For years I've read on forums about people finding moisture in their feed horns. Sometimes they'll report droplets on the interior of the lens, or sometimes puddling on inside of that lens with noticeable degradation of signal strength. There have also been reports of white deposits on the interior of the LNB's wave guide and the interior of the feed horn. The example below is a little unusual because the plastic seal on the feed horn interface to the LNB is intact and apparently kept moisture out of the feed horn. Therefore, no moisture was probably ever noted in the feed horn window. The owner of this system reported a complete loss of signal. I imagine that it had been degrading for some time but just had not been noticed.
On inspection, it is plain to see that the O ring that is supposed to seal the mechanical interface between the feed horn and LNB had been pinched during assembly and that is apparently how the moisture was allowed to infiltrate...
