This type of balun is to be found in most textbooks & many aerial designs.

Balun is 1/4 wave of scrap coax alongside the feeder.

        -------------
    B -@! dead coax !                    @=Solder
      ! ============@------------------
      \[                         Feeder
    B ---------------------------------

The feeder inner is connected to the dead coax braid BB are the 50 ohm balanced
feed points. The dead coax is what it says, a shorted out at both ends bit that
could even be copper pipe of the same diamater. It is connected to the feeder
1/4 wave back from the aerial where the braids join together. At the aerial end
the dead coax is connected to the feeder's inner.

This works by making the aerial connections look "balanced", as the +/- voltage
phases appear to radiate equaly from both halved of the balanced 1/4 wave lines
& hence cancel, the 2 balanced folded 1/4 waves look like a 1/2 wave where the
centre is at RF ground & therefore can be conneted to the feed coax outer
without any RF left on the outer of the coax skin.

73 De John, G8MNY @ GB7CIP