High performance frequency selective surface (FSS) filters have a myriad of applications in the field of 5G wireless communication and antenna/radome systems. The most common FSS is a two-dimensional (2D) periodic array of thin conducting or aperture (also termed slot) elements etched on a flat or curved dielectric structure. This type of structure usually exhibits frequency filtering characteristics similar to the frequency filters in traditional RF circuits. In this article, resonant frequency stability in a bandstop filter over varying incident angles and polarizations is demonstrated in a miniature fractal patch element FSS. In addition, to meet the wide passband demand of 5G wireless communication systems, a fractal slot element FSS is designed and demonstrated to have more than 30 percent bandwidth with an insertion loss less than 0.5 dB. The filter’s bandwidth is constant as the incident angle increases up to 60 degrees for both TE and TM polarizations.

The main themes of 5G wireless communication include a rapid growth of connectivity for a large number of devices and a huge increase in mobile data rates. Networks are required to support 1000x higher data volume per area, 10 to 100x more connected devices in real-time, and 10 to 100x higher data rates.¹ To meet these demands, industry research is focused in areas such as increased spectrum, improved efficiency and high-reliability communication links. To deal with stringent spectrum requirements, spatial filters are usually required. The design of multi-band spatial filters or FSS filters can be very challenging due to requirements for stable filtering performance with changes in incident angle and polarization.

PRIOR WORK

The most common FSS is a 2D periodic array of either thin conducting or aperture (slot) elements etched on a flat or curved dielectric structure.^2-5 Various FSSs with cross dipole patch elements have been used in multi-band communication systems;^6,7 however, the transmission performance changes drastically as the incident angle is steered from normal to 40 degrees. Thus, a large stop-to-passband ratio or band separation ratio is required to minimize RF losses. This is evident in a stop-to-passband ratio of 7:1 for a single screen FSS⁶ or 4:1 for a double screen FSS.⁷ Although the miniature fractal patch element FSS reported by Wu for dual band operation⁸ exhibits frequency stability with varying incident angles and polarizations at its first resonant frequency, it is unstable at the second resonant frequency occurring closer to the grating lobe region.^9,10

Much of the work for stabilizing FSS resonant frequencies has been through the use of cross or hexagonal loop slot elements^4,11 and complementary type miniature element FSSs (MEFSS)^12-15 for bandpass radome applications. Although stability of the passband center frequency can be achieved over various incident angles and polarizations,¹⁵ bandwidth is generally decreased for TE polarization or increased for TM polarization. To mitigate this, designs proposed by Munk⁴, Schneider and McCaan¹¹ have been used.

FRACTAL FSS FILTER DESIGNS

The design and analyses of the patch and slot FSS filters described in this article are based on an accurate integral equation formulation (IEF) combined with the method of moments (MOM).^2,16-19 This analytical approach is also known as the full wave analysis technique. The accuracy of this numerical approach has been verified by many comparisons with measured data.²

Fractal Patch Element FSS

These designs specifically address modern, multi-band wireless local area networks (WLAN), i.e., Wi-Fi systems, that generally operate in both the 2.4 and 5 GHz bands and are used to cover indoor environments such as hospitals, high-rise buildings and offices. Noise induced by unwanted outside electromagnetic interference (EMI) may cause life-support instruments to malfunction, endangering patients’ lives. To reduce or eliminate interference from nearby Wi-Fi systems, the Wi-Fi signals must be confined within specific physical areas. A traditional miniature fractal patch element FSS for a Wi-Fi system may exhibit a stable first resonant frequency at 2.4 GHz at various incident angles and polarizations; however, the second resonant frequency in the 5 GHz band is generally not stable, making it difficult to block Wi-Fi signals in both bands. A similar problem is observed in a Wi-Fi FSS design using multi-ring elements.²⁰ Two innovative FSS designs aimed at improving the stability of the second resonant frequency are shown in Figures 1 and 3.

The first (see Figure 1) is realized by etching a two screen fractal FSS on both sides of a thin dielectric substrate. Note the top FSS screen’s unit cell has one fractal loop patch element, while the bottom FSS screen’s unit cell has four, i.e., 2 × 2 fractal loop patch elements. These two screens have the same period, and their unit cells must be aligned exactly with each other. Transmission performance is shown in Figure 2 for a Duroid 6006 substrate with a dielectric constant of six. As the incident angle changes from normal to 60 degrees, the FSS provides at least 18 dB attenuation at both 2.45 and 5.8 GHz for both TE and TM polarizations.

The second design (see Figure 3) is a single FSS screen with two, concentric fractal loop (or double fractal) patch elements sandwiched between two dielectric slabs, with a dielectric constant of 2.2 and thickness of 3 mm. The period of the unit cell is 2 cm. Transmission performance is shown in Figure 4. At 2.45 and 5.2 GHz, at least 18 dB of attenuation is obtained for both TE and TM polarizations over incident angles varying from normal to 60 degrees. This is a considerable improvement with respect to previously published work.²⁰ There is also no difficult alignment required, as in the first design, making it more suitable for fabrication and mass production.

