 mm 6.1 mm 3.9 mm 15 mm 15 mm 2.5 mm 14.5 mm 14.0 mm III. DISCUSSION OF RESULTS The variation of return loss with frequency for different value of horizontal length (Y o ) of the L- strip for given value of L-strip vertical length (S) has been given in Fig.4. It is observed that the bandwidth of the antenna increases with decreasing value of horizontal length but, matching condition decreases. The resonance frequency shifts towards lower side as the value of Y o increases. In Fig.5 variation of return loss with frequency for different vertical length (S) of L-strip is shown. From the figure it is found that resonance frequency of the antenna decreases as the value of S decreases however, the bandwidth increases marginally with decreasing value of S. Fig.4. Variation of return loss with frequency for different value of L-strip horizontal length (Y o ) In Fig. 8 variation of return loss with frequency for different value of horizontal length (Y o ) of L- strip for U-slot loaded disk patch antenna is shown. It is observed that bandwidth of antenna remains almost invariable with Y o but as the value of Y o increases beyond 16 mm antenna exhibits as dual band resonator. The bandwidth of U-slot loaded disk patch antenna increases when vertical length (S=h 2 ) of L-strip increases (Fig.9). The optimum value of bandwidth is found to be 37.01 % at S=6.5 mm. Fig.10 shows the variation of antenna gain with frequency. From the figure it is found that gain is around 8 dbi from frequency range 3.121 GHz to 4.077 GHz which is supposed to be satisfactory however, it varies from 8.042 dbi to 6.513 dbi for entire range of operating frequency band (3.121 GHz- 4.414 GHz). Maximum gain of the antenna obtained from the simulation is 8.515 dbi at 5.03 GHz (Fig. 6) and it is varying from 8.195 dbi to 7.015 dbi for the operating frequency band (4.55 GHz to 5.37 GHz) of the antenna. E-theta and E-phi planes are shown in Fig.7. Good radiation pattern has been obtained at the central frequency 5.03 GHz.
67 Fig.7. Radiation pattern for disk patch antenna Fig.5. Variation of return loss with frequency for different value of vertical length (S) of L-strip Fig.6. Antenna gain versus frequency for disk patch antenna Fig.8. Variation of return loss with frequency for different value of horizontal length (Yo) of L-strip for U-slot loaded disk
68 (a) E θ -Plane Fig.9. Variation of return loss with frequency for different value of vertical length (S) of L-strip U-slot loaded disk patch (b) Eφ- plane Fig.11. Radiation pattern for L-strip proximity fed U- slot loaded disk patch antenna Fig.10. Antenna Gain versus frequency for U-slot loaded disk patch
69 Radiation pattern for the U slot loaded disk patch antenna is shown in Fig.11 for two different frequencies 3.14 GHz and 4.01 GHz. U-slot loaded patch design shows a better performance in terms of radiation pattern and offers more design alternatives compared to the rectangular patch design. 10. Shen, L. C., Resonant frequency of a circular disk printed circuit antenna, IEEE Trans. Antennas Propagat., Vol. 25, pp. 595-596, 1977. IV. CONCLUSIONS In this paper a design of L-strip proximity fed disk patch antenna embedded with U-shaped slot has been presented. It is found that antenna bandwidth and resonance frequency is highly dependent on L-strip dimensions. The maximum antenna gain for simple disk patch and U-slot loaded disk patch are 8.75 dbi and 8.274 dbi respectively with good radiation pattern. REFERENCES 1. Pues, H. F., and A. R. Van De Capelle, An Impedance-matching Technique for Increasing the Bandwidth of Microstrip Antennas, IEEE Trans. Antennas Propaga. Vol. 37, pp. 1345-1354, 1989. 2. Ansari, J. A., N. P. Yadav and Prabhakar Singh, Analysis of disk patch antenna with parasitic elements in single and multilayer structures, Microwave and Optical Technology Letters,Vol 52, pp. 865-870, 2010. 3. Ansari, J. A., Prabhakar Singh, S. K. Dubey, R. U. Khan and B. R. Vishvakarma, H-shaped stacked patch antenna for dual band operation, Progress In Electromagnetics Wave Research B, Vol. 5, pp. 291-302, 2008. 4. Lee, K. F.,K. M. Luk, K. F. Tong, S. M. Shum, T. Huynh, and R. Q. Lee, Experimental and simulation studies of the coaxially fed U-slot rectangular patch antenna, Inst. Elect. Eng. Microwave Antennas Propagation, Vol. 144, pp. 354 358, 1997. 5. Wong, K. L., and Y. F. Lin., Small Broadband Rectangular Microstrip with Chip-resistor Loading, Electronics Letters. Vol.33, pp.1593-1594,1997. 6. C. L. Mak, K. M. Luk, and K. F. Lee, Wideband L- strip fed microstrip antenna, in Proc. IEEE Antennas and Propagation Society Int. Symp., Orlando, FL, July 1999, pp. 1216 1219. 7. Y. X. Guo, C. L. Mak, K. M. Luk, and K. F. Lee, Analysis and design of L-probe proximity fed-patch antennas, IEEE Trans. Antennas Propagat., Vol. 49, pp. 145 149, 2001. 8. Wu, G. L., W. Mu, G. Zhao, and Y.-C. Jiao A novel design of dual circularly polarized antenna fed by L- strip, Progress In Electromagnetics Research, Vol. 79, pp. 39 46, 2008 9. IE3D simulation software, Zeland, version 14.05,2008.