Bazooka Baluns

The fundamental features of a Bazooka Balun are shown in Figure 1.

Figure 1. Bazooka or Sleeve Balun.

An equivalent diagram of this balun is shown in Figure 2.

Figure 2. Equivalent Representation of Sleeve Balun.

The green sleeve in Figures 1 and 2 acts as a transmission line, that is short circuited at the end. From Gauss's law, it is (basically) true that the current on the inside of the outer arm of the bazooka (green line) must be the opposite of that flowing on the outside of the coax (grey line). Hence, the current IC actually sees a short-circuited transmission line. If the length L of the sleeve is chosen to be a quarter-wavelength (at the desired frequency of operation), then the impedance that the current IC sees is infinite (this is the principle of a short-circuited quarter-wave transmission line - see the impedance page for a brief introduction to transmission line theory).

Since the impedance is infinite for traveling down outer side of the coaxial cable, the current flowing out of the inner conductor (red/pink line) must be equal to the current flowing out of the outer conductor (grey line). Consequently, we have balanced operation, and the sleeve has successfully given balanced operation to an inherently unbalanced transmission line.