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A batwing or super turnstile antenna is a type of broadcasting antenna used at VHF and UHF frequencies, 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 generate a horizontally polarized signal. The advantage of the "batwing" design for television broadcasting is that it has a wide bandwidth. It was the first widely used television broadcasting antenna.
Design and characteristics
Batwing antennas are a specialized type of crossed dipole antenna, a variant of the turnstile antenna. Two pairs of identical vertical batwing-shaped elements are mounted at right angles around a common mast. Element “wings” on opposite sides are fed as a dipole. To generate an omnidirectional pattern, the two dipoles are fed 90° out of phase. The antenna radiates horizontally polarized radiation in the horizontal plane. Each group of four elements at a single level is referred to as a bay. The radiation pattern is close to omnidirectional but has four small lobes (maxima) in the directions of the four elements.
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 gain (more of the energy radiated in horizontal directions and less into the sky or down at the earth), suitable for terrestrial broadcasting.
The "batwing" shape is used because of 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 stations (channels 2-6) in the early days of broadcast television.
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