Techniques for improving the bandwidth performance of an antenna assembly in a mobile device are provided. An example of an apparatus according to the disclosure includes a dielectric substrate having a first area and a second area disposed around the first area, a first radiator disposed on a surface of the dielectric substrate in the first area, the first radiator being configured to transmit and receive radio signals at an operational frequency, and a plurality of metamaterial structures disposed in a periodic pattern on the surface of the dielectric substrate in the second area and within a near field of the first radiator, wherein a maximum width of each of the plurality of metamaterial structures is less than half of a wavelength of the operational frequency.