Quad-band FSS for Electromagnetic Shielding

In this paper, a novel method to design Quad-band stop frequency selective surface (FSS) is proposed to shield medical/non-medical devices from electromagnetic radiation of wifi/Bluetooth, GPS,1G, 2G, 3G, 4G and 5G frequencies. The proposed device can also be used in security and surveillance devices to protect them from electromagnetic attack. The FSS unit cell consists of convoluted structures on FR4 substrate. The study was performed by designing individual frequency selective surfaces for 1G, 2G, Wi-Fi/Bluetooth, GPS, 4G and 5G, then combining it to form a single Quad-band FSS unit.

The current paper focuses on providing with an effective, low cost, Quad bandFSS design, with high attenuation against electromagnetic interference, and effective shielding from electromagnetic waves.
These designs offer greater advantage of stopping multiple range of frequencies, reduced cell size, ease of fabrication, stability with various incident angles and polarizations, and also helps in effectively isolating devices from interfering electromagnetic signals.
The basic principle of working of FSS is the geometry of structure and periodicity of unit cell in FSS results in resonance, this frequency is called resonant frequency (fr). For a good FSS design the distance between substrate and patch is kept at least 0.5 mm.
For an FSS structure to show stable resonance with different incident angles, interelemental spacing must be smaller than 0.4λ. In a convoluted FSS structure, if the unit cell dimensions remain as small as 4.48% of free space wavelength at resonant frequency, the structure shows stability with incident angles and polarization variations.

A. Design Simulated for various frequencies
The table 2 shows the periodicity of the simulated structure, with different frequencies, the dielectric material (substrate) used is FR4 (Lossy, ℇr=4.3), with thickness 1.6 mm. The copper (annealed) material is used as a patch material with uniform thickness of 0.035 mm.
The value of element size is calculated by finding λe, which is given by fc is the mid frequency of the frequency band to be stopped and c is the speed of light.
When ( < 1) the value of ∈ is given by: When ( ≥ 1) the value of ∈ is given by: Where, ∈ is relative dielectric constant of substrate, W is the width of element and H is the height of substrate (dielectric height). The width of element is dependent upon periodicity of unit cell [5].
Using the above parameters, for a frequency (f), the dimensions of patch can be estimated. Table 2 shows the target bandwidth, estimated frequency, periodicity and other properties of simulated design using equation 1 (i.e., ( < 1)). Where, given by + 2 , is lower and is the higher range of frequency band to be stopped. For a successful design the value of should be closer to FSS structure's resonant frequency, ,, therefore for further design discussions will be considered equal to .
Phase range is also an important parameter to decide Fractional Bandwidth of FSS. In previous studies it is demonstrated for a square loop based system that FBW is proportional to the change in phase range [17,18]. In this research, a method is developed to predict FBW by tuning simulated phase range ( 2) equal to the analytical phase range ( 1).
The conducting lines of FSS can be considered as a series combination of a capacitor and an inductor. If the overall capacitance of FSS is higher it acts as band stop FSS, when overall inductance is higher the FSS acts as band pass filter. Also, this can be correlated from the fact that patch-type FSS are mostly used in Band Stop Filter, while slot-types are preferred for Band Pass Filter [19,20].
Following are the various designs simulated for various frequencies:

C. Design Using Both Sides of Substrate
It was observed that various frequencies can be stopped using design at both sides of substrate, remarkable results are obtained for the following frequencies:

Results and Discussion
Scattering parameters are used to describe a relationship between input and output ports, using incident and reflected power, of a two-port system. • S11 should be ideally infinite, as it is the ratioof reflected power at input port to the incident power. More the reflection at the input port, lesser the power will be transmitted to the output port. For S11= 0 dB, all the power will be transmitted to output port, due to negligible reflection.Thus more negative the value of RLin (equation 4), better is the FSS device for band stop applications.
• Similarly, S21is the power at output port to the incident power at input port. Thus, for FSS band stop device S21should be ideally 0 dB; lesser the value of S21better is the device for FSS band stop design.
The results indicate the stability of design in both TE and TM Mode. The S11 (Blue), S21 (Red) and reflection parameter A. Results of Designs (a) to (f) The figure 6 shows the S-parameter results obtained for the simulated designs (a) to (f), for phase 0°.
For the simulated designs (a) to (f) the range of frequencies stopped, and value of scattering parameters (dB) is shown in table 3. From the results above it can be shown that by tuning the geometry of a single loop (using equation 1 or 2), the individual frequencies can be stopped without affecting other frequencies.
The value of S11 obtained in the range of -30.72 to -38.01 dB is showing comparatively good performance in comparison to previous reported work.

B. Results of Designs 2 (a) to (d)
For the simulated designs (g) to (j) following are the range of frequencies stopped, and value of scattering parametersin dB.  Table 5 shows the range of frequencies stopped, and value of scattering parametersin dB, for the simulated designs (m) to (o). The design 3(a) is stopping the following frequencies 2.38, 5.17, 7.91, 6.20 with good stability at TE and TM mode as shown in the graph similarly design 3(b) is stopping the following frequencies 5.99, 9.02, 0.84, with good stability at TE and TM mode as shown in the graph and design 3(c) is stopping the following frequencies 0.87, 1.65, 5.74, 9.09 with good stability at TE and TM mode as shown in the graph. This confirms that carefully designed single frequency loops when combined in two can give better frequency response on stop multiple frequencies.
On the other back hand designs i.e., designs on the both side of substrates. Can provide better shielding to wide bands of frequencies. And can be a viable option for future wide band application designs. The key to process success is carefully choose the design for single frequency, match the design for multiple frequencies, superimpose and then optimize the back design for the remaining frequencies. Closer the Omega r (resonance frequency) to previously superimposed design, better is the value of S11 parameters for band stop FSS.

Conclusion and Future Work
The various designs using copper (annealed) with resistivity, ρ= 8930 kg/m 3 , as a conducting line and FR4 (lossy) with relative dielectric constant, ℇr = 4.3, as a dielectric substrate are simulated.
Using different approach, it was observed that as the conductive lines in the patch increases the continuity in S11 parameter decreases, however in spite of that the transmission parameter remain almost near 0 dB, when the conductive lines were added on the back side of dielectric.
This indicates that a better band stop FSS can be obtained by individually finding out center frequency for single loops and then merging them together to form a dual side FSS structure.