Design of Wideband Two-Sided Bandpass Frequency Selective Surface for X, Ka, and Ku Band Application

. A novel wideband bandpass frequency-selective surface functioning at X, Ku, and Ka bands is proposed in this article. Te designed FSS has a metallic square loop and a circular ring, and they are printed on both sides of the FR4 substrate. Te proposed design FR4-based single-layer FSS is operating from 11.075GHz to 22.075GHz with a fractional bandwidth of 66.36%. Te parameters of the square loop and circular ring regulate the characteristics of the passband. Te optimum dimension of these parameters is obtained with parametric analysis. Te proposed structure is measured and fabricated. However, the measured results strongly agree with the simulated results, which authenticate the proposed design performance.


Introduction
In most RF circuits, the performance of the circuit totally depends on the characteristics of the flter.Such a fltering operation is performed by the frequency selective surface (FSS) [1] in a microwave wireless communication environment.FSS entails 2D or 3D structures with a defnite period.Te structure may be a slot or patch designed on a substrate made up of dielectric.Researchers widely use wideband FSS in many applications [2][3][4][5], including the radomes, Cassegrain antenna, shielding, absorbers, stealth applications, and so on, since most of the applications now operate over a wide operating range.Patch-based FSS achieves band stop performance, while bandpass performance is realized using slot-based FSS [1].Te major pullback limitation for the bandpass FSS is the narrow bandwidth [6][7][8].Te design and development of wideband bandpass FSS in the radomes and in the system with frequency reuse confguration, however, have had a signifcant research gap in recent years, according to the researchers.Few bandpass FSS with wideband are reported in [9].Tere is great demand for wideband bandpass FSS in various applications.In the literature, few bandstop FSS [10,11] and bandpass FSS [12,13] are reported for the application, as mentioned earlier.
To improve the bandpass FSS bandwidth, the researchers intended to develop techniques like 2, 2.5, and 3D.Te latter two [14][15][16] strongly attract investigators to the FSS design due to an additional degree of freedom compared to the 2D [17][18][19][20] topology.But the major limitations are the design and fabrication complexity.In [17], planar multilayer FSS is designed, but the high-quality factor results in narrow fractional bandwidth, and in [18], multilayer FSS with a stable Q factor consisting of the metal patch with wireframe is reported; it also results in a narrow band.A multilayer FSS can attain a maximum fractional bandwidth (FBW) of 20% by using nonresonant elements and adding more conducting layers to increase the bandwidth.Most of the 2D-based FSS results in narrow bandwidth due to the resonating behaviour.Tis can be overcome with nonresonant cascaded structures [21], and mushroom-shaped FSS with 15% FBW is reported in [22].Te hybrid unit cell is used to achieve FBW of 11.35%, as reported in [22].Tese two diferent structures are printed on adjacent sides of the substrates and have a high degree of design complexity.In [23], FSS based on a patch with VIA is designed, which ofers FBW of 110%, and this structure has very poor band rejection characteristics, multilayer FSS with narrowband responses is reported in [24].Another method of improving the bandwidth of the FSS is numerous steering conducting layers [25].Even though the 63% FBW is achieved, the fabrication complexity is higher.Te convoluting structures [26] are another approach, which can improve the bandwidth but has an asymmetric response due to diferent metallic topologies.In [27], metamaterial structure-based FSS is reported, but the gain is not stable in the operating X band.Triple band ultrawideband FSS designed with symmetrical structures on either side of the substrate, wherein they are not identical structures [28].It is possible to enhance the properties of frequency selectivity and miniaturisation by creating patches or etching slots.By incorporating slots and slits in the designed structure, it may increase the possibilities of wider range of applications.
Tereby, the proposed design performs as wideband (bandpass FSS).All the above methods in the literature can improve the bandwidth, but they also sufer from larger profles, fabrication difculties, and larger insertion loss.
In this article, a single-layer FSS with a wideband response is proposed for the X, Ku, and Ka band.Te proposed FSS resonates at 16.5 GHz, and it has good stability under various incident angles.Te unit cell has an overall dimension of 0.46 λ 0 × 0.46 λ 0 .Te structural parameters directly impact the resonance behaviour, and hence the optimum dimensions are obtained using the parametric analysis.Te signifcance of the proposed FSS structure is mentioned below: (1) Te motivation of the proposed work is to reduce the refections in the aircraft at a low cost, to design an FSS with minimum thickness for stealth applications, and to achieve wideband frequency.Te proposed singlelayer double sided FSS is compact with a minimum thickness and covers X, Ku, and Ka band with a fractional bandwidth of 66.36% as a wideband bandpass flter.(2) Te proposed prototype structure is fabricated and measured.(3) Te structure exhibits good angular and polarization stability in both TE and TM modes.(4) On either side of the passband, the refection coefcient is better than 20 dB.
Further, the structure of this manuscript is as follows: Section 2 describes the proposed unit cell design.Section 3 discusses parametric analysis with fuzzy verifcation.Te fabricated proposed structure is compared with the measured and simulated results in Section 4. Finally, concludes the work by providing a summary in Section 5.

