Flat Gain Broadband Metasurface Antennas

Modulated metasurface (MTS) antennas can provide a broadside pencil beam at the frequency where the cylindrical surface wave (SW) wavelength matches the period of the impedance modulation. For modulations with constant period, the mismatch between the SW wavelength and the period imposes a limitation on the gain-bandwidth product. However, this limitation can be overcome by shaping the local period as a function of the radial distance. Doing so, we generate an annular active region on the antenna aperture, where the SW-to-impedance interaction mainly occurs. Such active region moves from the antenna center to the circular rim as the frequency decreases. This article shows that one can optimize the profile of the local periodicity function to obtain broadside pencil beams over large bandwidths, while preserving the flatness of the gain versus frequency response and a good stability of the phase center. The antenna performances so obtained are really unique for flat antennas based on printed technology. Finally, we present a simple formula for the product between average gain and bandwidth, which gradually blends into the already known expression for modulations with constant period. This formula establishes an absolute limit of the gain-bandwidth product, which only depends on the wavelength-normalized antenna radius at the central frequency.