Effects of Two Dimensional Electromagnetic Bandgap (EBG) Structures on the Performance of Microstrip Patch Antenna Arrays Mr. F. Benikhlef 1 and Mr. N. Boukli-Hacen 2 1 Research Scholar, telecommunication, Faculty of Technology, University of tlemcen, Algeria. 2 Professor, Telecommunication, Faculty of Technology, University of tlemcen, Algeria. Abstract: Microstrip patch antennas became very popular in mobile and radio wireless communication because of their easy analysis and fabrication, and their attractive radiation characteristics. However, they have some drawbacks of low efficiency, narrow bandwidth and surface wave losses. A new solution method using electromagnetic bandgap (EBG) materials as substrates, has attracted increasing attention. Unlike other methods, this new method utilizes the inherent properties of dielectric materials to enhance microstrip antenna performance. In this paper, the effects of a two-dimensional EBG Structures (operating at 2.4GHz) on the performance of microstrip patch antenna arrays are investigated using the Ansoft High Frequency Selective Simulator (HFSS TM )in the first part. In the second part of the paper, two element microstrip patch antenna array on a uniform substrate suffer from strong mutual coupling due to the pronounced surface waves. Therefore, 2D-EBG Structures are integrated into the antennas to reduce the mutual coupling. Keywords: Electromagnetic band gap (EBG), Microstrip patch antenna (MPA), mutual coupling, Surface waves. INTRODUCTION In recent years, different types of antennas have been used in wireless services. Very popular planar antennas (e.g. patch ones) excel in a low cost, a low profile and a simple mass production. On the other hand, patch antennas exhibit a narrow bandwidth and low gain. Moreover, surface waves can be excited in the substrate, which decreases the antenna efficiency [1][2][3]. Electromagnetic band gap (EBG) structures became widely used in microwave- and radio engineering. They have drawn a lot of interests in the electromagnetics and antenna community recently. The EBG structures are periodical cells consisting of metallic and dielectric elements. Two major characteristics of EBG structures are to reflect incident plane waves in-phase rather than out of phase and to prohibit the propagation of surface waves in a certain frequency band. The suppression of surface wave propagation leading to the enhanced antenna gain and the suppressed back radiation improves the performance of antenna [4][5][6]. The aims of this paper are the design of a 2D-photonic crystal that gives the suitable bandgap to suppress the antenna surface waves, the design of a rectangular microstrip antenna with EBG structure to operate at 2.4GHz frequency with improvement in the maximum radiation pattern and the integration of the 2D- EBG into an antenna arrays system to evaluate the mutual coupling for both frequency domains. Band gap characterization of the EBG structure Indeed, the formation of the bandgap is dependent on the periodicity of the crystal, but it is also heavily dependent on the refractive index (dielectric constant) ratios between the base material (the substrate as a whole) and the impurities that form the crystal. Typically, the refractive index ratio must be at least 2: 1 (substrate-to-impurity) ratio for the bandgap to exist [7]. A photonic crystal essential behaves much like a bandstop filter, rejecting the propagation of energy over a fixed band of frequencies. However, once a defect is introduced such that it disrupts the periodicity in the crystal, an area to localize or trap electromagnetic energy is established. In this region, a passband response is created. This ability to confine and guide electromagnetic energy has several practical applications at microwave frequencies as filters, couplers, and especially antennas. The idea is to design a patch antenna on a 2D photonic crystal substrate, where the patch becomes the defect in the crystal structure. In this case, a crystal array of cylindrical air holes are patterned into the dielectric substrate of the patch antenna. In the paper, a purely dielectric EBG structure is used to suppress surface waves. The EBG structure is formed by holes drilled into the dielectric material Taconic CER-10(tm) with the dielectric constant 10, dissipation factor 0.0035, and thickness 3.125mm. Taconic CER-10(tm) is chosen due to the higher excitation of surface waves. The cut off frequency can be calculated using [8]: n. c f c = 4. h ε r 1 (1) Where c is the velocity of light in free space, h is the substrate thickness, and ε r its dielectric constant, n = 0, 2, 4, for TM modes and n = 1, 3, 5, for TE modes.. According to results, all higher order surface wave modes are safely away from the working (f c1 = 8GHz),it is only needed to eliminate the TM 0(excited mode). The EBG structure should be designed so that the antenna operating frequency should be located at the center of the Bandgap. To determine the suitable EBG structure dimensions and the appropriate TM bandgap for our antenna to work for Wi-Fi application frequency band. We studied Variation of Bandgap Length with the Filling Factor. The study is restricted for 0.4 Rc/a 0.5. The obtained results of 7472