FSS Unit Cell Design and Analysis
On the FR4 substrate, a symmetrical square loop and circular patch are designed to produce a wide bandpass flter.Figure 1 depicts the proposed FSS unit cell geometry.On either side of the substrate, the unit cell consists of a symmetrical square loop with a circular conducting ring.Te white area in the front view of Figure 1 is the FR4 substrate region, and the metallic region is represented in the black colour area.On both sides of the FR4 dielectric substrate, the identical structure is printed.Te L1 � 11.9 mm is the length of the metallic square loop's inner side; L2 � 12.5 mm is the length of the loop's outer side; and R1 � 0.6 mm is the radius of the circular conductive ring, which is employed as the identical structure of the unit cell.FR4 substrate of thickness T1 (0.4 mm) with loss tangent and 0.02 dielectric constant 4.4 and copper patch thickness T2 (0.035 mm) have been used.Te optimum dimension of the critical parametric is fnalized using parametric analysis.In Figure 1(a) the proposed FSS is shown in front and back views.In Figure 1(b), side view is shown, followed by the FSS design in the CST EM studio environment in Figures 1(c) and 1(d).
Fundamentally, frequency-selective surfaces are periodic structures distributed evenly in one or two dimensions to perform as flters, mostly the single unit cell dimension would be half of a wavelength [1].By considering this, for the required frequency, the overall dimension is obtained.Meanwhile, by introducing additional square patches and rings the desirable frequency band is achieved.Te evolution of each element and the improved results are plotted in Figure 1(e).
Te proposed unit cell has a simple design using a simple circular ring and a symmetrical square loop.Te symmetrical structure is chosen to obtain polarization stability at the unit cell level.Te resonant frequency varied by changing its elements perimeter [29].Te Q factor is the reciprocal of the fractional bandwidth.Q � f centre /(f 2 − f 1 ).Te Q factor for the proposed design is Q � 1.4977.
Te full-wave simulation of the proposed FSS is performed using the CST software, and the unit cell boundary condition is used to replicate the infnite structure.Floquet ports are used for exciting the FSS.Te transmission and refection coefcients at various angles of incidence (AoI) for both polarization of TE and TM modes are presented in Figure 2. It has been noticed that the proposed FSS shows a stable resonance up to 45 °.Beyond 45 °, the shift in frequency is very low.Te stable resonance stability at various AoI attributes to the unit cell's reduced overall size of 0.46 λ 0 × 0.46 λ 0 .(λ 0 -wavelength of free space @ lowest frequency).Te angular stability of TM mode nearly achieves a stable response comparing to the minimum variation in TE mode.Since the incident wave of the E-feld polarizes in Y-direction and H-feld polarizes in X-direction this could cause a minor deviation in the output observed.Typically, the free space impedance of incidence for TE mode is the ratio of impedance and cosθ.Similarly, the E-feld wave polarizes in X-direction, and the impedance of incidence in TM mode is the product of impedance and cosθ; hence, the H-feld of the AoI is comparatively similar.It manifests a better response in TM mode compared to the TE mode polarizations [30].With excellent stability in the resonant behaviour, the proposed FSS parameters such as L1, L2, T1, and R1 have excellent control over the passband.Further, in Section 3, the parameters of the simulated refection coefcient of the proposed FSS are discussed.

