Quantitative Analysis of the “Shimada” Isolating, 1: 1, Series-series, Equal-delay Balun

The “Shimada” isolating, 1: 1, series-series, equal-delay balun is analyzed using odd/even mode transmission line analysis and is shown to behave as a current balun over a broad bandwidth. Numerical results indicate that when the electrical length of the odd mode of the constituent bifilar transmission lines is near an odd-integer multiple of 90°, the general behavior of the device degrades from that of a current balun to simply that of a symmetric balun. Additionally, when the common-mode structure is lossless, a narrowband undulation in the short-circuit output currents occurs in the immediate vicinity of the same odd-integer multiples of the odd-mode quarter-wave frequency. However, this only occurs for short-circuit or very low impedance loads. Finally, we show that the widely used “Shimada” balun is equivalent to the conventional 1:1 isolating bifilar transformer with a compensating delay line added to the non-inverting output.