(A) study on electromagnetic lens design using thin planar metasurfaces for high antenna gain, low side lobe level and beam steering applications

Abstract

Electromagnetic (EM) Lens antennas with the ability to achieve high gain and directivity have attracted much attention. Various types of lenses has been proposed to enhance the gain and directivity of the antenna. In an EM lens antenna, the lens transforms the quasi-spherical wave emitted by a source antenna into near plane wave, producing a high gain. However, in cases where gain and directivity of the source antenna is improved but on the other hand side lobe level (SLL) becomes high. Therefore, an EM lens should possess twin properties i.e., achieving high gain and directivity along with suppressed SLL. In addition to that beam steering becomes necessary for mobile transmitter and receiver systems. Recently, researchers has address these problems by the development of ultra-thin metasurface based lenses. Thin Metasurface possess very attractive abilities to enhance the gain as well as steer the electromagnetic waves. It allows cost effective fabrications due to its flat shape which enable us to reduce size especially the thickness and promises for future applications in wireless communications.

A novel electromagnetic (EM) lens with high gain, suppressed side lobe level (SLL) and low profile is designed using thin planar metasurfaces. The proposed design consists of a source patch antenna and an EM lens based on two thin planar metasurfaces. The two thin metasurfaces with an air gap are placed at the focal distance, H of about $\lambda/2.25$ above the source patch antenna. This configuration form a thin planar lens in which the source patch antenna emitted quasi-spherical wave in order to transformed into plane wave. The total size of source antenna and an EM lens is same. The miniaturized EM lens antenna faces high SLL

which can be reduced by maintaining the uniform transmission phase coefficient and tunable transmission amplitude coefficient of metasurface unit cells. By implementing this topology on metasurfaces, the SLL of proposed EM lens can be reduced without affecting the boresight gain of source patch antenna. The measured results of proposed EM lens achieves 7 dB gain enhancement at designed frequency (5.8 GHz) with the SLL suppression of -24 dB. The total volume of planar lens antenna is $1.17\lambda \times 1.17\lambda \times \lambda/2.25mm^3$, which is very compact compared to other reported planar lens designs.

Moreover, in this study, two designs of thin metasurface electromagnetic lenses for high gain and beam steering applications are proposed at 5.8 GHz. These thin metasurfaces are fed by a traditional linear polarized source patch antennas. Firstly, the metasurface based lenses are characterized using GRIN standard function equations and designed using full wave EM simulation CST software. In first design, the unit cells are distributed over a metasurface in circular concentric zones, while in the second design the unit cells are distributed in linear vertical zones over a metasurface. Both the metasurfaces are fed by a source patch antennas. The metasurfaces are place 30 mm away from the source antenna so as to improve boresight gain. This distance is calculated by GRIN function and further optimized by full wave EM CST simulator. Once the boresight gain is improved for both the designs then next is the beam steering of the proposed metasurface lenses. It can be achieved by mechanically sliding the designed metasurfaces in horizontal direction (+x-direction or –x-direction) over source radiator to obtain different beam steering angles. This study demonstrate that the proposed mestasurfaces increase the gain of the source patch antenna from 5.1 dB to 14.98 dB and 5.15 dB to 15.1 dB and beam steering angle range of -23 to +23 degrees and -27 to 27 degrees for lens design 1 and 2, respectively.

Additionally, the proposed EM lens design has been implemented on different source antennas at 5.8 GHz and on millimeter-wave band (26.5-29.5 GHz). The different sources antennas are $2 \times 2$ circularly polarized (CP) array, $4 \times 8$ antenna array and a horn antenna. The presented EM lens proves reasonable performance on the aforementioned antenna arrays. Results shows the reasonable improved in gain and directivity of the antenna arrays.