How can I extend this outdoor signal cable with waterproof?
They make weatherproof aluminum junction boxes such as a Bell or Red Dot, with matching blank lid.  To avoid box penetrations, their mounting screws attach to "ears" that attach to the outside of the box.   The holes are threaded, as for pipe. 



To enter a cable into such a box, take a specimen of your cable (or its measurements) to a proper electrical supply house (which is not anything like an electronics supply house), and get a weatherproof strain-relief cord connector for it.  This has a rubber grommet that will compress to seal.  



Now if you have cut back your cables too much to fit the junction boxes available, you may need to use two junction boxes, and splice some additional wire between them.  You can optionally connect them with threaded pipe instead of having to buy two more strain reliefs.