Turnstile antenna

A cross dipole antenna is composed of two crossed dipoles arranged. It was the American engineer George Harold Brown as " Turnstile Antenna" (literally turnstile antenna) in 1935 patented ( U.S. Pat 2,086,976 ). The turnstile is (for example for FM radio) was used as receiving antenna for the all-round receiving horizontally polarized transmitter, as all around radiant transmitting antenna for these applications, as well as transmitting and receiving antenna for circularly polarized waves, such as in space.

The circular polarization is formed when the antenna electrically by 90 ° phase shift of the feeding of a mechanically rotated 90 ° relative to one another against the other dipoles. This phase shift or delay is realized by a λ/4-Umwegleitung, i.e., a piece of high-frequency cable, the length of which, taking into account the velocity factor of the one-quarter wavelength of the signal. Swapping the right or phase is fed dipoles can reverse the polarization direction ( left-circularly / right circularly ).

The circular polarization has advantages in satellite and Weltraumkommmunikation because no polarization fading occurs when the passage through the ionosphere, the polarization direction of the waves (see Faraday effect ) or the spacecraft is rotated in an unpredictable way ( with antenna) rotates.

A circularly polarized antenna can also be linearly polarized waves of arbitrary polarization direction is received, but with a constant loss of 3 dB from the correct circular polarization. Signals in the respective opposite circular polarization are, however, strongly suppressed, so may the independent use of the two polarizations in the same frequency range is possible.

As can be seen from the illustrated examples, the principle of the Kreuzdipoles can be applied by analogy with Yagi antennas, and log-periodic dipole antennas, in order to achieve higher antenna gains and larger bandwidths.

Crossed dipoles are used in the shortwave range, when it comes to special properties of the ionosphere. In the HAARP ionosphere is heated so with left of rotation, by radio broadcasters to radiate with right of rotation vertically upwards, because no energy is then lost through cyclotron resonance. The wave is reflected at several hundred kilometers height of the ionosphere and returns one with the opposite sense of rotation back to the ground, if the plasma frequency is not exceeded.