Thank you for your help!
At first, I'm not very sure if I have a monoblock antenna or not, I did not install it. I only know that I have one antenna for both satellites and 8 cables coming out of the antenna to my multiswitch.
Actually, the symptoms I have are quite similar to yours (BER in some situations), but the real problem is that, in some situations, for an unknown reason, the DM-7020 cannot find anymore a channel that it scanned before, displaying the message 'Service not Found'.
Sometimes, it's for an Astra channel, sometimes, it's for an Hotbird channel. When this message is displayed for an Astra channel, if I go to the 'Satfind' menu and look at the signal for Astra, I have BER and the signal cannot be locked. When the message is displayed for an Hotbird channel, in the 'Satfind' menu for Hotbird, I have BER and the signal cannot be locked.
This problem seems to appear only when I zap between channels. Once I can see a channel, if I don't change to another channel, it seems it can be locked indefinitely.
I don't know a lot about satellite antennas but I guess this is linked to a problem in the commands that are sent by the box to the antenna when swithching channels (polarity change, satellite chang, etc). For example, if I'm switching from channel A that I can see perfectly to channel B and then go back to channel A, channel A may not be found anymore so I guess that the antenna did not switch correctly to the configuration that is necessary to lock on channel A.
If I want to see a channel for which the message 'Service not Found' is currently displayed, I need to turn off the box and back on. Rebooting Enigma is not always enough. I also know that turning off the satellite receiver put the multiswitch in standby. So, may be, when restarting, the first command sent by the DreamBox is correctly interpeted by the multiswitch but not all the following ones...
So the question is what's wrong in my system, the DreamBox configuration or the Antenna or the multiswitch configuration ore something else?