Inteference Issues

Hi all,
I had dis problem while i was trying to align a 1.8m dish on AMC 12.The site is an existing site but the modem had to be upgraded to a 3100 series.
The problem now is that the modem refuses to lock even after i had changed the LNB several times and i now finally noticd that when i stand by the left side of the dish as if i am creating an ark beside it and reflecting the waves back to the feedhorn the voltage suddenly jumps to 16 and when i leave it goes bak to 2v.What kind of a solution can one prescribe to this problem