Country update, April 2002 - LNB got out of focus

Country update, April 2002 - LNB got out of focus

Correcting an Eb/N0 degradation on C-Band antenna

Since the antenna installation, we have monitored a constant degradation of the Eb/N0 ratio on the C-Band antenna. The degradation clearly shows on the graph below, between week 9 and week 12, we lost 1 dB, and there have been a couple of sudden loss during week 9 and 13.

Starting on week 13, the Eb/N0 ratio was down to 3dB where the receiver could not synchronize to the career any more, so we did not receive any data.

The firt control was to check that the receiver was sending the proper power to the LNB. Measured with a voltmeter, it shown 18 V, which is the expected value.

Second control was to call in the installer of the antenna to check the LNB. After inserting a spectrum analyzer between the LNB and the receiver, the measured signal level was around -68dB, while it was measured at about -61dB during the installation.

During the various manipulation, we came to a point where the signal level was back to a value close to th nominal level and the Eb/N0 read about 9dB.

At that moment, one of the technician noticed that he was holding the LNB at a very different height than it was when they first come in to look at the problem.

The LNB had slowly slided down during the time, on the direction marked by the arrow on the picture, so it was completely out of focus.

After readjusting the focus and the polarization, a plastic link was used to prevent further slidding, as it appears that the mounting device for the LNB is of cheap quality and not tight enough.

After adjustment, the signal level measured -61dB on spectrum analyzer, and the Eb/N0 ratio read 10.5dB, which is even better than after the initial installation.