My sirius portable radio keeps displaying "no antenna detected" but it's plugged in.

Several possiblities: one: antenna really is bad - borrow a friends Sirius or XM antenna and plug it into the base and see if the message goes away -if it does, replace antenna; if not, then your 'dock' or base could be bad - try removing and reinstalling the radio or try another base. If another base and antenna don't solve it, then the radio is shot and it's time for a new one. Customer service will ususally give you a great deal on a new XM radio IF you agree to keep it for 1 year. Let me know how it goes.