Design of a single layer single band 5G FSS as shielding materials

This paper deals with a novel Frequency Selective Surface (FSS) Electromagnetic shield that operates in the low 5G vertical applications. This FSS is designed for the purpose of serving as an electromagnetic shield to eliminate EMI. The FSS designed operates at a frequency of 5GHz with a bandwidth of 1.45GHz providing a shielding of more than 40dB. The proposed FSS uses a polyester sheet as substrate and uses silver for the metallization of the FSS. Since the proposed FSS is symmetric, it behaves similarly when a Transverse Electric (TE) or a Transverse Magnetic (TM) wave is incident. The structure was designed and simulated using CST software. The results show that this FSS blocks electromagnetic radiation at lower frequencies as intended.


Introduction
5G refers to a set of standards for the operation of mobile wireless.5G is a new generation of cellular networking.It promises high data speeds and seamless connectivity wherever we are [2].Compared to previous cellular technologies like 2G,3G or 4G; 5G operates at a much higher frequency.
The National Communications Commission has recently finished gathering public input on the potential use of the frequencies between 4.8-4.9GHzfor 5G vertical applications.5G has many uses for several industries like cellular communication.The various uses for 5G technology are immense.5G has the potential to enable technologies such as massive IoT, connected cars and many others which are yet to be determined.But there are some applications where transmissions in the 5G spectrum would not be desired.Stray EM waves in 5G frequencies interfere with the operation of existing antennas and sensors working in the same spectrum.It is known to interfere with ADAS and collision avoidance systems in cars [3].There also exists a considerable overlap in the operating frequencies of 5G devices and the frequencies of operation of aviation-grade radios.There exists a risk of interference between terrestrial communication and crucial aviation communication links and interfere with airplane traffic and could cause mishaps.With the proliferation of 5G systems in metropolitan areas and areas which witness high air-traffic, flights in those regions are experiencing a higher amount of interference in their mission critical systems, like radio altimeter, aircraft radio, radars etc. [4]- [9].The Federal Aviation Administration (FAA) had to set new operational directives to adapt to the changes [9].5G technology has also been postulated to affect human physiology.But research on the claim varies widely among the members of the scientific community [8].The author Wersényi, György proposes several experiments that could shed light on the interaction of the human body with electromagnetic radiation but also shares their limitations.This field of study is a highly contested one with diverse and sometimes even biased views held by different members.Still, there is a need for shielding of 5G frequency waves to protect critical equipment or biological tissues that might be affected by the transmission frequencies.Using a FSS based material as a shield can decrease the electromagnetic radiation emissions in frequencies that are not present in band used for communication while enhancing the susceptibility from external radiated fields, thereby improving the overall performance of the device [7].
A FSS (Frequency Selective Surfaces) is an assembly of miniature elements arranged in a periodic manner in an array to achieve bandpass or band stop EM characteristics [1].It is a burgeoning field and has witnessed intense study since its inception.In this paper a novel FSS based shield is designed and the characteristics of the same is studied.The structure of the FSS is elaborated in Section 2 and the results are elucidated in Section 3.

Structure of FSS
The FSS was designed using CST Microwave Studio Suite.The FSS designed is a single layer FSS with polyester as the substrate and silver as the metallized element.Silver is a good conductor of electricity and has an electric conductivity value of 6.3012 x 10 7 S/m.The FSS has a thickness (H2) of 0.06mm.The substrate material 'polyester' has electrical permittivity of 3.3.The thickness of the polyester sheet used is H1 = 0.1mm.The advantage of polyester is that they are flexible and hence the designed FSS material can be conformed onto any surface.
Figure 1 shows the top view of the FSS unit cell designed.The grey regions are the silver conductive sheets and form the unit cell of the FSS.The first step to design the FSS was to construct a simple cross structure of arm length L3 = 2mm and arm width L5 = 0.6mm and enclose it in square box of side length L2 = 5.6mm as shown in Figure 2(a).The result obtained by the simulation was not satisfactory as seen from Figure 3.The absorption characteristics did not show a minimum value at the frequency of interest.So inverted J shaped loading elements were added to the empty space between the cross and the square to fill it.To increase the attenuation in the transmission characteristics even more, meandered rectangular elements were added to the edges of the outer square; each of length L2 = 5.6mm and thickness L4 =0.2mm.The total length of the FSS unit cell became L1 = 8.6mm.All the measurements are mentioned in Table 1.The final FSS unit cell is shown in Figure 2(c).

