INVESTIGATION OF AN H-SHAPED MICROSTRIP PATCH ANTENNA WITH INDUCTIVE LOADING

Transcription

1 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY 99 VOL.4, NO., MARCH 009 INVESTIGATION OF AN H-SHAPED MICROSTRIP PATCH ANTENNA ITH INDUCTIVE LOADING PRADYOT KALA Department of ECE, Allahabad Agricultural Institute-Deemed University, Allahabad-INDIA, Abstract: Theoretical results of shorting posts loaded H-shaped planar antenna are presented. Based on the transmission line model, a study of resonant frequency, return loss and saving in patch area of the antenna are presented, and effects of each parameter are also illustrated. The results of resonant frequency and return loss obtained by given theory are compared with simulated and experimental results reported earlier. It is found that resonant frequency, return loss and saving in patch area of the designed antenna are very sensitive to the dimensions of middle section of the patch. This method is simple, computationally efficient and fast for the calculation and also has a good physical insight. Index Terms: Compact antenna, H-Shaped antenna, Shorting-post, Transmission Line model, Microstrip antenna. I. INTRODUCTION ireless communication is widely recognized as one of the fastest growing industries. It requires small size, low cost and high performance transceiver systems. For this segment, microstrip antennas have been proven to be preferable over other types of radiating element due to its nonelectrical characteristic, low profile and lightweight. They can be made conformal and well suited to integration with microwave integrated circuit. In terms of fabrication, such system offers simplicity, so as to allow mass production and cost-effective manufacturing as well as high performance. The ever increasing demand for compact wireless communication equipment explicitly necessitates research in compact antenna options and has sparked interest of many researchers world-wide. Different design techniques to reduce antenna size have been reported in the literatures [-0. It has been noted that reactive loading such as, shorting pin, stubs, slots, and capacitors lead to significant size reduction. Another approach based on the increase of electrical length by cutting slits in the radiating patch has also been used. H-shape antenna is one of the examples of this approach. However in the literature, only very few attempts have been made towards the analysis of H-shaped microstrip antennas [0-0. Experimental results of an H-shaped microstrip patch for single-frequency operation was firstly published by Palanisamy and Garg [0. They observed that the H-shaped patch antenna is smaller in size (about half), and is broadbeam but with narrow bandwidth. Singh et al [ proposed an H-shaped patch antenna loaded with multiple shorting pins which will find application in MMIC design. This antenna occupies approximately one tenth of the substrate area of a half wavelength patch antenna. Gao et al [ proposed a dual- JMOT ISRAMT

2 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY 00 VOL.4, NO., MARCH 009 frequency, compact antenna, which uses an H- shaped microstrip patch with a shorting pin. Compared with the conventional recgular patch antenna, this antenna can achieve both a significant reduction of antenna size and a dualfrequency operation with a single feed. Sheta and Mahrnoud [3 observed that compared to conventional microstrip patch antennas, H-shape microstrip antennas have less number of modes, absence of harmonic resonance, and the provision of pure reactive impedances at its harmonics that can result in increased antenna efficiency. Abdel et al. [4 studied a multi-band compact H-shaped microstrip antenna. The resonant modes of the H-shaped structure were analyzed using the concept of electric and magnetic walls at the planes of symmetry. It was shown that dual, triple, or quad-band operation is possible by the proper location of a coaxial feed. Many methods have been developed to achieve the analysis of arbitrary shaped microstrip antennas such as: transmission-line model, cavity model and the moments method [5. Titaouine et al. [5 proposed the cavity model in conjunction with the transmission line model to analyze arbitrary shaped microstrip antennas. This new approach is used to analyze an H- shaped microstrip antenna. The results brought in much more accurate formulation for the input impedance, resonant frequency, radiation resisce. Lee and Tseng [6 analyzed that when patch segments of different widths are adjoined to form a microstrip antenna, the resonant frequency is significantly lowered than that of a conventional recgular microstrip antenna with the same overall length. However, the radiation efficiency substially reduced as the antenna becomes smaller at a given frequency. u et al. [7 observed that compactness can further be achieved by enhancing capacice, which is produced by introducing a small gap between the edge of the patches and a supporting ground plane. Liu and Kao [8 proposed a simple design of a circularly polarized probe-feed H-shaped microstrip antenna by embedding dual slits in a square microstrip antenna. Ansari et al. [9 analyzed an H-shaped patch using equivalent circuit model. They found that the antenna exhibits dual resonance behavior which is very sensitive to the dimensions of the notch. In this paper, a theoretical investigation of an H- shaped microstrip patch antenna, loaded with multiple shorting post based on transmission line model, is presented. II. THEORETICAL FORMULATION The configuration of the probe-fed H-shaped microstrip antenna and its physical dimensions are presented in Figure. The antenna consists of an H-shaped microstrip patch, supported on a grounded dielectric sheet of thickness h and dielectric const ε r. The H-shaped patch can be divided into three parts consisting of a center conductor strip with length L and width and two identical conductor strips with length L and width on its two sides. One of the radiating edges of the patch is shorted by n numbers of equidist shorting posts. r s is radius of shorting posts and the feed point is located at the central line of the H-shaped patch, at a disce of y 0 from the shorted radiating edge. A. Calculation for Resonant Frequency The equivalent circuit of the shorting post loaded H-shaped microstrip patch antenna is shown in Figure. In this circuit, transformer is electrical equivalent of the junction of narrow and wide sections of the patch which maintains the current continuity at the junction. An inducce, connected at the narrow section of the patch, which allows for the distortion of the lines of magnetic flux due to the change in current density at the junction. On the other hand a capacice connected across the wide section side of the junction, represents the electric field energy stored in the partially open end of the wider microstrip section. The outer open ended portion of the patch is represented by an impedance open-end while X s represents an inductive reacce offered by a shorting pin. JMOT ISRAMT

