A New Miniaturized Low-Profile and Stable Dual-Band FSS with 2.5D Structure for ISM Bands

Abstract

Based on 2.5-Dimensional (2.5D) frequency selective surface (FSS) structures, a new miniaturized frequency selective surface structure is proposed in this work to operate as a bandreject filter in the 2.4 GHz (2400 to 2483.5 MHz) and 5 GHz (5150 to 5350 MHz and 5470 to 5725 MHz) ISM (Industrial, Scientific, and Medical) bands. The FSS has the metallization layer formed by a square loop with an internal cross loop, with each geometry being responsible for a rejection frequency range. The main characteristics of the structure are discussed, emphasizing the resonance frequency variation as a function of the increasing number of vias. Numerical results are obtained through simulations using the commercial software ANSYS HFSS. In addition, by analyzing numerical results for structures with different numbers of vias, it was possible to obtain expressions for calculating the first resonance frequency for each geometry. The cell size of the 2.5D FSS of the proposed cell is only 0.0883 λ₀, where λ₀ corresponds to the free-space wavelength of the lower resonance frequency. The FSS presents dual polarization, angular stability for angles up to 50° and its simulated response presented two resonant frequencies, the first at 2.65 GHz and the second at 5.34 GHz, with relative bandwidths of 38.11% and 35.96%, respectively. For comparison and validation purposes, the prototype is fabricated and measured. A very good agreement between the simulation and measurement results is observed.

Index Terms
Frequency selective surfaces; FSS; stopband; Wi-Fi; ISM

I. INTRODUCTION

Frequency selective surfaces (FSS) are structures typically composed of periodic arrays of resonant elements printed on a dielectric substrate [¹]. These elements are designed so that the circuit behaves like a spatial electromagnetic wave stop-band or band-pass filter, depending on whether these resonant elements are metallic patches or apertures, respectively, and are printed on a dielectric substrate [¹]-[³]. Some factors are very important in determining the frequency response of an FSS such as: dielectric properties of the substrate, geometry and shape of the element, the periodicity or distance between elements, and the radiation angle of the incident wave [³]-[⁷]. The main researches in this line are around FSS that have low cost, light weight and ability to integrate with other microwave circuits, and can be applied in reflector antenna systems, absorbers, radomes, missiles, satellites, among others [¹].

Different types of FSS structures and configurations have been proposed by researchers working in the field of microwave circuits and aiming at the development of high performance FSS, such as: 3-dimensional (3D) FSS [⁸]; single-layer selective filters and different geometric elements [²]-[³]; circuits with non-uniform arrangements [⁹]; multilayer or cascaded frequency selective surfaces [¹⁰]; FSS that have their elements based on fractal geometries [¹¹]; among others.

Studies show that inserting vias into single-layer FSS provides a miniaturization of the unit cell size while maintaining the resonance characteristics of the structure. This type of circuit is called 2.5D (2.5 Dimensional) and are composed of two or more layers interconnected by metallic vias between the dielectric [¹²]-[¹⁵].

Wei et al. [¹²] proposed a 2.5D dual-band FSS in which the elements are patch-type based on the Jerusalem cross and each geometry is responsible for a rejection band. The elements are miniaturized and the circuit response exhibits good bandwidths. The dielectric substrate used is FR4 with relative electrical permittivity equal to 4.4 and a thickness of 1.6 mm. The resonance frequencies of the structure are at 1.6 GHz and 3.6 GHz.

A frequency-selective surface with resonance frequency near 600 MHz with good angular stability is proposed by Yin et al. [¹³]. The structure is 2.5D ultrathin, the unit cell has dimensions of 0.014 λ₀ x 0.014 λ₀, and is composed of patch-like elements on the two faces of the dielectric interconnected by vias. The dielectric substrate used is FR4 of relative electrical permittivity of 4.3 and 0.025 loss tangent.

Yin et al. [¹⁴] proposed an FSS for the 900 MHz and 1790 MHz GSM bands. The structure is 2.5D miniaturized, has stable response for TE and TM polarization and dual-band behavior. The elements have dimensions of 0.072 λ₀ x 0.072 λ₀, are printed on FR4 of relative electrical permittivity of 4.3 and 0.025 loss tangent.

The main objective of this work is to propose a new miniaturized 2.5D FSS structure to operate in the ISM (Industrial, Scientific, and Medical bands) frequencies of 2.4 GHz (2400 to 2483.5 MHz) and 5 GHz (5150 to 5350 MHz and 5470 to 5725 MHz). The proposed FSS 2.5D geometry can be used technological applications such as Wi-Fi 6, as a signal blocker, and as absorbers, or can be integrated into microstrip antennas to improve their parameters. The numerical characterizations for the FSS, were obtained with the help of the commercial software ANSYS HFSS, where simple and 2.5D structures were analyzed with geometries based on square loop and cross loop, all surfaces are of the conductive patch type. The final proposed prototype was built and measured, and its results showed good agreement with the simulated ones.

