What is a Turnstile Antenna?

A Turnstile Antenna (also know as a crossed-dipole antenna) consists of a set of two identical dipole antennas placed at right angles (90º) to each other. 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.

Turnstile antennas have an omnidirectional radiation pattern with horizontal polarization. They usually operate at VHF and UHF frequencies from 30 MHz to 3 GHz. They are most often used for FM & TV broadcasting, military and general satellite communication applications.

Several turnstiles can be stacked along a vertical axis to provide higher directivity and gain. A pair of such stacked dipoles are also referred to as BAY. Multiple bays can be placed together to create an antenna with high directivity and gain. The polarization of these turnstile antennas depends on their mode of operation.  

A two-bay axial mode turnstile antenna is shown in the figure (left)

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.

For circular polarization, the transmitter radiating with right-circular polarization should have a receiver with the same right-circular polarization and vice-versa. If the polarization of the receiver and transmitter is not matched there will be a severe loss in gain.

Apart from providing high-gain and directivity, this antenna does have a limitation. The major disadvantage of a turnstile antenna is that its radiation power is 3 dB below the maximum radiation of a half-wave dipole radiating the same power. This can be overcome by using the super-turnstile antenna (it is also known as Batwing Antenna where the simple dipole elements in turnstile are replaced by flat sheets).