Turnstile antenna

A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase. The name reflects the notion the antenna looks like a turnstile when mounted horizontally. The antenna can be used in two possible modes. In normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis. A turnstile antenna, or crossed-dipole antenna, is a radio antenna consisting of a set of two identical dipole antennas mounted at right angles to each other and fed in phase quadrature; the two currents applied to the dipoles are 90° out of phase. The name reflects the notion the antenna looks like a turnstile when mounted horizontally. The antenna can be used in two possible modes. In normal mode the antenna radiates horizontally polarized radio waves perpendicular to its axis. In axial mode the antenna radiates circularly polarized radiation along its axis. Specialized normal mode turnstile antennas called superturnstile or batwing antennas are used as television broadcasting antennas. Axial mode turnstiles are widely used for satellite ground station antennas in the VHF and UHF bands, as circular polarization is often used for satellite communication since it is not sensitive to the orientation of the satellite antenna in space. The turnstile antenna was invented by George Brown in 1935 and described in scholarship in 1936. The patent history reveals the popularity of the turnstile antenna over the years. The antenna can be used in two different modes: normal mode and axial mode. In directions perpendicular to its axis the antenna radiates linearly polarized radio waves (horizontally polarized when the antenna's axis is vertical). This is called normal mode. The radiation pattern, a superposition of the two dipole patterns, is close to omnidirectional but actually 'cloverleaf shaped', with four small maxima off the ends of the elements. The pattern departs from omnidirectional by only ±5 percent. The radiation in these horizontal directions is often increased by vertically stacking multiple turnstile antennas (called 'bays') fed in phase. This increases the gain by strengthening the radiation in the desired horizontal directions but causes partial cancellation of the radiation in vertical directions, reducing power wasted radiated into the sky or down toward the earth. These stacked normal mode turnstile antennas are used at VHF and UHF frequencies for FM and television broadcasting. Since the first turnstiles invented by Brown operated in this mode, the normal mode turnstile is occasionally called the George Brown turnstile antenna. Off the ends of the antenna's axis, perpendicular to the plane of the elements, the antenna radiates circularly-polarized (CP) radio waves. This is called axial mode. The radiation off one end is righthand-circularly-polarized and the other end is lefthand-circularly-polarized. Which end produces which polarization is determined by the phase of the feed connections. Since in a directional antenna only a single beam is wanted, in a simple axial-mode antenna a flat conducting surface such as a metal screen reflector is added, a quarter-wavelength behind the crossed elements. The waves in that direction are reflected back 180° and the reflection reverses the polarization sense, so the reflected waves reinforce the forward radiation. For example, if the radio waves radiated forward are right-circularly-polarized, the waves radiated backwards will be left-circularly-polarized. The flat reflector reverses the polarization sense so the reflected waves are right-circularly-polarized. By locating the reflector λ/4 behind the elements the direct and reflected waves are in phase and add. Addition of the reflector increases the axial radiation by a factor of 2 (3 dB). Another common way to increase the axial mode radiation is to replace each dipole with a Yagi array.