A THIN WIDEBAND MICROSTRIP PATCH ANTENNA WITH TWO ADJACENT SLOTS

Transcription

1 some EBG-like structures and both positive and negative refractions have been demonstrated from the field plots. REFERENCES 1. V.G. Vaselago, The electrodynamics of substance with simultaneously negative values of and, Soviet Physics USPEKHI 10 (1968), D.R. Smith, W.J. Padilla, D.C. Vier, S.C. Nemat-Naasser and Schults, Composite medium with simultaneously negative permeability and permittivity, Physical Rev Lett 84 (2000), D.R. Smith and N. Kroll, Negative refractive index in left-handed materials, Physical Rev Lett 85 (2000), A. Garbic and G.V. Eleftheriades, A backward wave antenna based on negative refractive index L-C networks, Proc IEEE Int Symp Antennas Propagat IV (2002), C. Luo, S.G. Johnson, J.D. Joannopolous, and J.B. Pendry, Negative refraction without negative index in metallic photonic crystals, Optics Express 11 (2003), P.V. Parimi, W.T. Lu, P. Vodo, J. Sokoloff, S. Sridhar, Negative refraction and left-handed electromagnetism in microwave photonic crystals, cond-mat/ , P.E. Mayes, G.A. Deschamps, and W.T. Patton, Backward-wave radiation from periodic structures and application to the design of frequency-independent antennas, Proc IRE, vol. 49 (1961), K.-C. Chen, Y.X. Qian, H.-K.C. Tzuang, and T. Itoh, A periodic microstrip radial antenna array with a conical beam, IEEE Trans Antennas Propagat 51 (2003), Y. Hao, S. Sudhakaran, and C.G. Parini, Spatial harmonic effects on characterisation of left-handed materials, Asia-Pacific Microwave Conf (APMC 03), Seoul, South Korea, R.E. Collin, Foundations for microwave engineering, McGraw-Hill Kogakusha Ltd., New York, 1966, Sec R.E. Collin, Antenna theory Pt 2, McGraw-Hill, New York, 1969, ch R. Chatterjee, Elements of microwave engineering, Ellis Horwood Ltd., Chichester, UK, C.A. Belanis, Advanced engineering electromagnetics, Wiley, New York, 1989, ch. 8. Figure 6 (a) Electric-field plot showing positive refraction at 6.8 GHz (units are in V/m). (b) electric-field plot showing negative refraction at 7.6 GHz (units are in V/m). [Color figure can be viewed in the online issue, which is available at than in the previous case; hence, we expect that the bandgap will shift towards the lower frequency. The results in Figure 5 indicate the bandgap region is around GHz. It can be seen from the dispersion diagram (Fig. 5) that a positive refraction may occur at 6.8 GHz, but a negative refraction may occur at 7.6 GHz. Similarly, this is verified from the field plots in Figures 6(a) and 6(b). Different scales are used to provide a better view of the refraction phenomenon. The animation of phase obtained from the simulation also supports this result. All simulations are done using Ansoft s HFSS. CONCLUSION Negative refraction from EBG-like structures has been analysed in detail and it was found that the spatial-harmonics effect is neglected by the analysis method based on unit-cell analysis. Although demonstrated as valid for bandgap prediction, such an approach is not sufficient for evaluating pass-band behaviour, and therefore fails to identify negative refraction. In this paper we have presented an entire-cell approach in order to consider the structure itself as a cell; through numerical simulation, it was found that the effects of spatial harmonics can play a vital role in negative-refraction prediction. The theoretical predictions were verified with the simulation results in 2004 Wiley Periodicals, Inc. A THIN WIDEBAND MICROSTRIP PATCH ANTENNA WITH TWO ADJACENT SLOTS G. Yang, M. Ali, and R. Dougal Department of Electrical Engineering University of South Carolina Swearingen Building Columbia, SC, Received 1 November 2003 ABSTRACT: A thin (only 5% of the operating wavelength, ) wideband microstrip patch antenna with two closely spaced slots is introduced. When connected to each other, the two slots translate into two distinct resonant frequencies, resulting in wide impedance bandwidth (about 20% within 2:1 VSWR). Detailed design data, including resonance frequency, impedance, and bandwidth, are presented as function of the antenna and slot parameters. It is also demonstrated that such antennas can be designed at different frequency bands by appropriately adjusting the antenna and slot parameters Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: , 2004; Published online in Wiley InterScience ( DOI /mop Key words: microstrip patch antenna; wideband antenna, slots MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 41, No. 4, May

