I would suggest using Loctite 660 (Quick Metal) retaining compound. Sand or scuff the end of the telescoping part just far down enough for the new tip. Apply the Loctite 660 to the scuffed area and twist on the new tip. In 24 hours, you should have an antenna tip that won't be coming off any time soon.
http://www.amazon.com/LOCTITE-Retaining-Compound-Quick-Repair/dp/B000O03DK0