Preventing unwanted RF radiation
The main reason for the coax radiating RF is due to the manner in which RF energy travels along, or very close to, the surface of the cable.
This is called the “skin effect” thus when a balanced antenna such as a dipole is fed with coaxial cable the current travels along the centre core and the shield.
Because the shield is sandwiched between two layers of insulation RF energy is able to flow on the surfaces of both the inside and outside of the shield.
It is when current flows on the outside an imbalance of current is introduced as RF energy “sees” this part of the shield as another path on which it can travel and as it has nowhere to go. The unbalanced current “leaks” and is thus radiated.
To help prevent the RF “leaking” from the coax feedline a balun, choke or ugly balun may be used.
Some interesting methods of constructing an “ugly balun” can be found in the link above.
I constructed mine by winding approx 21 feet of coax around an empty coffee jar with a sheet of paper wrapped around it. The paper was to make it easier to slide the cable off the jar once the coil was wound.
Before removing the coil from the jar it was covered in black duct tape to ensure the coil kept its shape when removed from the jar.
Once removed the coil was completely covered with duct tape and a cable tie used to prevent the ends from pulling away.