Skew (satellite)

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The skew ( engl. Skew , skew ) upon receiving equipment for geostationary satellites, the deviation of the vertical polarization of the received signal from the geographical vertical to. For optimal reception, the LNB must be installed inclined by the skew angle in front of the satellite dish . That is why the term is used to skew LNB tilt (ger .: tilt , skew , tilt used). An even better efficiency of the overall system can be achieved with fixed offset parabolic reflectors by not installing an LNB, but an entire mirror inclined by the tilt value. Seen from behind the mirror in the direction of the satellite, the mirror or the LNB must be turned clockwise if the skew value is positive (counterclockwise if the skew value is negative).

Polarization control by magnetized ferrite rod
Historical mechanical polarizer

LNBs for circularly polarized signals (counterclockwise and clockwise) do not require any skew correction, since a circle looks the same regardless of the rotation. While circularly polarized signals predominate in the C and Ka bands , linear polarization (horizontal and vertical) dominates in the Ku band . In Europe, circular Ku-band signals from Russian programs can be received on Eutelsat 36B / 36C and Express AT1 .

Skew in satellite turning systems

If one looks at possible geostationary satellite positions from Central Europe , they describe an arc of a circle , ascending from the west on the horizon exactly to the south at the zenith , towards the east on the horizon again disappearing. Satellite rotating systems equipped with a so-called polar mount follow exactly this arc when the satellite antenna is moved. A polarization of communication satellites is normally aligned linearly to the equator , the correct polarization (skew) is set automatically in rotating systems. An exception are direct reception satellites such as the Astra 1M , whose polarization is often aligned with a reception target area and not with the equator. Some rotating systems therefore have a so-called polarizer that can also compensate for this deviation for each satellite. Rigidly mounted receiving systems, which today usually no longer have a polarizer, must, however, take the skew into account during assembly. In the case of satellites such as Astra , which are frequently used in Europe, this effect can almost be ignored due to its polarization alignment with its main target area; it is sufficient to mount the receiving system and the LNB perpendicular .

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