The Performance of Balancing Networks Derived From the Shunt-Series, Equal-Delay Hybrid

Abstract

Both voltage and current baluns can be derived from the shunt-series, equal-delay hybrid, but the voltage balun is known to be superior for broadband dipole antennas. To understand this behavior, it is necessary to distinguish between balance and symmetry. Both voltage and current baluns exhibit the appropriate balancing behavior as is shown here. However, the current balun lacks symmetry when driving high impedance loads. This is shown for the general case using analytical models, and for a representative shunt-series, equal-delay-hybrid configured as both balun types using numerical simulation and measurements. Additionally, it is shown that while the voltage balun provides symmetry it does not provide true voltage balance when the constituent transmission lines are not electrically short. While the current balun provides poor symmetry, it provides ideal current balancing action throughout a midband region, even if the constituent transmission lines are not electrically short. A broadband dipole antenna exhibits a high input impedance below its fundamental resonance. For this reason, the voltage balun topology provides better symmetry than does the current balun with antennas such as a 1.4-m biconical. The asymmetry manifests itself in vertical polarization measurements and is demonstrated with a vertical polarization inversion test using two 1.4-m biconical antennas.