S-Band Single-Layer Balun for Wideband Applications

In this paper, a wideband single layer Balun is presented that is based on an equal-split Wilkinson power divider and a wideband 180° phase-shifter. The wideband phase shifter is structured as a reference line and multi-sections of short-circuited quarter wave stubs that keep a 180° phase difference over the whole S-Band with a minimal amplitude imbalance. The design equations are simple; all line sections are integer factors of a quarter wavelength. The circuit is fabricated over a single-layer PCB with only one lumped resistor at the Wilkinson divider section. The design provides a flexible planner layout that can be adjusted to fit the available area. Measurements show a 0.75 dB insertion loss level with an amplitude imbalance of less than 0.6 dB between the balanced ports. The phase between the balanced ports is measured to be within $(180 \pm 7)$ degrees, and the isolation level is below -15 dB over the 66.7% fractional bandwidth.