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batwing antenna
A batwing or super turnstile antenna is a type of radio antenna named for its distinctive shape which resembles a bat wing or bow tie. Stacked arrays of batwing antennas are used as television broadcasting antennas due to their omnidirectional characteristics. Batwing antennas typically generate a horizontally polarized signal. The advantage of the "batwing" design for television broadcasting is that it has a wide bandwidth.
==Design and characteristics==
Batwing antennas are essentially an improvement of the slot antenna, where a vertical 1/2 wavelength slot (or stacked slots spaced properly and fed in proper phase for gain) is cut into a round conductor or cylinder. Then the slot is fed at its center on each side, creating a RF field in the horizontal field like a stacked set of ring or halo antennas. A common misconception is the batwing is derived from the turnstile or crossed dipole antenna; however, that is farther from the truth. The antenna radiates horizontally polarized radiation in the horizontal plane. Each group of four elements at a single level is typically referred to as a bay so the radiation can be omnidirectional.
To reduce power radiated in the unwanted axial directions, in broadcast applications multiple bays fed in phase are stacked vertically with a spacing of approximately one wavelength, to create a collinear array. This generates an omnidirectional radiation pattern with increased horizontal directivity (more of the energy radiated in horizontal directions and less into the sky or down at the earth), suitable for terrestrial broadcasting.
The most notable characteristic of a batwing antenna is its wide bandwidth of approximately 20% of operating frequency at a VSWR of 1.1:1. This makes the antenna design suitable for broadcasters who wish to use a single antenna to transmit multiple television signals and thus made the batwing the preferred antenna for lowband TV operators in the early days of broadcast television, which started on channels 2-6.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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