Balancing networks for symmetric antennas: part II-practical implementation and modeling

The practical aspects of implementing the three fundamental types of balancing networks presented in the first part of this paper, using a 180/spl deg/ 4-port hybrid network are investigated. It is shown, for the first time, that the use of attenuators between the hybrid and the antenna detracts from the ability of the balanced-to-unbalanced network (balun) to balance any of the three quantities: current, voltage, or forward power. It is shown that additional time delay or linear phase shift, even when made equal for each port, makes the implementation of a current or voltage balun difficult over a broad frequency range although narrowband operation is still possible. Thus, the placement of phase-matched coaxial lines between the balun and the antenna is not desirable. It is shown that the equal-delay hybrid is uniquely adapted for the realization of a voltage or current balun. Finally, the modeling of symmetric antennas as driven by each fundamental type of balun is presented.