Tracking on sidelobes |
The parabolic dish reflector in the antenna focuses the beam from the satellite toward the LNB receiver. In a perfect world, this beam focus would be just a single beam from the satellite to the center of the dish, but in the real world the dish has more than one lobe of gain toward the satellite. In fact there are several lobes spirallng off from the center of the dish.
With larger antenna sizes of 1 meter and above, and with strong satellite signals, it is possible for the antenna to begin tracking the satellite on one of the sidelobes.
If the threshold is set too low, the antenna could happily track the satellite when it is actually pointing a few degrees away. If the signal is strong enough, you could even get a good picture on some of the stronger channels, but the picture will break up into pixilation on some of the weaker channels.
When tracking on sidelobes, one usually sees the AGC drop from a normal ~2100 down to about ~1800 and the quality of the signal on the decoder signal strength meter drops down to about 50 or 60 percent.
To confirm this, go to the AZ-EL Antenna menu on the DAC and use the up, down and left, right buttons to drive the antenna in azimuth and elevation and watch the AGC to see if there is a stronger signal within a few degrees of where the antenna is tracking.
Imagine the above illustration in 3 dimensions with the sidelobes in a donut around the main beam, so the main beam could be above or below, or left or right of where you are tracking.
If this is the case, it is necessary to set the threshold above the sidelobes, so that the antenna will continue to search to find and track the main beam. This can be done by increasing the auto threshold in the setup menu from the default 0100 to about 0120 or 0150, or in extreme cases set it to 0000 to go to manual threshold and adjust the threshold manually in the Satellite menu so that it is above the sidelobe AGC but still below the real, mainbeam AGC.