INTRODUCTION:

A Turnstile antenna also known as a crossed dipole antenna consists of a set of two identical dipole antennas placed at right angles (90º) to each other. Turnstile antenna invented in 1936 by George Brown to create a circularly-polarized pattern. These antennas are fed with an in-phase quadrature signal (i.e., the signal to each dipole is +/- 90 degrees out of phase with each other)

FREQUENCY RANGE:

The frequency range of turnstile antenna is 30MHz  to 3GHz Which belong to the VHF and UHF bands

WORKING:

To increase directivity and gain, several turnstiles can be stacked along a vertical axis. BAY refers to a pair of stacked dipoles. Multiple bays can be combined to form an antenna with excellent directivity and gain. The polarization depends on their mode of operation.

MODES OF OPERATION:

Turnstile antennas have two modes of operation:

  1. Normal Mode: In this mode of operation, the antenna radiates horizontally polarized waves which are perpendicular to its axis. 
  2. Axial Mode: In this mode of operation, the antenna radiates circularly polarized waves along its axis i.e., parallel to the Earth’s surface.

SUPER TURNSTILE ANTENNA:

Because the radiation output of a turnstile antenna is 3dB lower than that of a halfwave dipole radiating the same power, a super turnstile antenna is designed to overcome this limitation. In Super-turnstile, the simple dipole parts of the turnstile are replaced with four flat sheets. The Super-turnstile array is designed in such a way that 1 to 8 bays can be built on a single mast. Super-turnstile antenna can also be called Batwing Antenna.

RADIATION PATTERN:

omnidirectional radiation pattern with horizontal polarization   is found in Turnstile antenna

(a) The 3-D view  (b) its vertical-plane pattern (c) horizontal-plane pattern at the resonant frequency 

Radiation pattern of single turnstile antenna

ADVANTAGES:

  • Produces higher gain by stacking
  • Better directivity

DISADVANTAGES:

The radiation power is 3dB less than that of a half-wave dipole radiating the same power.

APPLICATIONS:

  • FM & TV broadcasting
  • Military Communication
  • Satellite Communication
  • Spacecraft antenna
  • Batwing or super turnstile array
  • Stacked arrays

CONCLUSION:

The stacked turnstile antenna is the basic turnstile antenna with dipoles stacked above each other vertically. It is commonly used for broadcasting TV due to its omnidirectional characteristics.

Double stacked turnstile is used for Ham Radio purposes. Another type of the stacked turnstile is the batwing antenna or super-turnstile antenna

The advantages of this antenna over the regular turnstile is  stacking allows for a strong directivity and increases the gain horizontally