Reconfigurable Metasurface for Electronic Beam Scanning Antennas

This paper presents an electronic beam steering antenna for radar applications obtained by overlapping a reconfigurable transmitarray metasurface to a conventional fixed-beam monopulse antenna. The transmitarray is an active metasurface including pin or varactor diodes to change the antenna beam pointing direction by electronic control. Some configurations have been analyzed and compared. Results are promising for new low-cost radar antennas without mechanical gimbals as an alternative to AESA antennas. Phased-array antennas are progressively substituting mechanically gimballed antennas for radar and communication applications. Actually, phased-array technology leads well-known advantages in comparison to conventional antennas with mechanical scanning. On the other hand, phased-array antennas involve some drawbacks in terms of costs and complications for which they are not suitable for some application. Therefore, new solutions for electronic beam scanning of antennas are sought. Active transmitarray metasurface is a solution for this problem. The transmitarray is a dielectric multi-layer metasurface that, placed in front of an antenna, re-radiates the wave coming from the antenna changing its pattern and then modifying beam pointing, gain, beam-width and/or polarization. An active transmitarray includes electronic components (i.e. diodes) that allow an electronic control of the beam. Therefore, classical fixed beam antenna can become electronic-beam antenna simply by overlapping on it a dielectric multi-layer metasurface with embedded low-cost off-the-shelf components. The transmitarray metasurface is divided into sub-wavelength unit cells, and each one includes some active components able to modify phase distribution of the re-radiating surface and, consequently, antenna beam pointing direction. Several different unit cells have been analyzed, with pin or varactor diodes as active components. Varactor diodes allow analogic phase variation, but involve too many losses, especially over X-band. Pin diodes, conversely, allow only digital phase variation (2 diodes for bit), but they imply low losses. So, the best choice for high frequency bands are pin diodes, although they involve phase errors on the array. In fact, simulations have shown that in a wide array, 2-bit phase quantization (4 pin diodes per unit cell) is satisfactory: side-lobes grow, but essentially performances remain acceptable. Full beam control of the antenna, i.e. 2-D beam scanning, requires one control line for each cell. This is difficult to be implemented due to limited area available per unit cells. In this case, two stacked transmitarrays are possible, each one controlled by row and therefore scanning in one plane only. This solution, however, is complicated and presents double losses. In alternative, a hybrid solution is possible: one transmitarray with pin diodes controlled for row allows elevation scanning, whereas transmitarray rotation assure azimuth scanning. Simulations of monopulse radar antenna with 2-bit pin-diode transmitarray have been carried out. Results have shown good performance in terms of beam-width, side-lobes (despite quantization) and scan performances.