Cross polarity adjustment in DTT-SAT applications
In the case of having signals transmitted using the OVB-S2 standard in the same frequency but different polarity (e.g. DTTSAT services via Hispasat), the adjustment of the LNB skew angle is critical. Only with a meticulous procedure and the help of a field strength meter as the H45, with Link Margin measurement, the installation can be fitted with guarantee.
The dish alignment and the adjustment of the reception system is divided into two phases:
1. Dish alignment: azimuth and elevation setting .
1.1. Power the LNB at 18V and 22KHz using the field strength meter H45 (fig.1).
1.2. Rotate the LNB to "5 pm" as show in fig.2.
1.3.Set the ELEVATION and AZIMUTH of the dish until getting the maximum level in any frequency (fig.3).
2.LNB skew angle adjustment. This process is divided into
2.1. Adjustment using a spectrum analyzer: center the frequency (11 .970MHz / SPAN 200M Hz) and the spectrum should be as show in fig.4.
Adjust the LNB skew angle until getting a minimum level in this frequency
2.2 Fine adjustment reading the LINK MARGIN at 11 .615MHz Horizontal Adjust the LNB skew angle reading the LINK MARGIN at 11.615 MHz Horizontal (18V). This adjustment will be made rotating the LNB (softly) until getting the MAXIMUM value of the LKM (>3dB) (fig .5)
2.3 Final check of the LINK MARGIN. Once optimized the LNB skew angle, verify the LINK MARGIN at 12.631 MHz Horizontal High / Vertical High and 12.671 MHz Vertical High should be LKM>3dB (fig.6)