On one occasion I needed a longer cable. The other occasion, last week, I fitted a system to a car and wanted to make the hole in the body work as small as possible.
It's 50ohm co-ax. I fold back the screen on both ends for about 15mm. Slip some shrink over the cable outer. Shorten the inner and tin just about 1mm on each centre conductor leaving a wee blob on one of them. Next, solder the tips together and quickly push them together while they are still hot. They will push up inside the core until the cores butt and become virtually continuous. The wee blob of solder retains the heat for long enough for you to do this, (if you're quick
Ensure it's well done, bring back the screen from one end and smooth it. Bring back the screen from the other end to form an overlap. Give it a wee touch of solder just to ensure a sound conductor, then slide the shrink over the top and shrink it.
If it is going to be subject to the weather, I would also make a sleeve from silicon tube because it makes a completely watertight seal.