It depends if you want to pick up anything on the ground. At 18km your ability to pick up the tower may be in question as well, depending upon your relative elevation and path. My advice would be to wait until you move, use what you've got out there and report back here with your findings, particularly what you can no longer hear that you were hearing before (now). That will tell us what type of gain you are going to need, if any. You may find that since you can still pick up planes on short final or takeoff and maybe the tower, too, it might not be worth the bother. On the other hand if you are in a superior location, like the top of a mountain or right underneath a giant water tower reflector, you might capture more than you did before. Let's just wait and see.