Sounds like a GA29 which was a replacement for the GA28. If this
is the antenna
, then yes, it can be opened up. I'm a service
tech and have replaced the coax in many of these antenna's. It is no simple task. The top part of the antenna does unscrew but the threads are coated with silicone rubber. You will have to open it with a strap wrench which means you need to make a special mount to hold the lower section of the antenna as it takes a lot of torque to get the top to start turning loose from the rubber. I used an old brass 1"-14 thread mount that I drilled and tapped a 1/4-20 hole into the side of the threads. I screw this up into the base of the antenna and align the tapped hole with the slot in the side of the antenna to allow me to put a bolt in. Then clamp the modified mount into a vise. This allows you to unscrew the top without allowing the antenna to just unscrew from the mount. Your not likely going to be able to just clamp the lower part of the antenna into a vice as a tremendous amount of torque is needed to get the top loose. You can try it, but you might just end up crushing/damaging the lower part of the antenna in doing so. Once you get it open, you have to unsolder a small circuit board that the cable connector is directly soldered to. Then you can get to the nut that holds the connector to the base. You will also need to find a replacement crimp ferrule to be able to re-crimp the braid of the coax to the connector which also means you should have the proper crimp tool, and you have to trim down the cable dielectric in order to fit up into the connector when you redo the cable end.
BTW, you first have to remove a small screw which is under a label on the underside of the antenna before trying to unsrew the top. This allows air to equalize inside the unit.