An antenna system comprises first and second support structures, and a plurality of first, second and third antenna elements. Each of the first and second support structures has four sides. Two adjacent sides are perpendicular to each other so that both the first and second support structures form a closed loop. The first support structure is disposed outside of and surrounds the second support structure. Some first antenna elements are disposed on two symmetric sides of the first support structure, and other first antenna elements are disposed on two symmetric sides of the second support structure. Some second antenna elements are disposed on another two symmetric sides of the first support structure, and the other first antenna elements are disposed on another two symmetric sides of the second support structure. The third antenna elements are disposed on the four sides of the second support structure.