An antenna includes a radiating element with a shape substantially conforming to a quadrilateral, a grounding and feed-in element, substantially surrounding the radiating element and having an opening formed near to a fourth side of the radiating element, wherein the grounding and feed-in element is electrically connected to a ground at one side of the opening and is electrically connected to a signal feed-in terminal at another side of the opening, a first connection element, having a terminal electrically connected to a first side and the fourth side of the radiating element, and another terminal electrically connected to the grounding and feed-in element, and a second connection element, having a terminal electrically connected to a third side and the fourth side of the radiating element, and another terminal electrically connected to the grounding and feed-in element.