Parametric Effect, Fuzzy Logic FBW Validation, and Equivalent Circuit of the Proposed FSS
Figure 3 shows the refection coefcient of the proposed FSS for various L1 values.It is observed that the resonance frequency decreases with increasing patch length.Patch length reduction will result in a larger dielectric gap between the patches, decreasing the capacitance that increases the resonant frequency and providing a good correlation between the circumference of the ring and the efective wavelength at the resonance frequency.Hence, L1 = 11.9 mm is chosen for the fnal fabrication.
In Figure 4, the refection coefcient for various values of L2 is plotted, it is observed from the fgure.As we increase the unit cell size, the transmission pole at 15.9 GHz is reduced, and achieving good impedance bandwidth from 11.075 GHz to 22.075 GHz.Hence, L2 = 12.5 mm is chosen for the fnal fabrication.Figure 5 depicts the S11 of the proposed FSS for various R1 values.Te fgure shows that R1 = 0.6 mm has good impedance bandwidth since the large ring will provide a larger current path, increasing the wavelength and reducing the frequency.From Figure 6, it is observed that T1 = 0.4 mm features efective impedance matching, and hence it is chosen for the fnal fabrication.By varying the values of the parameters, we have achieved a better fractional bandwidth of 66.36% for both the TE and TM modes of refection coefcient (S11).Te optimum dimensions for the unit cell structure are represented in Table 1.International Journal of Antennas and Propagation in a wide range from 11.075 GHz to 22.075 GHz by providing 11 GHz and a fractional bandwidth of 66.36%.Te proposed FSS simulations S11 and S21 are presesnted in Figure 7.

Bandwidth Validation Using Fuzzy Logic Empirical
Formula.Te correctness of the fractional bandwidth is validated with the help of fuzzy logic.Tis method uses mathematical interpretations by varying the variables R1, L1, and L2 one at a time.Te calculated fuzzy bandwidth is shown in Table 2.In Figure 8, empirical relations between fractional bandwidth and fuzzy bandwidth are presented.

Equivalent Circuit of the Proposed FSS.
In the proposed FSS, the top-layer metallic square loop is modelled by a parallel RLC circuit with resistance R1, capacitance C1, and inductance L1.Te unit cell's central square loop and circular conductor are separated by a gap, which is represented by C3.L3 represents the centre circular metallic ring.Te FR4 substrate is modelled using the transmission line component based on the telegrapher model.Te substrate has series inductance L5 and L6 and shunt capacitance C5 and C6.Te proposed FSS equivalent circuit is presented in Figure 9(a).Similarly, R2, C2, and L2 represent the bottom-layer square loop.C4 represents L1 = 7.9 mm L1 = 9.9 mm L1 = 11.9 mm   [31] is represented in Figure 9(b).Te lumped element values are represented in Table 3. Figure 10 depicts the proposed FSS's surface current and E-feld distribution at the resonant frequency.Te feld distribution is taken under normal prevalence.Te fgure shows that more feld is accumulated around the square loop and the centre circular conductor ring.
Tis clearly shows that the area of the square loop and the circular ring is the reason for the passband characteristics.

