An apparatus having multiple mushroom structures is disclosed. Each of the multiple mushroom structures includes: a ground plate; a first patch provided parallel to the ground plate with a separation of a distance to the ground plate; and a second patch provided parallel to the ground plate with a separation of another distance to the ground plate, which another distance being different from the distance from the first patch to the ground plate, wherein the second patch is a passive element which is capacitatively coupled with at least the first patch.