A turnstile antenna is a set of two dipole antennas aligned at right angles to each other and fed 90 degrees out-of-phase. The name reflects that the antenna looks like a turnstile when mounted horizontally. When mounted horizontally the antenna is nearly omnidirectional on the horizontal plane. When mounted vertically the antenna is directional to a right angle to its plane and is circularly polarized. The turnstile antenna is often used for communication satellites because, being circularly polarized, the polarization of the signal doesn't rotate when the satellite rotates.
The principles of the turnstile antenna are also applicable to Yagi and Log-periodic antennas.