Monopole antennas are provided. A monopole antenna transmitting radio signals within a specific frequency range includes a substrate, a ground, a first sleeve portion, a second sleeve portion, a first conductive element, a second conductive element and a cable. The first conductive element and the ground are formed on the substrate. The first and second sleeves electrically connect the ground and project from a side of the ground in a first direction. The first conductive element comprises a feed end and a connection portion adjacent to an edge of the substrate. The second conductive element connects the connection portion and projects from the edge of the substrate substantially in the first direction. The cable connects the ground and the feed end to transmit the radio signal.