It's a question I've often wondered about being primarily a control line flyer. It used to be everybody routed the antenna out behind the wing and up to the tip of the vertical fin.

On my kit-built LT-40, I ran the antenna down through the bottom of the fuselage as per instructions (about 3" behind the firewall). It runs along the bottom of the fuselage and ends about 4" short of the rear of the fuselage. I used one of those Dubro clips and a thin wire s-hook at the very rear of the fuselage. A rubber band is used between the plastic clip and the wire s-hook.

I considered routing it inside the fuselage but I substituted metal pushrod for rudder and elevator. I've heard it's not a good idea to have your antenna run parallel to anything which conducts a signal.

RCKen had good description and photos of an antenna tube installed in the fuselage of his "Build it Now" LT-40 thread.