3 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY 0 VOL.4, NO., MARCH 009 JMOT ISRAMT

4 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY 0 VOL.4, NO., MARCH 009 A simple transcendental equation for the unknown wave number k can be obtained by imposing continuity of both the electric and magnetic fields at the junctions which may be derived as jx N jb yr jx j jx N jb y R where y L and y R N j jb y L n G jb jx s j Y 0 n G jb jx s j Y0 G jb j Y0 G jb j yl Y0 ( kl ) ( kl ) 0 [ k( L h) [ k( L h) [ k( L h) [ k( L h) () () (3) G and B are conducce and suscepce of open ends of the patch respectively, n is number of shorting posts, and x is normalized reacce ( x X 0 ) which allows for the distortion of lines of magnetic flux due to change in current density at the junction between the two strips and can be approximated as [ x e π. e. k.ln csc. e (4) hile b is normalized suscepce (b B c Y0 ) which may be taken to represent the electric field energy stored in the partially open end of the wider microstrip patch section and is thus situated on the wider strip side of the junction and is given by [6 4k b where k e k e m m k ym and k ym e mπ sin e k k m ym e (5) mπ (6) and X s is inductive reacce offered by a shorting post [ X s h λ0 0 π γ ln (7) λ 0 π rs ε r Since both characteristic impedances have been normalized here by taking them as unity, the ideal transformer :N appears explicitly in the circuit representation. If the characteristic impedances of the two transmission lines involved are defined as 0 (for narrow part) and 0 (for wider part), respectively, then N 0 e (8) 0 e where e and e if width of the strip is too large compared to substrate thickness otherwise. e and e (9) JMOT ISRAMT

5 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY 03 VOL.4, NO., MARCH 009 where and are the fringing field widths along the edges of narrow and wide patches respectively and for approximation it may be assumed as half of substrate thickness. R Bc N jx 0 R 0 j j 0 R ( kl) ( ) kl (6) Using above equations and approximations, the resonant frequency of the patch is thus evaluated by jx R (7) N R3 f r k c (0) 4π ε r ( kl) ( ) kl R4 j 0 R 3 0 (8) Bc 0 j R4 where c is the speed of light. B. Calculation for Return Loss From Figure, the input impedance offered by the patch may be given by jx () in p A B where X p denotes the self reacce of the probe and is given by [3 R 4 (9) G jb Using the values of input impedance, the return loss can be computed using the following relations reflection coefficient in 0 ρ (0) o where o is the characteristic impedance of the coaxial feed. Thus in X p 0π π h ε r λo () return loss 0log 0logρ () ρ A and B are the impedances offered by the left and right hand part of the patch respectively where probe is connected and may be written as j [ ky0 [ ky L 0 A 0 (3) 0 j L 0 j [ k( L y0) [ k( L y ) R 0 B 0 (4) 0 j R 0 where L is left hand side impedance of section- A and R is right hand side impedance of section-b. L (5) n G jb jx s C. Calculation for Patch Area The patch area for a given H-shape antenna is A H L L () and the patch area of a simple recgular patch which resonates at the resonant frequency of H- shape patch antenna is [ A R f r c ε eff l (3) Thus, the saving in patch area in percentage may be written as JMOT ISRAMT

