Wideband mosaic frequency selective surface for dichroic surface, absorber and directive antenna applications

Abstract

Acquiring a highly optical transparent feature on the wideband frequency selective surface (FSS) is challenging. Generally, the wideband response can be achieved using the multilayer FSS configurations, leading to high fabrication cost and thick end product. The innovation of the FSS structural element allowed the wideband frequency response to be achieved by using the single-layer configuration, but unsuitable for optical transparency application due to the wide trace width of the structural element. The main objective of the thesis is to assess the performance of mosaic frequency selective surface (MFSS), which comprises a combination of the Koch fractal on the inner and outer loops of the basic double hexagonal loops structural element. The narrow trace width of the MFSS allowed the structural element to be further utilised for optical transparency application. The performance comparison between the opaque MFSS (with FR4 substrate) and the transparent MFSS (with polycarbonate substrate) was studied and investigated thoroughly. The opaque MFSS and transparent MFSS (~ 70.3 % of transparency level) yielded wideband bandstop and bandpass frequency responses with a low cross-polarisation, Tyx (-37 dB), yet the angular stability was limited to only 25°. The MFSS performance under the influence of the array and substrate configurations (MFSS, MFSS2 and MFSS3) were also investigated in detail. With increased array and substrate configurations, broader bandwidth (BW) was achieved for the first and second bandstop frequency responses, while vice versa for the bandpass frequency response. The optical transparency of the MFSSs remains constant with a low Tyx (< -35 dB). Within the lower frequency region, the MFSS2 and MFSS3 had a high angular- and polarisation stability of up to 30° and 40°, respectively. However, the scan blindness phenomenon became more obvious within higher frequency regions. The MFSS structural element was further proposed for dichroic surface, absorber, and directive antenna applications. The complementary MFSS (CMFSS) structural element was applied for the dichroic surface application. The design requirement was to permit the reflection and the transmission of the EM wave at 183 GHz and 325 GHz, respectively, and the Tyx levels were < -68 dB. The CMFSS was also highly insensitive towards angular- and polarisation variations of up to 45°. For the absorber application, the metallic MFSS periodic array was replaced by the resistive MFSS (RMFSS) periodic array, which was fabricated on the thin Polyethylene Terephthalate (PET) substrate backed with a metal plate to establish a very thin wideband absorber. Even though the wideband absorption with fractional bandwidth of 132 % is achieved by using the multilayer MFSS absorber, yet the relative thickness was only λL15⁄. The MFSS absorber was also insensitive to angular- and polarisation variation of up to 35°. Lastly, the monopole antenna was associated with the studied MFSS absorber to have a low-profile directive antenna. With the MFSS absorber, the maximal directivity with high front-to-back ratio was obtained by placing the absorber at only a short distance from the antenna (d = 2λfr25⁄). This absorber can be used to avoid or control the perturbation near the monopole antenna. The simulation process (Computer Simulation Technology Microwave Studio (CST MWS)), the fabrication process (laser etch and screen-printing techniques), and the measurement process (free space and radiation pattern measurements) varied depending on the applications. The simulated results were in good agreement with the measured results, which validated the utilisation of MFSS for various applications.