II. ELEMENT DESIGN

The final proposed FSS is designed to behave as a spatial band-stop rejection filter for the 2.4 GHz and 5 GHz ISM bands, which is used by Wi-Fi systems. The unit cell of this structure is composed of two 2.5D concentric geometries, one in the shape of a square loop and the other in the shape of a cross loop, with 0.0883λ₀ x 0.0883λ₀ (λ₀ – wavelength in free space for 2.45 GHz) of unit cell size. Fig. 1 shows the design of this surface, its main parameters are: a_x = a_y = 10.0 mm, L_x = 9.5 mm, L₁ = 1.78 mm, L₂ = 2.38 mm, L₃ = 1.25 mm, L₄ = 1.1 mm, D = 0.3 mm, w = 0.6 mm and h = 1.57 mm.

Fig. 1
FSS element design.

The 2D geometries were chosen from a literature search for simple geometries in loops shapes to enable the transformation to 2.5D. The initial dimensions were defined based on [¹⁶], [¹⁷] and considering some manufacturing constraints, such as minimum via diameter and minimum copper strip thickness size.

A resume of the evolution process of the FSS geometry design until arriving at the proposed FSS is shown in Fig. 2. Initially the geometries were analyzed separately, and to obtain miniaturization in each geometry the initial dimensions of the 2D structure were established and maintained, subsequently the increase in the number of vias gradually in each FSS was analyzed.

Fig. 2
Evolution process of the unit cell of the FSS.

The simulations of the loops shown in Fig. 2 are presented in Fig. 3. With the insertion of vias and the increase in their number the resonance frequency decreases, providing the miniaturization of the unit cell.

Fig. 3
Simulated transmission coefficient for: (a) square loop 1, 2 and 3 and (b) cross loop 1, 2 and 3.

Based on computational methods of surface fitting and the numerical simulations, two equations were obtained that optimize the design by estimating the first resonance frequency according to the number of vias.

For the square loop three parameters are used, these are f_r2D (resonance frequency of the 2D structure in GHz), d_vias (distance between vias in mm), N_vias (number of vias) and ε_ref (effective dielectric constant). (1) demonstrates this relationship.

(1) $$f_{\mathbf{S}\mathbf{L}}(GHz)=\frac{5.61{\mathrm{\hspace{0.33em}e}}^{-{\left(2.95N_{vias}\right)}^2}+\left(f_{r2D}-9.71\right)e^{-\left(0.63d_{vias}\right)}}{\sqrt{{\epsilon }_{ref}}}+4.174$$

For the cross loop, the parameters f_r2D(GHz), N_vias, and ε_ref are used in the approximate calculation of the resonance frequency, with f_CL(GHz) calculated by:

(2) $$f_{CL}(GHz)=\frac{0.6f_{r2D}{e}^{-0.1396N_{vias}}}{\sqrt{{\epsilon }_{ref}}}+5.04$$

The value of ε_ref is approximated by:

(3) $${\epsilon }_{ref}=\frac{{\epsilon }_{refCPW}+1}{2}$$

ε_refCPW being the value of the effective dielectric constant of a coplanar waveguide, CPW (coplanar waveguide), without the ground plane [¹⁸]. For this work, it was calculated as ε_refCPW = 1.808 and ε_ref = 1.404.

Table I describes the number of vias for each geometry and the comparison of the simulated frequency and those obtained by applying (1) and (2). Good agreement between them is observed for the first resonance frequency. However, it should be noted that this procedure is limited, even though it provides good initial values.

Table I
NUMBER OF VIAS IN EACH GEOMETRY

Geometry	Number of Vias	Simulated Frequency (GHz)	Estimated Frequency (GHz)
Square loop	0	4.78	–
Square loop	4	4.12	4.25
Square loop	12	3.40	3.68
Square loop	16	3.19	3.25
Square loop	20	2.94	2.91
Cross loop	0	10.05	–
Cross loop	6	7.12	7.25
Cross loop	12	5.89	5.99
Cross loop	18	5.40	5.46
Cross loop	24	5.30	5.22

III. SIMULATED RESULTS

The commercial software ANSYS HFSS was used for the simulations. The simulations were performed using periodic boundary conditions, and with the electromagnetic waves incident in the z direction and with TE and TM polarizations.

After reaching the desired frequency response with the square loop 3 (2.45 GHz band) and cross loop 3 (5 GHz band), the two were joined together forming the proposed FSS. For the proposed FSS, the angular stability with respect to angle θ of plane wave incidence was analyzed. The bandwidth in this work was set to -10 dB. These results are presented in Fig.4. The structure maintains the stable frequency response with increasing angle θ, keeping its rejection band for any angle up to 50°.

Fig. 4
Simulated transmission coefficients of the proposed FSS under different incident angles for TE polarization.