6 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY 04 VOL.4, NO., MARCH 009 AR AH Saving 00 % (4) A R III. DESIGN SPECIFICATIONS In this work, a compact H-shaped microstrip antenna is proposed which is loaded with multiple shorting posts at one of the edge of the structure to achieve more compactness and its results (resonant frequency and return loss) are compared with existing experimental and simulated results [. The physical and electrical parameters of the designed microstrip antenna are as follows: Substrate thickness h mm Length of H-shape L 9.55 mm Length of middle section L 4.55 mm idth of H-shape mm idth of middle section, 3,5,7,9 and mm. Position of probe y 0 mm (from shorted end). Relative permittivity and permeability of substrate materials are: ε r. and µ r.0 Using above data, computations were carried out for various parameters such as resonant frequency, return-loss and saving in patch area and the data thus obtained are plotted in Figures 3-7. negligence of mutual coupling between all the radiating edges. Figure 4 shows the variation of resonant frequency with / for different numbers of shorting posts. The ratio of resonant frequencies from n0 to n0 for different values of / is varying from The ratio of resonant frequencies from / 0. to /.0 varies from.3 (for n 0) to.96 (for n 0), Resonant Frequency [GHz n 0 theoretical n 0 measured [ n 0 simulated [ / Figure 3: Variation of resonant frequency with / for n0, L.5 mm. IV. RESULTS AND DISCUSSION The variation of resonant frequency, return loss and saving in patch area of an H-shape microstrip antenna for different width of middle section and different number of shorting posts are plotted in Figures 3-7. Figure 3 shows the variation of resonant frequency with / for the patch loaded with 0 shorting posts. It is observed that the resonant frequency of the patch decreases as / decreases. The results are in good agreement with measured and simulated results obtained by Singh et al [, especially at higher values of /. The possible reason of deviation of resonant frequency for lower values of / is Resonant Frequency [GHz n 0 n n n 3 n 4 n 5 n 6 n 7 n 8 n 9 n 0 theoretical / Figure 4: Variation of resonant frequency with / for different numbers of shorting posts. JMOT ISRAMT

7 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY VOL.4, NO., MARCH and maximum variation of resonant frequency is.57 GHz to 8.85 GHz. Figure 5 shows the variation of return loss with frequency for different width of middle section and n 0. From this figure it is observed that the theoretical results are in good agreement with experimental as well as simulated results obtained by Singh et al [. The possible reason for the deviation of resonant frequency at lower values of can be explained as was explained earlier in the case of Figure 3. Figure 6 shows the variation of resonant frequency with L /L for different width of middle section and n0. It is observed that lowest possible resonant frequency is obtained when mm and L /L0.3. It is also observed that the point of lowest resonant frequency is shifted towards lower values of L /L for higher values of. Return Loss [db mm 3 mm 5 mm 7 mm mm Theoretical Measured [ Simulated [ Frequency [GHz Figure 5: Variation of return loss with frequency for different width of middle section and n0. Resonant Frequency [GHz L / L Figure 6: Variation of resonant frequency with L /L for different width of middle section and n0. Figure 7 shows that the variation of saving in patch area in percent with L /L for different width of middle section and n0. From this figure it is observed that the loading of shorting posts saves approximately 50% patch area for mm (for recgular patch loaded with 0 shorting posts). On other hand saving in patch area increases as L /L and decreases, which is maximum up to 9% ( mm and L /L0.) compared to a simple recgular patch resonate at same resonant frequency. Area reduction in % mm 3 mm 5 mm 7 mm 9 mm mm L / L Figure 7: Variation of saving in patch area in percent with L /L for different width of middle section and n0. JMOT ISRAMT