Fractal Slot Element FSS

Contrary to patch element FSSs, cross or hexagonal loop slot element FSSs are usually implemented to provide a passband with fast roll-off skirts for 5G wireless communication or antenna/radome applications. Multiple dielectric slabs are needed to stabilize the bandwidth of the passband, and two or more FSS screens are needed for a flatter passband and sharper roll-off.^4,11 To provide wider bandwidth, we introduce a novel FSS design with fractal loop slots, as illustrated in Figure 5. Figure 6 shows the simulated transmission performance. The 0.5 dB passband bandwidth is about 34 percent, which is greater than previously published designs^4,11 for both TE and TM polarizations, as well as incident angles varying from normal to 60 degrees. One can further sharpen the roll-off skirts by adding another slotted screen.

CONCLUSION

Novel FSS filters with miniature fractal patch elements have been designed for 5G multi-band wireless communications, while a fractal slot element FSS was designed and demonstrated to have greater than 30 percent bandwidth with an insertion loss less than 0.5 dB for wideband antenna/radomes. Both patch and slot FSSs exhibit angular stability and polarization independent features as the incident angle is varied from normal to 60 degrees. They are low volume, lightweight and can be easily fabricated with conventional printed circuit board techniques. These designs may also be scaled to THz and infrared frequency bands. There are a myriad of applications in advanced communication and radar systems to be further explored.

References

1. B. Bangerter, S. Talwar, R. Arefi and K. Stewart, “Networks and Devices for the 5G Era,” IEEE Communications Magazine, Vol. 52, No. 2, February 2014, pp. 90–96.
2. T. K. Wu, (ed.), Frequency Selective Surface and Grid Arrays, Wiley, N.Y., 1995.
3. J. C. Vardaxoglou, Frequency Selective Surfaces: Analysis and Design, Research Studies Press, Taunton, England, 1997.
4. B. Munk, Frequency Selective Surfaces, Wiley, N.Y., 2000.
5. B. Munk, Finite Antenna Arrays and FSS, Wiley, N.Y., 2003.
6. V. D. Agrawal and W. A. Imbriale, “Design of a Dichroic Cassegrain Subreflector,” IEEE Transactions on Antennas and Propagation, Vol. 27, No. 4, July 1979, pp. 466–473.
7. G. Schennum, “Frequency-Selective Surface for Multiple Frequency Antennas,” Microwave Journal, Vol. 16, pp. 55–57, 1973.
8. T. K. Wu, “Improved Dual Band FSS Performance with Fractal Elements,” Microwave and Optical Technology Letters, Vol. 54, No. 3, March 2012, pp. 833–635.
9. T. K. Wu, “Improved THz/Infrared Frequency Selective Surface with Minkowski Fractal Elements,” International Conference on Wireless Information Technology and Systems (ICWITS) and Applied Computational Electromagnetics (ACES), March 2016.
10. T. K. Wu, “Single-Layer FSS for Wi-Fi Applications,” IEEE International Symposium on Antennas and Propagation and USNC-URSI Radio Science Meeting, July 2017.
11. S. W. Schneider and J. F. McCaan, “Frequency Selective Surfaces,” Chap. 56, J. Volakis, (ed.), Antenna Engineering Handbook, 4th ed., McGraw-Hill, N.Y. 2014.
12. K. Sarabandi and N. Behdad, “A Frequency Selective Surface with Miniaturized Elements,” IEEE Transactions on Antennas and Propagation, Vol. 55, No. 5, May 2007, pp. 1239–1245.
13. X. D. Hu, X. L. Zhou, L. S. Wu, L. Zhou and W. Y. Yin, “A Miniaturized Dual-Band FSS with Closed Loop and its Complementary Pattern,” IEEE Antennas and Wireless Propagation Letters, Vol. 8, December 2009, pp.1374–1377.
14. M. Al-Joumayly and N. Behdad, “A Generalized Method for Synthesizing Low-Profile Band-Pass Frequency Selective Surfaces with Non-Resonant Constituting Elements,” IEEE Transactions on Antennas and Propagation, Vol. 58, No. 12, December 2010, pp. 4033–4041.
15. H. Li, C. Yang, Q. Cao and Y. Wang “An Ultrathin Bandpass Frequency Selective Surface with Miniaturized Element,” IEEE Antennas and Wireless Propagation Letters, Vol. 16, June 2016, pp. 341–344.
16. C. C. Chen, “Scattering by a Two-Dimensional Periodic Array of Conducting Plates,” IEEE Transactions on Antennas and Propagation, Vol. 18, No. 5, September 1970, pp. 660–665.
17. S. W. Lee, “Scattering by Dielectric-Loaded Screen,” IEEE Transactions on Antennas and Propagation, Vol. 19, No. 5, September 1971, pp. 656–665.
18. C. C. Chen, “Transmission of Microwave Through Perforated Flat Plates of Finite Thickness,” IEEE Transactions on Microwave Theory and Techniques, Vol. 21, No. 1, January 1973, pp. 1–6.
19. R. Mittra, C. Chan and T. Cwik, “Techniques for Analyzing Frequency Selective Surfaces–a Review,” IEEE Proceedings, Vol. 76, No. 12, December 1988, pp. 1593–1615.
20. D. Ferreira, I. Cuiñas, R. F. S. Caldeirinha and T. R. Fernandes, “Dual-Band Single Layer Quarter Ring FSS for Wi-Fi Applications,” IET Microwaves, Antennas and Propagation, Vol. 10, No. 4, March 2016, pp.435–441.