The “Ugly” Balun
Preventing unwanted RF radiation with an “Ugly” balun or more correctly, an RF choke. One of the problems that can occur with simple wire dipoles is that of the feeder (in this case coaxial cable) radiating some of the RF energy which can lead to interference and other problems.
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. All the choke does is present a high resistance path to the RF current and prevents it flowing along the feeder. This isn’t a guaranteed method, and some trial and error, or even a different method may be required to achieve a satisfactory outcome.
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.
Construction is really simple, all you need is a length of coax, something to use as a former and cable ties (zip ties) or tape to maintain the coil shape when finished.
Almost any round object can be employed as a former for the coil, a lot of amateurs use empty 2 litre cola bottles. A neat trick to make things easier if using the empty plastic bottle method is to fill it with water and freeze it. The ice in the bottle makes it easy to wind the cable around without the bottle crushing. When finished, simply allow the ice to melt, pour out the water and if not already done so, remove the coil from the bottle.
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. If aesthetics isn’t a problem, there’s no reason why you can’t leave the coil on whatever you used as a former, the only restriction here being, the former must not be metal or conductive.