During the night due to the occuring ionosphere the radiation of a vertical mast produces a fading zone in which the reception is distorted.
The fading zone is defined as an area where the difference of levels between ground wave and sky wave field strength is less than 6 dB.
Due to different phases of these field strengths - the path along the ionosphere is longer than along the ground- distortions of reception occur.
Hence the idea came up to install a horizontally polarized antenna, which radiates the whole power towards the ionosphere. There the power is reflected towards the earth where now nearly everywhere the skywaves are much stronger than the groundwaves.
The cross dipole is a very efficient horizontally polarised antenna. The additional circular polarisation uses a distinctive behaviour of the ionosphere.
The correct sense of rotation of the wave being incident to the ionosphere enables a nearly complete reflection of the energy. The loss in the ionosphere is little.
During day time one of the masts is utilized as vertical radiator in order to provide a ground wave coverage.
The cross dipole in Arganda is an ionospheric antenna which consists of five guided supporting masts and the two horizontal dipoles arranged orthogonally atop of the masts. The cross dipol antenna has a circular polarisation, which means that the wave of one dipole lags the wave of the other dipole by 90 degrees.
A right or left rotating field occurs depending on which dipole lags.
The correct sense of rotation of the polarization is important. The wave being incident to the ionosphere splits into an ordinary and extraordinary wave. The extraordinary wave is attenuated nearly completely while the ordinary wave is reflected. When standing below the antenna and looking towards the ionospere a right sense of rotation is necessary to incite the ordinary wave.