An array antenna comprises a first substrate comprising a first plurality of ceramic layers; a second substrate comprising a second plurality of ceramic layers; a bottom ground plane stacked on the bottom of the second ceramic substrate; a plurality of quasi-cavity-backed patch antennas mounted on a top surface the first substrate, each of the patch antennas including a radiating element and two grounded grid-like conductor walls; and a mixed feeding network coupled to each of the patch antennas. The array antenna working at mm-wave frequency band can provide high radiation efficiency and low loss from feeding network by using quasi-cavity-backed patch elements and a mixed feeding network configuration.