Cross dipole
A crossed dipole is an antenna that consists of two crossed dipoles . It was patented in 1935 by the American engineer George Harold Brown as the English turnstile antenna (literally translated as "turnstile antenna").
application
The cross dipole is used as a receiving antenna for all-round reception of horizontally polarized transmitters (e.g. for VHF radio), as an all-round transmitting antenna for these applications and as a transmitting and receiving antenna for circularly polarized waves, e.g. B. in space travel.
Horizontal polarization
For terrestrial transmission , crossed dipoles are used as omnidirectional antennas with horizontal polarization.
Circular polarization
The circular polarization is created by feeding the dipoles 90 ° out of phase with each other. This phase shift is realized by a λ / 4 detour line, i. H. a piece of high-frequency cable, the length of which, taking into account the shortening factor, corresponds to a quarter of the wavelength of the signal. The polarization direction can be reversed (left circular / right circular) by swapping the dipoles that are fed in directly or out of phase.
Circular polarization has advantages in satellite and space communication because polarization fading does not occur if the direction of polarization of the waves is rotated in an unpredictable manner when passing through the ionosphere (see Faraday effect ) or the spacecraft rotates (with antenna).
A circularly polarized antenna can also receive linearly polarized waves of any polarization direction, but with a constant loss of 3 dB compared to the correct circular polarization. Signals in the opposite circular polarization are, however, strongly suppressed, so that u. U. the independent use of the two polarizations in the same frequency range becomes possible.
Extension of the principle
The principle of the cross dipole can be applied analogously to Yagi antennas and logarithmic-periodic dipole antennas in order to achieve higher antenna gains or larger bandwidths.
Crossed dipoles are used in the medium and short wave range when special properties of the ionosphere are important. With HAARP , the ionosphere is heated up with a left-hand rotation, with radio transmitters you radiate vertically upwards with a right-hand rotation, because then no energy is lost through cyclotron resonance . The wave is reflected by the ionosphere at a height of a few hundred kilometers and returns to the ground with the direction of rotation reversed if the plasma frequency is not exceeded.
literature
- Karl Rothammel , Alois Krischke : Rothammels Antennenbuch . DARC Verlag, Baunatal, ISBN 3-88692-033-X , chap. 23.2.1.8 - Cross Dipole (Turnstile).
Web links
Crossed dipoles as medium wave transmitting antennas in Europe:
Individual evidence
- ↑ Patent US2086976 : Antenna system. Applied September 20, 1935 , published July 13, 1937 , applicant: RCA, inventor: George H. Brown.
- ↑ Radio frequency pumping of ionospheric plasma ( Memento of the original from December 17, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 14.6 MB)