Simulation and Measured Results
To validate the proposed FSS, the unit cell is simulated using the electromagnetic (EM) simulation software CST Microwave Studio.The proposed unit cell is simulated with CST Software Suite with Floquet ports and boundary conditions to simulate an infinite array.
The CST solver uses Finite Integration Technique (FIT).FIT method was presented by Weiland in 1977.It discretizes the Maxwell-Heaviside equations which are in continuous form and represents them in dual-orthogonal grids [5,6].Weiland introduced this novel technique as a Maxwell's equations analogue to ensure the physical consistency of computed fields.This method, an extension of the Finite Difference Time Domain (FDTD) approach and akin to the Finite Element Method (FEM), yields computationally efficient and memory-effective solutions.The advantage of using the FIT method is that each further calculation of the time step requires one matrix-vector multiplication and as a result is advantageous as there are many matrix solvers capable of performing calculation at high speed [5,6]

Transmission Parameters
To achieve resonance at the frequency 5GHz, the FSS unit cell is designed as a simple cross in a square box.The frequency response of the FSS designed was inadequate.So, loading elements were used to give a staggered swastika shape with inverted J-shaped stubs.This increased the attenuation meanwhile resulting in shift of resonant frequency to 7GHz, which is greater than desired frequency range.
To lower the resonant frequency, meandered rectangular loading elements were used.The meandered rectangular elements were added to the outsides of the FSS to shift the operating frequency to the lower ranges.This resulted in the FSS meeting the design parameters.The transmission characteristics as shown in Figure 3 showed increased absorption (minimum transmission) at the 5G frequency of 4.9 GHz.The simulated FSS has a transmission minimum of 45 dB at 4.98GHz.The bandwidth if the resulting structure is 1.45GHZ.The FSS has good polarization stability due to the symmetric shape of the structure.The polarization stability exhibits remarkable performance under normal incidence conditions, primarily attributable to the inherent symmetry present in the unit cell.

Surface Currents
To study the characteristics of the FSS with better clarity the surface current distribution of the FSS at the resonant frequency is plotted.The surface current distribution on the metallized regions of the FSS without the meandered rectangles (i.e.) the second stage of evolution of the design is shown in Transmission Coefficient (dB) The surface current distribution on the metal part of the FSS with the meandered rectangles in both TE and TM modes is also studied as shown in Figure 5

4 Fig 3 .
Fig 3. Transmission parameters of the different FSS evolutions designed Fig 4a and 4b.The figures highlight the distributions at the resonant frequency in TE and TM modes at its resonant frequency of fr = 7.1 GHz.When an incident EM wave falls on the FSS array, the energy of the wave in a particular frequency is dissipated as surface currents.This results in the attenuation of the incident wave.(a)(b) Fig 4. Surface current for wave incident on FSS without meandered stubs with angle of incidence (theta) equal to zero (a) TE mode (b) TM mode

Fig 5 .
Fig 5. Surface current for wave incident on FSS with meandered stubs with angle of incidence (theta) equal to zero (a) TE mode (b) TM3.3.Angular StabilityThe designed FSS was tested for angular stability to check position insensitivity to the incident wave and has been plotted inFig 6.This is tested by choosing different incident angles of the EM wave and testing the transmission parameters of the FSS.A parametric sweep was performed on the FSS by the CST software from an incident angle of 0 • to 60 • .The angle of incidence was increased by 5 • for every iteration on simulating, it has been realized that as the incident angle increases, the attenuation of the transmitted signal increased marginally without a change in the frequency of interest.

Fig 6 .
Fig 6.Graph depicting the transmission parameters for different angles of incidence

Table 1 .
Table with dimensions