You just have to ensure that the inner and outers are connected correctly on the Male plugs - with no shorts. That cable carries power and if you short it you can zap your STB.
A good idea is to try the box without your extension first.
If some one has told you that the LNB is broken why don't you believe them? New LNBs are very cheap (a few pounds each) but if it is a Sky dish make sure you get the correct LNB as they are not standard.
BTW I'd put future questions in the satellite receivers section.It was only luck that I looked here - its for electrical stuff rather than electronic.
t
January 2008