2 1. INTRODUCTION Microstrip antennas are very popular due to their smaller size, conformal nature, and ease of integration with other components. They are especially attractive for many current wireless applications, including global positioning systems (GPS) and wireless local-area networks (WLANs). Their main limitation is their narrow impedance bandwidth, which is due to their lower profile and small size. For many wireless applications, wideband or multiband operation is generally required. For instance, to support the two 5 6-GHz WLAN bands ( GHz and GHz) a wideband or dual-band design will be needed. While such a bandwidth requirement can be satisfied by employing a stacked patch design, it would require multiple substrates and will generally be thicker. Miniature size and small thickness in small wireless devices, are very important so that antennas can be packaged within such devices [1 3]. Another alternative can be to introduce slots within the patch element in order to create multiple resonant frequencies. When these operating frequencies are generally close to each other, with the midband VSWR under a reasonable value (2:1, for instance) the resultant bandwidth can be wider. Rectangular patch antennas with U-shaped slots are described in [4 6] for wideband performance. Most of these designs require relatively thicker substrates. For instance, the impedance bandwidth achieved with U-slot patch antennas is about 25% for a substrate thickness of 0.1 [8]. In [7], several E-shaped wideband patch antennas are presented for operation in the GHz band. The bandwidths achieved were 21.2% and 32.3% for antenna heights ranging from 0.1 to In this paper, we focus instead on achieving wideband performance for an antenna on a thin substrate ( 0.05 ) because it is much easier to accommodate a thinner antenna into present-day wireless communication devices, both as an external appendage as well as an internal component. We consider Rohacell foam (equivalent to air in terms of dielectric constant) as the substrate material. We introduce two closely spaced slots along the diagonal of the patch, as shown in Figure 1. The slots are placed at the center of the patch and are connected to each by a gap x. Wide impedance bandwidth is achieved primarily by controlling the slot parameters L x, L y, and x. A detailed study of the antenna characteristics as functions of the antenna and slot parameters has been conducted using HFSS [8]. Prototype antennas have also been fabricated and tested to validate the simulation data. 2. ANTENNA DESIGN The geometry of the proposed antenna is shown in Figure 1. The slots are positioned at the center of the patch. Hence, slot edges 2 Figure 1 Geometrical configuration of the antenna Figure 2 Computed VSWR vs. frequency (parameters L 23 mm, W 20 mm, L x 6.25 mm, L y 7.0 mm, and f 3 mm) and 2 are equidistant from the patch edges. Similarly, all the other similar slot edges are also equidistant from the patch edges. The presence of the slots translates into two resonant frequencies, which in turn results in wideband performance. The slots also reduce the antenna resonant frequency. Thus, the resultant antenna is smaller than a conventional microstrip patch operating at the same frequency. The antenna parameters are as follows: length L, width W, height h, and feed position f. The slot parameters are L x and L y. 3. RESULTS 3.1. Initial Study of Slotted Antenna Initially, a mm patch antenna on a 3-mm-thick foam substrate was studied using HFSS [8]. A vertical 50 probe feed was considered with the probe location from the patch edge given by f (see Fig. 1). The probe feed was a 1-mm-wide vertical metal strip lying on the yz plane with a 1-mm-wide y dimension and a 3-mm-high z dimension. The slots were created using the perfect magnetic conductor (PMC) boundary condition. As a starting point, slot dimensions of 6.25 mm (L x )and7mm (L y ) were considered. Computed VSWR versus frequency data for this case are shown in Figure 2. It is apparent that the antenna has two resonances: one at 5 GHz and the other at 5.6 GHz. If we consider a 2:1 VSWR as the upper limit to define the operating bandwidth, the present antenna does not have any noticeable bandwidth. This is further evidenced from the plot of Figure 3, which shows that the impedance knot is away from the center of the Smith chart, thus indicating a poor impedance match. To improve the impedance matching of the antenna, a parametric study was conducted. First, the effect of slot 1 (Fig. 1) was studied. Initially, metal was added separately to each side of slot 1. For instance, when metal was added to side 1, all the other sides were unchanged. The results of adding 1 mm of metal to each side of slot 1 are shown in Figure 4. The term initial means initial data while A1,...,A4 indicates adding metals separately to the respective sides. Clearly, adding metal to side 3 has a positive effect on the antenna bandwidth characteristics. Any other change seems to have adverse or negligible effect. 262 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 41, No. 4, May

