An antenna unit is provided. The antenna unit includes a first substrate, a first conductive layer, a second conductive layer, a first planar conductive ring and a feed conductor. The first substrate includes a first surface and a second surface, wherein the first surface is opposite to the second surface. The first conductive layer is disposed on the first surface. The second conductive layer is disposed on the second surface, wherein a main opening surrounded by a plurality of first conductive vias electrically connecting the first and the second conductive surface is formed on the second conductive layer, and the main opening defines a radiation cavity and center frequency. The first planar conductive ring surrounds the radiation cavity. The feed conductor feeds a wireless signal to the antenna unit. Both the first planar conductive ring and the feed conductor are placed between the first conductor layer and the second conductor layer.