It seems to me that this one came up before. The small DC fan motors use a brush and commutator system that makes a lot of sparks and electrical noise. This will be picked up by your antenna on the roof, but might not affect your cable reception. With digital TV, too much noise on the signal will blank out the screen.
The classic way to filter this out is to wire a small ceramic capacitor (.001 uFd, 20 volts or greater) across the power wires as close to the motor as possible. This shorts out the AC signals (radio frequency waves are AC signals) on the power wires while not affecting the DC power. A radio noise filter from Radio Shack might help too.
Some people have tried switching the fan out to a computer-type "muffin" fan. This type of fan doesn't produce electrical noise as it uses a different type of motor without a commutator.