3 Figure 3 Computed input impedance (parameters L 23 mm, W 20 mm, L x 6.25 mm, L y 7.0 mm, and f 3 mm) Next, we studied the effect of removing metal from slot 1. The results of this study are shown in Figure 5. As seen in the figure, removing metal from side 1 made the antenna electrically longer and the low-frequency resonance shifted to a lower-resonance frequency. The effect of removing metal from side 2 seems beneficial. Similar studies conducted with slot 2 produced similar results. It was thus concluded that adding metal to slots had a negligible positive effect, while removing metal had some positive effects. Nevertheless, simply by adding or removing metals to and from the slots was not enough to obtain a reasonable impedance bandwidth. Figure 5 Computed VSWR data for the proposed slotted patch antenna when metal (1 mm) is removed from each side (1, 2, 3, and 4) separately while all other sides remained unaltered (parameters L 23 mm, W 20 mm, f 3 mm, and h 3 mm) 3.2. Further Improvement of Antenna Impedance Match Effect of Slot Gap x. Next, we studied the effect of slot gap x on the performance of the antenna. Computed VSWR versus frequency data for this case are shown in Figure 6, and the corresponding impedance data are shown in Figure 7. The slot dimensions are 6.25 mm (L x )and7mm(l y ). When x 0, this indicates that the slots are not connected anymore and also means that L x 6.25 (3.5 x)/ 2 when x 3.5 mm, while L x 6.25 ( x 3.5)/2 when x 3.5. It is observed that when x 0, the antenna is single band and resonant at 6.1 GHz. These results are outside the scale of Figure 6. Also, according to Figure 6, as x increases to 0.5 mm the antenna shows two distinct Figure 4 Computed VSWR vs. frequency data for the proposed slotted patch antenna when metal (1 mm) is added to each side (1, 2, 3, and 4) separately while all other sides remained unaltered (parameters L 23 mm, W 20 mm, f 3 mm, and h 3 mm) Figure 6 Computed VSWR vs. frequency with gap x as a parameter MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 41, No. 4, May

4 Figure 8 Computed VSWR vs. frequency with L x as the parameter Figure 7 parameter Computed input impedance vs. frequency with gap x as a resonances at 4.9 and 5.7 GHz. The antenna has about 250-MHz bandwidth centered at 4.9 GHz, while its bandwidth is about 450 MHz centered at 5.7 GHz. The midband VSWR is only as high as 2.6, while the total band extends about 1000 MHz (17%). As x increases to 1.5 mm, the VSWR response does not improve or degrade significantly. With a further increase in x to 3.5 mm, the VSWR curve moves higher and exceeds the 2:1 limit. Increasing x even further to 5.5 mm further deteriorates the VSWR bandwidth. Clearly, a narrow gap ( x 0.01 ) is required to achieve dual-frequency resonance with reasonable bandwidth for the two bands separately. Further investigation is required in terms of lowering the midband VSWR Effect of Antenna Height h. Antenna performance as function of substrate height is shown in Figure 11. From a height of 2.5 to 3.5 mm, the antenna easily retains its wideband characteristics. The antenna may also perform at larger antenna heights with wider bandwidth; however, the antenna and slot dimensions would have to be adjusted properly in order to achieve those performances Comparison of Computed and Measured Data. To demonstrate the scaling of the design, another antenna measuring mm was also analyzed. Computed VSWR data and measured results for this antenna and the mm antenna are shown in Figure 12. Agreement between the computed and measured data is quite good. The slotted-patch concept can be easily scaled for application at other frequency bands. The measured bandwidth for the mm patch is 19.2% with a substrate height of 0.053, while that for the mm patch is 20% for a substrate height of Effect of Slot Length, L x and Width, L y. Since the midband VSWR is generally higher, we decided to investigate the effects of slot length and width on the antenna s performance. Considering antenna parameters of L 20 mm, W 23 mm, x 1.0 mm, h 3 mm, and L y 7 mm slot length in the x direction, L x was varied as 4, 4.5, 5, and 