English: Closeup of the top of the ABC AM radio tower in Hamersley, Western Australia. The circular structure at top is called a "capacitive hat" or "top hat". Antenna towers at these frequencies are mast radiators; the structure of the mast itself serves as the antenna. Often, because of expense or height restrictions, the mast cannot be built as tall as it should be for optimum radiation. The radial screen of the "capacitive hat" adds capacitance to the top of the antenna, increasing current in the top part, making the tower act electrically as if it was taller, so it has higher gain.