Design of Band-stop Frequency Selective Surface

　　1 Introduction

　　Frequency selective surface (FSS) is a two-dimensional periodic array structure, which is composed of periodically arranged metal patch units or periodically arranged aperture units on a metal screen. This surface can present total reflection (patch type) or full transmission characteristics (aperture type) near the unit resonant frequency, which are called band-stop or band-pass FSS respectively. The actual band-stop section is composed of one or more layers of FSS patch layers separated by dielectric layers. In order to ensure the stability of the frequency response of FSS relative to the incident angle and polarization, the metal patch layer is usually embedded in a multi-layer dielectric layer. In addition, two or more layers of FSS patch layers stacked back to back can produce good passband characteristics (flat passband, steep sideband).

　　2 Design of band-stop frequency selective surface

　　The usual band-stop filter is composed of two layers of FSS metal layers and a dielectric layer in the middle. The dielectric sheet in the middle determines the flatness of the transmission curve passband, and the FSS metal layer determines the bandwidth and resonant frequency of the transmission curve. The thickness and dielectric constant of the dielectric sheet are very important. The thickness of the dielectric sheet is typically taken as 0.5

,

is the wavelength of the center of the stop band. The dielectric layer between the two FSS layers improves the stability of the band-stop filter relative to the angle of incidence. Although the higher the dielectric constant, the better from the perspective of stability, a high value also introduces high transmission loss. Thus, the value of the dielectric constant must be comprehensively considered according to the design requirements.

　　The goal of this paper is to design a FSS structure that can pass signals in the GPS & DCS1800 mobile communication band and reject signals in higher frequency bands. Figure 1 shows the geometry and configuration of the designed frequency selective surface.

　　It consists of two layers of FSS separated by a foam spacer. The circular ring cell shape is chosen for its good band stability. The FSS metal layer is embedded in a dielectric constant

The thickness of the foam gasket is close to 0.5

=

, where the resonant frequency of FSS is

，

The design process of this band-stop filter will be analyzed using the Smith chart and transmission line theory.

　　Figure 1 Two-layer circular FSS structure

　　3 Analysis and design using Smith chart and transmission line theory

　　The design starts with a layer of FSS, which consists of a circular FSS metal layer embedded in a dielectric layer. The dielectric constant of the dielectric layer is 3.5 and the loss factor is 0.0026. The structure is analyzed and designed by the simulation software Ansoft DesignerTM. Figure 2(a) shows the impedance of a layer of FSS in the frequency range of 1GHz to 12GHz, and the corresponding transmission and reflection frequency responses are

and

It is shown in Figure 2(b). The equivalent circuit of a layer of FSS transmission line and the corresponding formula are shown in Figure 2(c). For comparison, five points are marked in Figure 2(a), representing the passband at 1GHz, 2GHz, and the stopband at 5GHz, 7GHz and 9GHz. It can be observed that the admittance trajectory

　　Figure 2 (a) Admittance trajectory of a layer of FSS

　　(b)

　　Figure 2(b) A layer of FSS

and

　　Figure 2(c) Equivalent circuit and formula of a transmission line of one layer of FSS

　　Located in the circle where the conductance is equal to 1, it implies that the real part of the orthogonal admittance is equal to the admittance of air (that is,

=1/377 mhos), which can also be obtained from equations (1.2) and (1.3). From Figure 2(a), it can be observed that the reflection coefficient at 9 GHz

, which is similar to the case in Figure 2(b) at 9 GHz

are consistent.

　　Figure 3 shows the result of adding an extra layer of 25.4mm thick gasket to a layer of FSS structure. Comparing Figure 3(a) with Figure 2(a), it can be seen that there is only a small change at 1GHz, and the admittance value rotates 1200 at 2GHz. This can be explained by equation (2.2). The points at 7GHz and 9GHz move to the edge of the admittance diagram. This suggests that there is a high reflection coefficient at these frequency points, which can also be seen from Figure 3(b). A second layer of FSS is added to the gasket, and the results are shown in Figure 4. As can be seen from Figure 4, the high frequencies are concentrated on the left side of the admittance diagram, and most of the frequency points are concentrated at the edge of the table, which means that the reflection band is widened with the addition of the second layer of FSS.

　　Figure 3 (a) Admittance trajectory of one layer of FSS plus one layer of gasket

　　Figure 3(b) s11 and s21 of one layer of FSS plus one layer of gasket

　　Figure 3(c) Equivalent circuit of a transmission line with one layer of FSS and one layer of gasket

　　4 Conclusion

　　The goal of this paper is to design a band-stop FSS structure that allows GPS & DCS1800 signals to pass through and rejects higher frequency signals. A two-layer FSS structure separated by a foam spacer was designed and the frequency response was simulated using simulation software. The FSS structure was analyzed using Smith charts and transmission line theory.

　　Figure 4 (a) Admittance trajectory of two layers of FSS plus a layer of gasket

　　Figure 4(b) s11 and s21 of two layers of FSS plus a layer of spacer

　　Figure 4(c) Equivalent circuit of a transmission line with two layers of FSS and one layer of gasket