Experimental and Measured Results
Te simulated results of the proposed FSS are validated, and the prototype of the designed FSS is fabricated using the PCB photolithography thickness of various designs are compared using the wet etching method.Te fabricated FSS is 300 mm × 300 mm and fabricated using FR4 substrate with 24 × 24-unit cells.
Te total number of 564-unit cells fabricated on either side of the FR4 substrate and the proposed prototype is represented in Figure 11.In the mid of the RF absorbing wall, the fabricated FSS prototype is placed.It is excited by a transmitter horn antenna situated 2 metres apart and is received by a receiver horn antenna, respectively, as shown in Figures 12(a Te absorbers in the RF absorbing wall avoid the measurement error by decreasing the EM difraction at a large incident angle.In order to measure the range of frequency between 11.075 GHz and 22.075 GHz, two different kinds of horn antennas are used.Te horn antenna used has an operating range between 1∼12 GHz and 12∼18 GHz.Figures 12(a      8 International Journal of Antennas and Propagation of the small discrepancy between the measured and simulated fndings.Te comparison of the suggested FSS's simulated and measured refection coefcients is shown in Figure 13.Te measured AoI is represented in Figure 14, lower shift and a higher shift in the frequency are noted due to the fabrication impact.
Table 4 demonstrates the comparison of the proposed work with existing work in the literature.Te thickness of various designs is compared and reported as a 45% thickness reduction from the existing work.Table 5 is the proposed design structure which is associated with square loop and circular ring hence results in obtaining wideband frequency and is found to be novel compared to the literature.
Te wavelength considered for computation in the comparison table concerns a lower cut-of frequency.Hence, it is   International Journal of Antennas and Propagation more suitable for stealth applications as it demands reduction in thickness, compact design, minimum refection, and reduced weight.Te overall unit cell size is 0.46 λ 0 × 0.46 λ 0 Te proposed work successfully achieves wide bandwidth and minimal thickness, which is the biggest hurdle for stealth applications.Te designed FSS could be used for stealth, radomes, and EM shelter applications.As a future enhancement, conformal substrate can be used instead of FR4 substrate.

. Conclusion
Te unit cell size for the proposed single-layer FSS is 0.46 λ 0 × 0.46 λ 0 .Te proposed FSS structure well maintains stability in resonance due to its miniaturized unit cell.Apart from that, the metallic square loop and circular conductor exert considerable infuence over the passband properties.Hence, the square loop's area is altered to achieve working frequency.On a FR4 substrate, the designed FSS is fabricated and measured.Te proposed prototype is fabricated and measured; the measured results agree with the fndings of simulations, validating the proposed structure.Te proposed FSS resonates at 16.5 GHz, and it has good under various incident angles.Te proposed work helps to reduce the refections in the aircraft at a low cost, designed an FSS with minimum thickness for stealth applications and achieved wideband frequency.Te proposed single-layer double Sided FSS is compact with a minimum thickness and covers X, Ku, and Ka bands with a fractional bandwidth of 66.36% as a wideband bandpass flter.Te wide bandpass flter resonates in the X, Ku, and K band in the simulation since it is measured for 1-18 GHz, and the experimental result lies in the X and Ku band.Te overall thickness is reduced.Furthermore, the FSS prototype shows a stable response for both polarizations.Te designed and developed single-layer FSS includes the following features, which have been shown to be useful in applications for radars, EM shelters, and stealth.

Figure 2 :
Figure 2: Transmission coefcient and refection coefcient of TE and TM mode polarization for several AoI up to 60 °.

Figure 7 :
Figure 7: Simulated results of S11 and S21 of the proposed design.

Figure 8 :
Figure 8: Empirical relations between fractional bandwidth and fuzzy bandwidth.

Figure 9 :
Figure 9: (a) Equivalent circuit of the proposed FSS.(b) Simulation results of S11-ECM in ADS vs. CST.

Figure 11 :
Figure 11: Fabricated prototype of the proposed FSS.
ring in front and back side 10 International Journal of Antennas and Propagation

Table 2 :
Fractional and fuzzy bandwidth.

Table 3 :
Lumped elements value of ECM in ADS tool.

Table 4 :
Comparison of the proposed work with the existing work.

Table 5 :
Comparison of the structure with proposed and existing work.