5.5 mm. Computed VSWR data for this case are shown plotted in Figure 8. The effect on the lower part of the resonance frequency is strong; the resonance frequencies of 5.1, 5.0, 4.95, and 4.8 GHz change as L x changes to 4, 4.5, 5, and 5.5 mm, respectively. Therefore, adjusting L x is not very effective to lower the midband VSWR and improve antenna bandwidth. Similarly, L y was varied at 6.5, 7, and 7.5 mm when all the other parameters were kept constant. A value of L x 4.5 mm was selected because it showed better performance than the other values. Computed VSWR data for this case are shown in Figure 9. As is apparent from the figure, L y has a significant effect on antenna VSWR and bandwidth. This is further evidenced from the impedance plot shown in Figure 10. Increasing L y to 7.5 mm makes the antenna operate from 4.85 to 5.7 GHz (850 MHz of bandwidth) within a VSWR of 2:1. Therefore, the final antenna parameters selected are L 20 mm, W 23 mm, x 1.0 mm, h 3 mm, L x 4.5 mm, and L y 7.5 mm. Figure 9 Computed VSWR vs. frequency with L y as the parameter 264 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 41, No. 4, May

5 Figure 10 parameter Computed input impedance vs. frequency with L y as the Figure 12 Comparison of computed and measured VSWR data for two antennas (first antenna is mm with slot dimensions mm, f 4 mm, h 3 mm; second antenna is mm with slot dimensions mm, f 3 mm, h 3.8 mm) that of a square patch, the antenna will start to generate left-hand circular polarization Radiation Pattern The computed radiation patterns for these two antennas are shown in Figure 13. As expected, the radiation pattern for the mm antenna shows that the patterns are broadside to the antenna axis. The peak gain is 8 dbi. The co- and cross-polarized components differ by 10 db. Radiation patterns for the mm antenna are shown in Figure 13(c) and 13(d). These patterns clearly indicate that the field components have very nearly equal magnitudes. Thus, as the antenna dimensions approach close to 4. CONCLUSION A new, thin ( 0.05 ), slotted microstrip patch antenna with wideband characteristics has been presented. Wideband operation is achieved by utilizing two closely spaced slots in a rectangular microstrip patch antenna. When connected to each other, the two slots translate into wide impedance bandwidth ( GHz). For two prototypes, mm and mm, the measured bandwidth is 20% for an antenna height of Computed radiation-pattern data for the longer antenna show that the co- and cross-polarized components differ by about 10 db in both planes. Figure 11 Computed VSWR vs. frequency as function of antenna height h Figure 13 Computed normalized radiation patterns. [Color figure can be viewed in the online issue, which is available at MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 41, No. 4, May

6 This can be further improved by increasing the aspect ratio W/L of the antenna. However, as the aspect ratio decreases (for the mm antenna), the two field components become comparable. Fora21 21 mm antenna with the proposed type of slot, we found that the antenna generates left-hand circular polarization [9]. The 3-dB axial-ratio bandwidth obtained was 2.8%. The impedance bandwidth is much wider. Note that if the slots are reversed, right-hand circular polarization results without any change to the antenna s impedance bandwidth. REFERENCES 1. M. Ali, R.A. Sadler, and G.J. Hayes, A uniquely packaged internal inverted-f antenna for Bluetooth or wireless LAN application, IEEE Antennas Wireless Propagat Lett 1 (2002), K.V. Kumar, M. Ali, H.S. Hwang, and T. Sittironnarit, Study of a dual-band packaged patch antenna on a PC card for 5 6-GHz wireless LAN applications, Microwave Opt Technol Lett 37 (2003), M. Ali, T. Sittironnarit, H.-S. Hwang, R.A. Sadler, and G.J. Hayes, Wideband/dual-band packaged antenna for 5 6-GHz WLAN application, IEEE Trans Antennas Propagat (2004, to appear). 4. K.F. Tong, K.M. Luk, K.F. Lee, and R.Q. Lee, A broadband U-slot rectangular patch antenna on a microwave substrate, IEEE Trans Antennas Propagat 48 (2000), J.Y. Sze and K.L. Wong, Slotted rectangular microstrip antenna for bandwidth enhancement, IEEE Trans Antennas Propagat 48 (2000), C. Kidder, M. Ling, and K. Chang, Broadband U-slot patch antenna with a proximity coupled double -shaped feed line for arrays, IEEE Antennas Wireless Propagat Lett 1 (2002), F. Yang, X.X. Zhang, X. Ye, and Y. Rahmat-Samii, Wideband E- shaped patch antennas for wireless communications, IEEE Trans Antennas Propagat 49 (2001), Ansoft HFSS, Ansoft Corporation, 9. M. Ali, R. Dougal, G. Yang, and H.