Design of dual-band balun with tapped stubs

This paper presents a new type of stub tapped balun for dual-band applications. In the design, a tapped stub is used to realize 90deg phase shift at two frequencies, resulting in a dual-band quarter-wavelength transmission line. The dual-band line, as an impedance transformer, is then implemented into a branch-line balun for dual-band operations. Explicit design equations for the new balun are derived using the ABCD-matrix and even-odd mode method. To verify the design concept, a microstrip balun operating at 2.45/5.25 GHz is fabricated on printed circuit board. Measurement results match well the theoretical predictions.