If your antenna is a typical quarter wave whip i.e about 20" high, there should not be continuity between the braid and the center wire when the cable is disconnected from the
radio. If the antenna is supposed to provide some gain then it''s possible that phasing stubs etc. could provide a path to the braid. Disconnect the co-ax from the ant and see if the short still exists. if it does, the problem is obviously in the coax. If you assembled the co-ax cable double check the connectors for a possible short. i.e. disconnect one connector and check, if still shorted, check the other end the same way.
There is always the possibilty of water contamination shorting out one of the connectors.
Also make sure no screws or sharp edges have cut into the co-ax.
Jim