This article proposes an explanation of how the balun used on some Tonna Yagis works.
It appears Tonna no longer manufactures these antennas, I do not know if this design is novel. I do not recall seeing them used by other manufacturers, they may be protected by patent.
Above is a pic of the balun structure on a 2m antenna.
Above, the manual shows that the black sleeve on the balun sleeve would be slid up over the coax connector, making a neat finish. There are slightly different versions for 70cm antennas. Continue reading (How) does this balun work? (Tonna / F9FT balun for Yagis)