Coaxial versus Bifilar Transmission Lines as Constituents of Transmission-line Transformers and Baluns

A broad and useful class of transmission-line transformers (TLTs) is based on the series/shunt interconnection of commensurate, two-conductor transmission lines combined with integral common-mode (CM) chokes. Therefore, such a section of uniform transmission line with integral CM choke has aptly been termed a “fundamental building block” of this class of TLTs. Various transmission-line geometries for this building block can be employed provided the characteristic impedance and phase velocity of the desired mode satisfy the requirements of the design procedure. In particular, both bifilar transmission line and coaxial transmission line geometries are frequently used interchangeably. However, published analyses of this “building block” treat the bifilar+ geometry exclusively. We show that the characteristics of the fundamental building block when implemented with coaxial transmission line are quite different from those obtained when bifilar transmission line is employed. This has implications for many transformer, balun, and hybrid designs. Two new models are presented for the coaxial “building block”: one which is appropriate when the CM structure behaves in a lumped manner and one when it behaves in a distributed manner. We show analytically the effect of the shield on TLT operation, specifically for a simple 3-port choke/current balun topology. Then, two 3-port, series-series, 1:1 baluns implemented with coaxial and bifilar media are analyzed using the model and also with a commercial finite element simulator. It is seen that little performance difference exists between the bifilar and coaxial implementations. This is shown to be due the series-series balun topology effectively employing both a coaxial delay line and an “inverted” coaxial phase inverter. The degraded performance of the latter cancels the performance gain obtained by the former.