Is this for power antenna?

Yes, you can purchase the rebuild kit and fix this problem..... however.... I have had the same problem with my power antenna (non CB) and have repaired twice.  The internal cable that pushes up the mast and pulls it back down is probably broken, so when the motor is pushing up the antenna it never gets the "resistance" when fully extended to shut off the motor.  Over the years I have found the repair to last anywhere from 1 to 2 years.  After the last failure I just replaced the entire unit.  There are also several UTUBE's on this repair.

Your CB antenna replacement use to be available but is now discontinued however you may be able to find one