IV. EXPERIMENTAL RESULTS

To verify the performance of the proposed designed FSS, a prototype was fabricated and measured. The structure was built using the same substrate as for the simulations, FR4, and has dimensions of 200 mm x 200 mm, containing a 20 x 20 element arrangement as is illustrated in Fig. 5(b). The measurement system mainly consists of two A.H. Systems Inc.-SAS-571 horn antennas (a transmitting and a receiving), cables, and a Rohde & Schwarz-ZVB14 vector network analyzer (VNA). The FSS prototype was placed between the transmitting horn antenna (Antenna 1), which was connected to the output port of the VNA (Port 1), and the receiving horn antenna (Antenna 2), which is connected to the input port of the VNA (Port 2), at a distance to ensure far-field condition and plane-wave incidence on the FSS, as shown in Fig. 5(a). Then, the VNA was used to perform the measurement of the transmission coefficient (S21) from the transmitting antenna (Port 1) to the receiving antenna (Port 2).

Fig. 5
(a) Photograph of the measurement setup; (b) Photograph the fabricated prototype.

Fig. 6 shows the comparison of measured and simulated transmission coefficient values for the proposed FSS for TE and TM polarizations. The FSS presents dual polarization and its simulated response presented two resonant frequencies, the first at 2.65 GHz and the second at 5.34 GHz, with relative bandwidths of 38.11% and 35.96%, respectively. The measured values for this FSS are 2.71 GHz and 5.5 GHz for the resonant frequencies and 34.87% and 36.00% for the bandwidths.

Fig. 6
Comparison between simulation and measurement. (a) Polarization TE and (b) Polarization TM.

Fig. 7 presents the measurement for different angles θ of incidence of the electromagnetic wave. As in the simulation the measured results show that the structure has optimal angular stability and for angles up to 50° the rejection bands of the structure are maintained. Table II summarizes the simulated and measured data for the proposed FSS.

Fig. 7
Measured transmission coefficients of the proposed FSS for different incident angles for TE polarization.

Table II
COMPARISON OF MEASURED AND SIMULATED RESULTS

θ	First Resonant Band
	Frequency (GHz)			BWstart (GHz)		BWend (GHz)		BW %
	Sim.	Measur.	Er %	Sim.	Measur.	Sim.	Measur.	Sim.	Measur.
0	2.65	2.71	2.21	2.01	2.12	2.98	3.07	38.11	34.87
30°	2.65	2.66	0.38	1.93	2.13	3.00	3.07	42.64	37.03
50°	2.66	2.75	3.27	1.70	1.94	3.13	3.20	53.38	45.81
θ	Second Resonant Band
	Sim.	Measur.	Er %	Sim.	Measur.	Sim.	Measur.	Sim.	Measur.
0	5.34	5.50	2.91	4.45	4.46	6.37	6.44	35.96	36.00
30°	5.30	5.45	2.75	4.38	4.42	6.54	6.67	40.75	40.45
50°	5.25	5.32	1.32	4.20	4.15	6.86	7.30	47.05	59.21

Table III presents a comparison of the unit cell size proposed in this work with other results found in the literature for applications at the 2.45 GHz frequency. The proposed FSS presents high performance in relation to the miniaturization of the unit cell with size equal to 8.83 % of the wavelength in the free space of the lowest resonance frequency, besides presenting dual-band behavior and angular stability.

Table III
COMPARISON OF EXISTING FSS IN THE LITERATURE FOR 2.45 GHZ FREQUENCY

References	Structure	εr	Dual-band	Dual-polarized	Unit cell size
[19]	2D	4,4	No	Yes	0,104λ0 x 0,104λ0
[20]	2D	4,4	Yes	Yes	0,163λ0 x 0,163λ0
[21]	3D	2,78	No	Yes	0,195λ0 x 0,195λ0
[22]	3D	–	No	Yes	0,300λ0 x 0,300λ0
This work	2.5D	4,4	Yes	Yes	0,0883λ0 x 0,0883λ0

V. CONCLUSION

In this paper, a miniaturized 2.5D dual-band FSS is proposed for the 2.45 GHz and 5 GHz bands. The scattering properties of the 2.5D FSS have been investigated. The insertion of vias into the structures exhibited desirable characteristics, allowing tuning of the frequency response (resonant frequency, bandwidth) without changing the physical dimensions of the unit cell of the FSS. The proposed frequency selective surface has concentric geometries based on square and cross loop, with unit cell dimensions equal to 0.0883λ₀ x 0.0883λ₀, which in comparison with the designs listed in Table III, confirms a superiority of the proposed FSS with respect to miniaturization of unit cell size.

In addition the FSS exhibits good angular stability for angles from 0° to 50° and dual polarization. Measured results were obtained to validate the theoretical formulation described in this paper. A good agreement between numerical simulations and measurements was achieved.

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Publication Dates

• Publication in this collection
  23 Sept 2022
• Date of issue
  Sept 2022

History

• Received
  10 Mar 2022
• Reviewed
  24 Mar 2022
• Accepted
  09 Aug 2022