Abstract: Embodiments of millimeter-wave antenna structures are generally described herein. The antenna structure may include an a radiating-element layer comprising a patterned conductive material, a ground layer comprising conductive material disposed on a dielectric substrate, and a feed-line layer comprising conductive material disposed on a dielectric substrate. In some embodiments, the antenna structure may include an air-gap layer disposed between the radiating-element layer and the ground layer. The air-gap layer may include spacing elements to separate the radiating-element layer and the ground layer by a predetermined distance. In some other embodiments, the radiating-element layer may be disposed on a radiating-element dielectric substrate which may include one or more cavities between the radiating-element layer and the ground layer.