Balancing networks for symmetric antennas .I. Classification and fundamental operation

Emphasis has been placed on the quality of a metrology antenna system that allows it to be represented accurately using analytical or numerical models. Central to this capability is the efficacy of the balancing networks, sometimes referred to as balanced to unbalanced transformers (baluns). In this paper, we classify three fundamental types of baluns and show that all three can be derived from the 180/spl deg/, four-port hybrid network. Balanced antennas driven from coaxial feed lines and operated in the presence of an asymmetric scatterer present an intrinsically unbalanced load to the balun. We show that in such situations the current balun is the only appropriate balun to employ. For numerous antenna metrology applications, in particular, site attenuation measurements, 180/spl deg/, four-port hybrid networks have been employed as baluns. Network relations are derived relating the so-called voltage and current baluns to the 180/spl deg/, four-port hybrid network. It is shown that, in addition to acting as a 0/spl deg/ or 180/spl deg/ power divider, the hybrid network can serve as either a voltage or a current balun depending on the termination at the isolated port. In contrast to the traditional approach, it is shown that when using a 180/spl deg/ hybrid as a balun, the isolated port should be terminated in such a way that the hybrid behaves as a current balun as opposed to a 180/spl deg/ power divider. This will yield experimental results that can be more readily reconciled with models.