I was searching for 1:1 balun designs that would be efficient at VHF yet could easily be made from commonly available materials when I came across these two:
While the design on the right seems pretty straightforward, the workings of the left one are a bit of a mystery to me. A quarter-wave shorted stub will, as far as I understand, look like an open circuit at the resonant frequency, so it seems like it shouldn't have any effect, at least if treated as a perfect transmission line...
So how (if at all) does it function as a balun, and what are the necessary conditions?
- Is it important for the coax from which the stub is made to have the same impedance as the antenna (75 ohms)?
- Is the crossover (i.e. connecting the shield of the stub to the feedline's center conductor and vice versa) important, or will the balun also work without it?
- Does the stub have to be oriented in a particular manner relative to the main feedline in order for it to work?