This small project wasn’t meant to happen until I faced serious side problems with the coaxial cable connected with my EAntenna HF fan dipoles. The problem can be summarized like this: Movement of the coaxial cable near the antenna input was severely influencing the SWR across all dipoles, despite the presence of the EAntenna choke balun. So much, that sometimes I was questioning whether tuning could ever be predicted or even achieve it at all. The problem was also analysed here.
The idea of the ‘ugly’ balun was meant to be the solution to this problem. It’s not the first time an ‘ugly’ balun gave this life-saving solution to antenna detuning problems due to the presence of the coaxial cable. I decided for first time to give it a go to see whether it could assist my problem too.
An ‘ugly’ balun is effectively an inductor which aims to reduce and ‘faint’ the skin current flowing on the outside (i.e. shield) of a coaxial cable. This unwanted current flow on the coaxial is caused by the balanced-to-unbalanced transition between a dipole (symmetrical) antenna and a coaxial (unbalanced) cable. This current not only affects the original radiation pattern of an antenna but it also affects the resonance frequency (i.e. SWR of antenna) as the coaxial cable becomes part of the antenna too! The presence of the ugly balun aims to insert a virtual inductor between the dipole antenna input and the coaxial cable in order to eliminate the unwanted current caused by this asymmetrical condition.
Details of the construction:
- Plastic (PCV) tube 63mm diameter, diameter of PVC is not critical. Length of PVC tube depends on the number of turns and the diameter of the coaxial.
- Cable I used for the ugly balun: RG58
- Length of coaxial: 6.4m (anything between 19-20 feet long) for the operation of the choke effect (this length is fixed no matter what cable of cable you are using, even if you had used the fatter RG213, you will still have to twist 6.4m of cable, just make sure though it’s a 50 Ohm cable to match your system), plus allowance for the connectors, so I cut 7m.
- Length of the PVC tube, based on the above information, ended up to be around 30cm or so. I’ve added some extra PVC length to allow the installation of the UHF connector plus some extra space in case I needed to open holes to insert fixing screws etc (see photos below).
So, take one end of the coaxial cable, give some allowance to solder a connector, fix it on the PVC pipe, then start turning it around as if you make a air wound inductor. Make sure that each turn is touching the next. When you reach the necessary length, that is 6.4m, for the implementation of the ‘inductor’ part, stop there and fix it. Use the remaining length to attach it to your connector or to whatever you want to connect with.
As soon as I had all the materials in hand, it took me around an hour or so to complete the construction of the ugly balun.
Testing: I connected the ugly balun between the end of the coaxial cable up in the roof and the entry point of the dipoles (the original EAntenna balun was removed as it wasn’t effective). Any type of balun, including the ugly balun, must be inserted at the feed point of the antenna, don’t forget this. The result was amazing: The SWR across all bands remained locked despite the movement of the coaxial cable underneath the dipole and next to the antenna mast! The ‘ugly’ balun cancelled all asymmetrical currents and the dipoles operated as they were supposed. No matter how long or how I’ve placed the coaxial cable in relation to the dipole, the SWR remained locked in place. Weeks of dipole tuning solved in a hour, the ‘ugly’ balun succeeded!