English: Yagi antenna for UHF HDTV reception. The antenna's main lobe is off the right end of the antenna; it is most sensitive to stations in that direction. Each of the metal crossbars along the antenna support boom is called an element, which acts as a half-wave dipole resonator for the radio waves. The antenna has one driven element which is attached to the TV; it is behind the black box (left). The black box is a preamplifier which increases the power of the TV signal before it is sent to the TV set. The 17 elements to the right of the driven element are called directors; they reinforce the signal. The 4 elements on the V shaped boom (left) are called a "corner reflector"; they serve to reflect the signal back toward the driven element. Yagi HDTV antennas use a corner reflector to increase the bandwidth of the antenna. The rest of the antenna increases the gain at higher channels, while the corner reflector increases the gain at lower channels.