A New FSS Design Proposal for UWB Applications Rossana M. S. Cruz*(1), Adaildo G. D’Assunção(1), and Paulo H. da F. Silva(2) (1) Federal University of Rio Grande do Norte, Department of Electrical Engineering, Campos Universitário, Lagoa Nova, CEP: 59072-970, Natal – RN , Brazil, Phone: +55(84) 3215-3700 Emails:
[email protected],
[email protected] (2) Federal Institute of Education, Science and Technology of Paraíba, Microwaves and Applied Electromagnetism Group, Av. Primeiro de Maio, 720, Jaguaribe, CEP 58015-430, João Pessoa - PB, Brazil
Phone: +55(83)32083055, Fax: +55(83)32083088 Email:
[email protected]
ABSTRACT: This work presents a new proposal for Frequency Selective Surface (FSS) design for Ultra-Wideband (UWB) applications. The new FSSs consist of an array composed by the association of two patch elements per cell: a square loop and a crossed dipole. These structures are called Crossed Loops and have the objective of increasing the bandwidth of the square loop and the crossed dipole, when analyzed separately. Simulated results of the transmission coefficients are obtained using the Ansoft DesignerTM commercial software. Some of the FSSs tested were fabricated in order to make an experimental analysis with the measured results and to validate the new proposal. In particular, the presented results for the transmission coefficient of a built FSS prototype indicate a percent bandwidth 52.4%. INTRODUCTION The technology of frequency selective surfaces [1–4] has a long history of development. The analysis got started in the mid-1960s because the FSSs present a great potential in both commercial and military sectors. Over the past few decades, they have been studied by many researchers [5-7] and used for a variety of applications in microwave and optic bands, for example, to provide operation multiple frequency bands. A plane FSS is a periodic structure of infinitely many identical cells. The structure acts as a filter for an incident electromagnetic plane wave. The periodic surfaces have shown so far exhibit perfect reflection or transmission only at resonance. However, many applications call for a resonant curve with a flat top and faster roll-off. There are essentially two ways to accomplish this goal [8]: i) use two or more periodic surfaces cascaded behind each other without dielectrics; ii) use dielectric slabs sandwiched between cascading periodic surfaces, which leads to the so-called hybrid periodic surfaces. This work suggests a new way to design a periodic surface in order to obtain flat top curves that are characteristic of wideband applications. The new structure investigated consists of two patches per cell and is used as a starting point to model wideband frequency selective surfaces supported by a single dielectric layer. In this case, we consider the low cost FR-4 fiberglass substrate with relative permittivity εr = 4.4, thickness d = 1.5 mm and dielectric loss tangent tan(δ)= 0.02. The array formed by square loop and crossed dipole elements (Crossed Loops) is printed on the fiberglass substrate in order to be investigated in terms of some electromagnetic parameters such as the transmission coefficient, resonant frequency and bandwidth. Simulated results of the transmission coefficients are obtained using the Ansoft DesignerTM commercial software. Some of the FSSs tested were fabricated in order to make an experimental analysis with the measured results and to validate the new proposal. THE CROSSED LOOP FSS The conventional bidimensional FSS structure is shown in Fig. 1(a). The basic configurations are made of periodic metallic patch elements printed on a dielectric substrate with relative permittivity εr and thickness d. The arrays are composed by cells with periodicity tx and ty, along x and y axis, respectively. Each array acts as a stop-band filter where the electromagnetic waves incident on the structure are reflected around the resonant frequency.
