This has be done carefully as shorting the braid to the inner conductor may damage your pa finals and will do nothing to help reception. Discard any 'solder-free' crimp plug that comes with the antenna - they're sh*te. I'd recommend you purchase a good quality PL259 plug that can be soldered (these usually have dark brown insulation and are available as one-offs in most chandelries). Cut off the outer layer of the coax about an inch from the end then roll the braiding back over the outside. Then, cut about half an inch of the innner to expose the conductor. Tin this with solder and then assemble the plug (don't forget to put the threaded bit on beforehand). Push the cable in, then (with difficulty), twist so the braiding section goes in as well. When the conductor is sticking out the end, cut it flush then put a big blob of solder on the end making sure there are no gaps. Try to ensure the solder at this point doesn't trickle down inside too much as it may short with the braid. After the solder's cooled, smooth it off carefully with some sandpaper then put some sealent around any braid that's sticking out the bottom of the plug. If you know a marine electrical engineer borrow his or her VSWR meter and check your ratio is less that 1:3. If it's off the scale you most likely have a short. If all's ok, make a few test calls to the Coastguard, have a listen to make sure the rx's ok and then Bob's your uncle.