-S. Hwang, Wideband (5 6-GHz WLAN band) circularly polarized patch antenna for wireless power sensors, IEEE Antennas Propagat Soc Int Symp Dig 2 (2003), Wiley Periodicals, Inc. FREQUENCY-SELECTIVE SURFACE BASED BANDPASS FILTERS IN THE NEAR-INFRARED REGION Srikanth Govindaswamy, 1 Jack East, 1 Fred Terry, 1 Erdem Topsakal, 2 John L. Volakis, 3 and George I. Haddad 1 1 Solid State Electronics Laboratory Electrical Engineering and Computer Science Department University of Michigan Ann Arbor, MI Department of Electrical and Computer Engineering Mississippi State University Mississippi State, MS Radiation Laboratory Electrical Engineering and Computer Science Department University of Michigan Ann Arbor, MI He is also a Professor and the Director of ElectroScience Laboratory Electrical Engineering Department The Ohio State University Columbus, OH Received 7 November 2003 ABSTRACT: A spatial bandpass filter resonant at 1.5 m and based on a frequency-selective surface (FSS) was analyzed and fabricated. It consists of circular apertures arranged in a hexagonal lattice and was modeled using a hybrid finite-element/boundary-integral method, which accounted for metal thickness and conductivity. Measurements demonstrate the accuracy of the design Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: , 2004; Published online in Wiley InterScience ( DOI /mop Key words: arrays; bandpass filters; frequency-selective surfaces; electron-beam lithography; periodic structures 1. INTRODUCTION Frequency-selective surfaces (FSSs) consist of elements arranged in a planar periodic array to create a bandpass or bandstop filter. The properties of the filter can be varied by choosing the appropriate element type and its dimension and volumetric structure, and the spacing of the elements. The substrate also influences the behavior of the filter. FSSs have been used extensively in the microwave and millimeter region; hence, there is a good understanding of their behavior [1 3]. Of particular interest in this paper are FSSs operating in the near-infrared region. Analyses of FSSs at near-infrared frequencies were done in [4 9]. However, models of the metallization surface and the dielectric regions have been inadequate. Also, fabrication challenges exist because the critical dimensions of the FSS are less than 1 m. In this paper, we consider a new model for the metallization region of the FSS, which is volumetric and recognizes the plasmalike behavior of the metal at infrared wavelengths. Measurements based on a newly fabricated FSS are also given to verify the proposed model and related analysis. Using electron-beam lithography, the FSS is constructed as an arrangement of circular holes designed to operate as a bandpass filter in the optical and near-infrared frequencies. In the following sections, we present the analysis and validation along with the results, which led to the fabrication choices. The fabrication process is also discussed. 2. BANDPASS FILTER MODELING The bandpass filter was modeled using FSDA-PRISM, a code described in [11 13]. This code uses a hybrid finite element/ boundary integral (FE/BI) to analyze the electromagnetic scattering and radiation characteristics of infinite periodic planar arrays and FSS configurations. The finite-element formulation is used within the volumetric region with the boundary integral employed to terminate the mesh. Prismatic elements are employed for volume discretization in the FE regions and triangular elements in the BI region. The FE region is used to model the inhomogeneous sections in the thick metal layers, whereas multilayered uniform sections are modeled using the multilayer Green s function [13], and the BI-computation is accelerated by using the fast spectraldomain algorithm (FSDA) [12]. The filter consists of circular apertures arranged in a hexagonal lattice. In the ideal case, the circles should have a diameter equal to about /2 and separated by, where is the desired resonant wavelength. However, the dimensions must be scaled if the filters are fabricated on top of a substrate. The key to our modeling approach is the treatment of the metal layer (which incorporates the circular-aperture FSS) as a thick dielectric layer with appropriate dielectric constants and thicknesses [14]. The employed real and imaginary values of the electrical permittivity used in our model are given by r j i n 2 k 2 j2nk, where n and k are the real and imaginary values of the refractive index. These values are given in [15] for the wavelength range of 266 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 41, No. 4, May