Is your signal dropout all day long, or only at certain times of the day? When I originally installed my outdoor antenna, the signal was OK. It was only after extended listening, that I noticed three periods per day of extended signal loss, each lasting about an hour. Since Sirius has three birds, and they are not stationary, I made the connection, and moved the antenna.
By "professionally installed" I assume you mean they charged you for installation. If they are professional, they should be in contact with you to check how well the system is working for you. If you are having antenna placement probblems, they should correct it.