Coax Balun
By G8MNY

This type of balun is to be found in most textbooks and many aerial designs.
The balun is a quarter wave of scrap coax alongside the feeder (a pipe will do).

Illustration by G4TUT. Not to scale

The feeder inner is connected to the dead coax braid. The two Blue lines represent 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 diameter. It is connected to the feeder a quarter 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 equally from both halves of the balanced quarter wave lines and hence cancel. The 2 balanced folded quarter waves look like a half wave where the centre is at RF ground and therefore can be connected to the feed coax outer without any RF left on the outer of the coax skin.

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