8 INTERNATIONAL JOURNAL OF MICROAVE AND OPTICAL TECHNOLOGY VOL.4, NO., MARCH V. CONCLUSION In the present work, it is demonstrated that the proposed inductively loaded H-shaped microstrip patch antenna shifts the resonant frequency to lower side by (i) increasing the number of shorting posts, (ii) decreasing / and (iii) optimizing the value of L /L, this in turn reflects its potential for size reduction and hence its suitability for wireless communication. VI. ACKNOLEDGEMENT The author would like to thank Prof. A. K. Srivastava and Prof. (Col.) Gurmit Singh, C.E.T., A.A.I.-D. University, Alaahabad, India for their valuable suggestions in the preparation of this manuscript. REFERENCES. J. L. Volakis and J. M. Jin, A scheme to lower the resonant frequency of the microstrip patch antenna, IEEE Microw. Guid. ave Lett., vol., pp. 9-93, 99.. R. aterhouse, Small microstrip patch antenna, Electron. Lett., vol. 3, pp , J. George, M. Deepukumar, C. K. Aanandan, P. Mohanan, and K. G. Nair, New compact microstrip antenna, Electron. Lett., vol. 3, pp , J. George, C. K. Aanandan, P. Mohanan, and K. G. Nair, Analysis of a new compact microstrip antenna, IEEE Trans. Antennas Propagat., vol. 46, pp. 7-77, H. R. Kan and R. B. aterhouse, Size reduction techniques for shorted patches, Electron. Lett., vol. 35, pp , C. S. Lee and K. H. Tseng, Size reduction of microstrip antennas, Electron. Lett., vol. 37, pp , L. Desclos, Y. Mahe, S. Reed, G. Poilasne, and S. Toutain, Patch antenna size reduction by combining inductive loading and short-points technique, Microwave Opt. Technol. Lett., vol. 30, pp , B. Paul, S. Mridula, C. K. Aanandan, and P. Mohanan, A new microstrip patch antenna for mobile communications and Bluetooth applications, Microwave Opt. Technol. Lett., vol. 33, pp , M. Pauison, S. O. Kundukulam, C. K. Aanandan, and P. Mohanan, Analysis and design of a dual port compact microstrip antenna, Microwave Opt. Technol. Lett., vol. 3, pp. 5-7, V. Palanisamy and R. Garg, Recgular ring and H- shaped microstrip antennas an alternatives to recgular patch antenna, Electronics Letters, vol., pp , D. Singh, P. Gardner, and P. S. Hall, Miniaturized microstrip antenna for MMIC applications, Electron. Lett., vol. 33, pp , S. C. Gao, L.. Li, T. S. Yeo, and M. S. Leong, A dual frequency small microstrip antenna, IEEE AP-S Symp. Dig., pp , Abdel Fattah Sheta and Samir F. Mahrnoud, A novel H-shaped patch antenna, Microwave Opt Technol Lett, vol. 3, pp. 6-65, A. F. Sheta, A. Mohra, and S. F. Mahmoud, Multiband operation of a compact H-shaped microstrip antenna, Microwave Opt. Technol. Lett., vol. 35, pp , Titaouine, Mohammed, and Djahli, Farid, Simple and efficient approach for the analysis of arbitrary shaped microstrip antennas, Recent advances in European Antennas Research within COST 84, pp.33-36, C. S. Lee and K. H. Tseng, Radiation efficiency of electrically small microstrip antennas with width discontinuities, IEEE Trans. Antennas Propagat., vol. 53, pp , u, D. Lacey, G. Drossos, L. E. Davis, T.. Button, and P. Smith, Comparative study of miniaturized HTS microstrip H-shaped antennas with and without enhanced capacice, Physica C, pp. 8-87, C. Liu and P.C. Kao, Design of a probe-fed H- shaped microstrip antenna for circular polarization, Journal of Electromagnetic aves and Applications, vol., pp , J. A. Ansari, Satya Kesh Dubey, Prabhakar Singh, R. U. Khan, and Babau R. Vishvakarma, Analysis of compact H-shaped microstrip antenna, Microwave Opt Technol Lett, vol. 50, pp , B. Davor, R. Bojan, Small H-shaped shorted patch antennas, Radioengineering, vol. 7, pp. 77, H. M. Altschuler and A. A. Oliner, Discontinuities in the center conductor of symmetric strip transmission line, IRE Transactions on Microwave Theory and Techniques, vol. MTT-8, pp , I. J. Bahl and P. Bhartia, Microstrip Antennas. Dedham, MA: Artech House, K. Carver and J. Mink, Microstrip Antenna Technology, IEEE Trans. Antennas Propagat., vol. 9, pp. - 3, 98. JMOT ISRAMT