Design of rf/microwave planar crossovers using pure-series-connected lumped elements

In this paper, an ultra-compact planar crossover is presented and designed by using the pure-series-connected lumped elements. The basic structure of the crossover is inspired by the window-shaped 0-dB branch line coupler. Due to the perfect symmetry, the structure is analyzed by the even-odd mode theory and each lumped element can be determined analytically. Finally, the proposed crossover is fabricated. The measured results shows that 20 dB isolation and return loss can be achieved at the operating frequency, while the fractional bandwidth of 15-dB isolation/return loss is about 13.5%.