Thank you for the diagram. That is a 4:1 halfwave coaxial balun which has a low loss.
It has a narrow bandwidth, so you can use it only for one TV band, either VHF-High or UHF, but not both. It must be cut to the proper length for each band as you found out.
A conventional ferrite core balun is needed to cover VHF and UHF, but it has a little more loss than a halfwave coaxial balun.
In post #2 Yurii asked you what balun were you using before the improvement?
If you can not measure it, you can not improve it.
Lord Kelvin, 1883