Wideband Active Region Metasurface Antennas

Modulated metasurface (MTS) antennas with broadside beam rely on the interaction between a radially modulated equivalent impedance and a surface wave (SW) with cylindrical wavefront, launched by a point source. At the frequency where the SW wavelength matches the period of the impedance modulation, the -1 indexed (leaky) mode of the Floquet-mode expansion in cylindrical coordinates provides a broadside beam. The mismatch between the SW wavelength and the period of the modulation imposes a limitation on the product bandwidth-gain. Here, we overcome this limitation by exponentially stretching the radial period of the impedance modulation. Doing so, an annular active region is generated on the antenna aperture, which moves from the antenna center to the circular rim as the frequency decreases. This mechanism enables a broadside beam over an extreme large bandwidth. We, therefore, significantly extend the applicability of these antennas, e.g., to requirements of 30 +/- 1.5 dB gain over 30% bandwidths. Here, an analytical formulation is proposed to treat the active region migration and edge outgoing by a Fresnel-type transition function. This function predicts in closed form the antenna bandwidth and average gain. A more accurate gain versus frequency response is also introduced by an integral formula that accounts for the frequency-dependent amplitude distribution of the aperture fields. The theory is validated by full-wave simulations and by measurements of a prototype realized by subwavelength elliptical patches. The presented results show that these antennas can provide a performance difficult to reach by any other flat antennas based on printed technology.