Turnstile antennas

A turnstile antenna, or crossed-dipole antenna,[1] 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.[2][3] 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.

1. TURNSTILE ANTENNAS - BY ALANKRIT MISHRA

2. Turnstile antenna, also called crossed dipoles is another type of array antenna. The shape of this antenna symbolizes the turnstile, which is used at the entrances of few places. A basic turnstile consists of 2 horizontal short dipoles placed normal to each other Dipole antenna – It is the simplest form of radio antenna , consisting of a conductive wire rod that is half the length of the maximum wavelength the antenna is to generate. Dipole means two poles.

3. Frequency range : 30 MHz to 3 GHz (VHF and UHF bands) Construction and working : Two identical half wave dipoles are placed at right angles to each other. The two dipoles are energized with currents of equal magnitude but in phase quadrature. To provide high directivity, several turnstiles may be stacked along a vertical axis. The polarization of these antennas depends upon their mode of operation. The pair of such dipoles frequently stacked is known as Bay.

4. In the figure shown, the two bays are spaced half wavelength apart and the corresponding elements are fed in phase. The radiation produced by the combination of bays results in better directivity.

5. Radiation pattern : The individual field patterns of the two dipoles are figure of eight fitted by 90 degree. The total field pattern as a function of and time is given by, Which reduces to , It produces circular patterns in the θ plane  

6. At any value of θ the maximum amplitude of E is unity at some instant during each cycle. Hence, RMS field pattern is circular. At any instant of time the pattern is a figure of 8 of the same shape as for a single infinitesimal dipole. An instantaneous pattern is shown in the above figure for ωt=135ᴼ.As a function of time this pattern rotates completing 1 revolution per cycle. Considering the clockwise rotation, the phase of the field as a function of θ is given by, θ+ωt=constant And if the constant is zero, ω t =- θ

7. The field in the axial direction (normal to the crossed infinitesimal dipoles )has a constant magnitude given by, Practically, turnstile is preferred to operate in horizontal plane using λ /2 dipoles(George brown turnstile antenna). The field pattern in the plane of turnstile is not perfectly omnidirectional as shown in the figure

8. The relative field as a function of θ and time is expressed by, Although the θ plane pattern with λ /2 elements differs from the pattern with infinitesimal dipoles, the radiation is circularly polarized in the axial direction from the λ /2 elements provided that the currents are equal in magnitude and in phase quadrature. In order that the two dipoles be in phase quadrature, the dipoles may be connected to separate non resonant lines of unequal length. Another method of obtaining quadrature currents is by introducing reactance in series with one of the dipoles.

9. Modes of operation: 1.Normal mode : In normal mode of operation, the antenna radiates horizontally polarized waves which are perpendicular to its axis. 2.Axial mode: In axial mode of operation, the antenna radiates circularly polarized waves along its axis i.e. parallel to the axis. SUPER TURNSTILE ANTENNA In order to obtain a very low VSWR over a considerable bandwidth, the turnstile antenna has been modified as super turnstile. Vertical metal plates can be used instead of dipoles known as super turnstile. There is almost zero field along the axis of the turnstile.a metal plate of approximate dimension 0.7 λ by 0.5 λ can replace a λ /2 dipole.

10. The metal plate can be replaced by tubing or grid of conductors to reduce the wind resistance. The super turnstile is most suitable for tv transmission for frequencies from 50MHz. Directivity can be increased by stacking super turnstiles one above the other.

11. ADVANTAGES : 1.High gain is achieved by stacking. 2.Super turnstile produces high gain output. 3.Better directivity is achieved. DISADVANTAGES : 1.The radiation power is 3dB below the maximum radiation of a half wave dipole radiating the same power. APPLICATIONS : Used for VHF communications,FM nad TV broadcasting,military communications and satellite communications.

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