Variation of the first TM bandgap Length with the Filling Factor are shown in figure 1, the antenna operating frequency is 2.4GHz. Figure 2: Dispersion diagram of the designed EBG structure. (In MATLAB) Figure 1: Variation of length of the first TM bandgap with respect to the filling factor. By inspection of the results of Figure 1, we find that as the filling factor increases, _ The normalized center frequency increases because it is inversely proportional to the equivalent dielectric relative permittivity which is reduced by the insertion of the air holes. _ The bandgap length (BGL) increases since of the surface wave suppression gets stronger by increasing the filling factor. A trade off should be made to design a photonic crystal of low lattice constant (low normalized center frequency) and wide bandgap. Therefore, to fit the desired requirements we selected (Rc/a)=0.48. where (Rc=18.5mm) is the radius of holes, (a=38.541mm) is the lattice constant. The dispersion diagram that characterize our found EBG structure is shown in Figure 2. It reveals the first and second bandgaps for TM polarization. The dispersion diagram that characterize our found EBG structure is shown in Figure 1. It reveals the first and second bandgaps for TM polarization. It is seen from the Figure 1, two TM-band gaps are appeared, the first one is between [2.06-2.74] the second is between [3.8-4.05]. Antenna design and configuration In order to identify and verify the improvement of the performance of microstrip antenna on EBG substrates, designed a conventional antenna and the proposed antenna. The width of the rectangular patch antenna is usually chosen to be larger than the length of the patch, L to get higher bandwidth. The antenna is designed to operate at frequency 2.4GHz In this paper, we use Taconic (tm) dielectric material as patch substrates whose dielectric constant is 10. The antenna is fed by a coaxial probe. The point of excitation is adjustable to control the impedance match between feed and antenna, polarization, mode of operation and excitation frequency. Table1 shows the important parameters for the geometrical configuration of the patch antenna. Table II: Geometrical configuration of the patch antenna. Antenna part Parameter value Patch lenght 17.75mm Wide 26.7 mm Patch substrates Dielectric constant 10 TaconicCER10(tm) Height 3.125mm Loss tagent 0.0035 2D-EBG structure Radius 18.5mm Lattice constant 38.54mm Before proposing the EBG patch antenna we should determine the needed number of periods of our photonic crystal, Translight software is used to calculate the TM polarization transmission and reflection coefficients for different periods of extension. The results are depicted in Figure 3. 7473
(a) Figure.4: Proposed EBG patch antenna Simulation results and discussion It is a common practice to evaluate the system performances through computer simulation before the real time implementation. A simulator Ansoft HFSS based on finite element method (FEM) has been used to calculate return loss, radiation pattern and gains. Figure 5 shows the simulated results of the return loss of the conventional antenna with and without EBG structures. (b) Figure.3: TM polarization photonic crystal responses for different periods of extension (a)transmission response (b) Reflection response. We note that for a number of periods less than 5 periods, there is a transmission or reflection response. To obtain a transmission response about 0% (or reflection response about 100%) the crystal structure must extend at least five periods (5a ) (5a). Figure 3 shows the proposed EBG patch antenna. Figure 5: Return losses of the patch antenna with and without EBG It is seen from the Figure 4, the return loss for the conventional patch antenna is 27.2dB at 2.4GHz and for the proposed patch antenna is -32.2dB at 2.405GHz. From simulation results we have observed that the minimum loss get at 2.4 GHz for conventional antenna and 2.405GHz for the proposed antenna. Thus the return loss of the proposed microstrip patch antenna is 15.6% less compared to the conventional microstrip patch antenna. 7474
The simulated results for gain that are obtained from conventional antenna and the proposed antenna on EBG substrates are shown in Figure 6-a and Figure 7-b. Figure 7: Array of two patches in dielectric substrate including a 2D-EBG (a) The computed results by using HFSS TM are shown in (Figure 8). The EBG structures of vacuum holes resonate at 2.4 GHz with return loss is 26 db. The mutual coupling of the antennas without the EBG structure is 38.4 db. In comparison, the mutual coupling of the patches with the EBG structure is only 40.23 db. An approximately 2 db reduction of mutual coupling is achieved. (b) Figure 6: (a) Gain of the conventional rectangular patch antenna (b) Gain of the rectangular patch antenna with EBG. From the simulated results, it is shown that the gain of the proposed patch antenna on EBG substrates is 8% more than the conventional patch antenna. Mutual coupling reduction Now, to evaluate mutual coupling, a two-element array in the Taconic CER-10(tm) dielectric substrate is studied, (Figure 6). The separation between edges has been chosen to be 67.05mm and the total separation between elements is 75mm which is 0.75λ 0. Ground plane size is (200*250)mm 2. With these dimensions only two periods of the 2D- EBG(a=38.541mm,r=18.5mm) can be placed in E-plane. Figure 8: Computed results of microstrip antennas with and without the EBG structure. In table II, we studied mutual coupling bandgap with respect to the filling factor. Results show that BGL increases since of the surface wave suppression gets stronger by increasing the filling factor. The computed results by using HFSS TM are shown in table 2. It is observed that all EBG structures with different filling factors (Rc/a) inserted between the patch antennas reduce the mutual coupling. When BGL >700 MHz, mutual coupling is 40.4 db implies that mutual coupling reduction is linked with wide band gap. 7475