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Fig. 1. (a) The conventional geometry of a FSS; (b) transmission coefficient for the crossed dipole: fr = 10 GHz. The dimensions of the crossed dipole and the square loop used in this work are shown in Figs. 1(b) and 3(b), respectively. The unit cell dimensions remain the same for both elements (tx = ty = 15 mm). The crossed dipole was designed to resonate at 10 GHz and the square loop was designed to resonate at frequencies from 6 GHz to 9 GHz, depending on the length l (in mm) of the square loop aperture. The variation on the dimensions of the length l was accomplished to observe the bandwidth (BW) and the resonant frequency (fr) presented by the structures. After that, the new structures formed by the two patch elements described above were designed in order to obtain an increase in the bandwidth. The crossed loop configuration is shown in Fig. 5(a). RESULTS Initially a parametric analysis was carried out to verify the behavior of FSS spatial filter design with square loop patch elements. Figure 3 shows the results of the transmission coefficient for the square loops as a function of the slot length l. We can observe that the electromagnetic parameters (fr and BW) are inversely proportional to the geometric parameter l. Therefore, an increase in the square loop slot dimensions implies a reduction at resonant frequency and bandwidth. The obtained values of these parameters are summarized in Tab. 1. In order to optimize the narrow bandwidth of the crossed dipole structure, (about 5.6%), some variations in the crossed dipole width (W) dimensions were accomplished. New crossed loop FSSs were then designed based on these variations and the results are shown in Fig. 4. It can be observed that for the same crossed loop structure (l = 6.25 mm), the variation of the crossed dipole width also allows a significant increase in the range of resonant frequencies and a desired flat top characteristic is observed for all resonance curves. The values for the resonant frequencies and the bandwidths are directly proportional to the geometric parameter in this case and they are shown in Tab. 2. In order to validate the proposal, some FSS prototypes had been constructed and measured using a vector network analyzer (model N5230A, Agilent Technologies). Figure 5 (a) shows the measured and simulated results for a built crossed loop FSS prototype considering for l = 9.0 mm, L = 9.0 mm and W=0.9 mm. Each of the arrays consists of a plate with 20 cm of height for 15 cm of width, as can be seen in Fig. 5(b). We can observe a very good agreement between measured and simulated results in the range 7.0 ~ 14.0 GHz. Tab. 3 shows the corresponding values of measured and simulated resonant frequency and bandwidth. CONCLUSION This work considered the design and analysis of a new periodic array consisting of the combination of two patches per cell, which were called crossed loop elements, and were printed on a single layer of the low cost FR-4 fiberglass dielectric substrate. The analysis was performed with the aid of the Ansoft DesignerTM commercial software for simulations and some of the FSS structures designed were fabricated and experimentally analyzed. It was confirmed that the crossed loop arrays enable a significant increase in the bandwidth of the FSSs, making possible their use in ultra-wideband applications. Besides, the authors propose a manner of analyzing and obtaining
periodic surfaces that present resonant curves with a flat top and faster roll-off, two important characteristics of UWB structures. The measured results validated the new proposal and the parametric investigation of the square loop slot and the crossed dipole width dimensions. The variations of l and W became an attractive way to fine-tune the location of the resonant frequency as well as the bandwidth for the new FSS structures. Those geometric parameters must be taken into account for the specifications of such band-stop spatial filters. REFERENCES [1] T. K. Wu, Frequency Selective Surface and Grid Array. New York: John Wiley & Sons, Inc., 1995. [2] J. C. Vardaxoglou, Frequency Selective Surfaces: Analysis and Design. New York: Wiley & Sons, Inc., 1997. [3] C. K. Lee, Modeling and design of frequency selective surfaces for reflector antennas, Ph.D. dissertation, Kent University, 1987. [4] R. Mittra, C. H. Chan, and T. Cwik, “Techniques for analyzing frequency selective surfaces – a review”. Proc. IEEE, vol. 76, n. 12, pp. 1593-1615, 1988. [5] J. P. Gianvittorio, et al., “Self-similar prefractal frequency selective surfaces for multiband and dual-polarized applications”. IEEE Transactions on Antennas and Propagation, v. 51, n. 11, pp. 3088–3096, nov. 2003. [6] P. H. da F. Silva, and A. L. P. S. Campos, “Fast and accurate modeling of frequency selective surfaces using new modular neural network configuration of multilayer perceptrons”. Microwaves, Antennas and Propagation, IET, v. 2, n. 5, pp. 503-511, 2008. [7] A. L. P. S. Campos, E. E. C. Oliveira, and P. H. da F. Silva, “Miniaturization of frequency selective surfaces using fractal Koch curves”. Microwave and Optical Technology Letters, vol. 51, n. 8, pp. 1983-1986, 2009. [8] B. A. Munk, Frequency Selective Surfaces – Theory and Design. New York: John Wiley & Sons, Inc., 2000.
Fig. 3. (a) Transmission coefficients for the square loop FSSs as a function of the length l; (b) square loop unit cell configuration. Tab.1. Values of resonant frequencies and bandwidths for the square loop FSSs. Length l (mm) 6.25 7.00 8.00 9.00
fr (GHz) 8.97 8.06 7.01 6.16
BW (GHz) 4.07 3.51 2.89 2.36
BW (%) 45.37 43.55 41.23 38.31
Fig. 4. Transmission coefficients for the crossed loop FSSs as a function of the crossed dipole width (W). Tab.2. Values of resonant frequency and bandwidth for the crossed loop FSSs as a function of the crossed dipole width (W). W (mm) 0.90 1.10 1.30 1.50 2.00
fr (GHz) 12.29 12.54 12.80 13.13 13.66
BW (GHz) 6.45 6.49 6.50 7.63 8.08
BW (%) 52.48 51.75 50.78 58.11 59.15
Fig. 5. (a) Comparison of measured and simulated transmission coefficients for the crossed loop FSS fabricated; (b) photograph of the crossed loop FSS with l = 9 mm. Tab. 3. Values of resonant frequency and bandwidth for the crossed loop FSSs fabricated. Length l (mm) 9.00 (measured) 9.00 (simulated)
fr (GHz) 9.37 9.12
BW (GHz) 4.78 4.78
BW (%) 52.41 52.41
