Found this on the first Google search I tried:

"High Mount Roof Antenna
The Civic Hybrid has a high mount roof antenna. Its location helps prevent interference that may occur from the advanced electronics in other parts of the vehicle. The antenna is isolated from interference sources and enhances reception. The antenna can pivot 90 degrees on its center mounted roof mount and quickly unscrews for car washes. Non-hybrid Civic sedans and coupes use a rear window glass integrated antenna."

Will post more info as I googlocate it....