Table II: Obtained S21 results with by using HFSS TM a (mm) Rc(mm) BGL (GHz) S21 (db) 30.962 12.385 0.106-40.04 31.538 12.930 0.154-40.08 32.462 13.634 0.226-40.09 33.275 14.308 0.286-40.03 34.388 15.131 0.374-40.04 35.625 16.031 0.451-40.10 37.000 17.020 0.530-40.12 38.388 18.042 0.602-40.15 41.475 20.323 0.714-40.39 43.025 21.513 0.758-40.36 To decrease the coupling between two patches, it is suggested to modify the designed EBG antenna by replacing its ground plane with a suitable well-designed metallodielectric EBG structure and study the resultant novel EBG antenna, informing that the Sievenpiper mushroom structure. The operation mechanism of this EBG structure can be explained by an LC filter array [9]: the inductor L results from the current flowing through the vias, and the capacitor C due to the gap effect between the adjacent patches. One unusual but important feature of mushroom-like EBG structures is the inphase reflection characteristics. The frequency where reflection phase is zero is the resonance frequency of the structure. At this frequency, the structure behaves like an artificial magnetic conductor which does not exist in nature. With the reflection phase ranging from +90 degrees to -90 degrees, the reflected wave interferes with the incident wave in-phase rather than out of phase. Using HFSS simulation tools, EBG structures are designed to include in-phase reflection for a normally incident plane wave on their surfaces. The Fig.9 shows the unit cell model and simulation setup of these EBG structures. The Reflection phase characteristic of the EBG structure is show in figure 10. (a) (b) Figure 9: Mushroom like EBG (a) simulated model, (b)ebg structures dimensions g m=0.5mm, w m=11mm Figure 11: Simulated results of reflection phase for EBG structure 7476
The simulated center frequency and bandwidth of EBG structures show that the center frequency is the frequency point with a reflection phase of 0 is 2.4 GHz and bandwidth is defined as ±90 crossings for the reflection phase is 87.4 MHz. Differents columns of these mushroom-like patches are inserted between the antennas and were originally designed to have a gap at approximately 2.4 GHz figure 11. The computed results by using HFSSTM are shown in Figure 12 CONCLUSION In the paper, the EBG concept was applied to the design of a simple patch antenna with operating frequency 2.4 GHz to suppress the surface wave propagation in the dielectric substrate and enhance the gain, the proposed patch antenna on EBG substrates is 8% more than the conventional patch antenna. In the second part, the EBG structures were inserted into two patch antenna to reduce the mutual coupling; an approximately 2 db reduction of mutual coupling is achieved. When the filling factor gets higher than 0.40, more significant band gaps appear. The EBG structure can be utilized to reduce the antenna mutual coupling between array elements. The lowest mutual coupling is obtained in the mushroom-like patches structure case as a 5 db reduction is achieved.. REFERENCES Figure 11: Array of two patches in dielectric substrate including a 2D-EBG. Figure12: Computed results of microstrip antennas with and without the EBG structure. It is observed that for the antennas without the EBG structure, the mutual coupling is 38.4 db. In comparison, the mutual coupling of the antennas with the EBG structure (1 coln.) is 40.57 db while the EBG structure (2 coln.) only 41.82 db and the EBG structure (3 coln.) only 42.86 db. From this computed demonstration, it can be concluded that the EBG structure can be utilized to reduce the antenna mutual coupling between array elements. The reduction of mutual coupling improved by increasing number of columns of mushroom-like patches structure. [1] Ittipiboon, A., Garg, R., Bahl, I., Bhartia, P. Microstrip antenna Design Handbook. Norwood: Artech House, 2000 [2] Jing Liang, and Hung-Yu David Yang, Radiation Characteristics of a Microstrip Patch over an Electromagnetic Bandgap Surface, IEEE Transactions on Antennas and Propagation, Vol. 55, June 2007, pp1691-1697. [3] K.L. Wong, Compact and Broadband Microstrip Antennas. New York: Wiley, 2002. [4] D. Sievenpiper, L. Zhang, R. F. J Broas, N. G. Alexopolus and E.Yablonovich, "High-impedance electromagnetic surface with a forbidden frequency band," IEEE Trans. Microwave Theory Tech., vol. 47, pp. 2059-2074, Nov. 1999. [5] Yang, L., Fan, M., Chen, F., She, J., Feng, Z. A novel compact electromagnetic-bandgap (EBG) structure and its applicationsfor microwave circuits. IEEE Transactions on Microwave Theory and Techniques, 2005, vol. 53, no. 1, p. 183 190. [6] Y. D. Yang, N. G. Alexopoulos, E. Yablonovitch. «Photonic band gap materials for high-gain printed circuit antennas.» IEEE Trans. on Antennas and Prop., 45(1),, 1997. [7] Rumsey, I., M. Piket May, and P. K. Kelly, Photonic bandgap structures used as filters in microstrip circuits, IEEE Microwave and Guided Wave Letters, Vol. 8, 336 338, 1998. [8] Lin, Q., Zhu, F., He, S. A new photonic bandgap cover for a patch antenna with a photonic bandgap substrates. Journal of Zhejiang University, 2004, no. 5, p. 269 273. [9] Sievenpiper, D., L. Zhang, R. F. J. Broas, N. G. Alexopolus, and E. Yablonovitch, High-impedance electromagnetic surfaces with a forbidden frequency band, IEEE Trans. Microwave Theory Tech., Vol. 47, 2059 2074, Nov. 1999. 7477