Design and Experimental Validation of Highly Compact Branch-Line Balun For Quad-Band Applications

In this communication, a novel design approach to design of highly compact quad-band branch-line balun (BLB) is presented along with the implementation and experimental validation of the structure. A quad-band transmission-line is obtained by adding one coupled-line between two-series transmission-lines used to replace the traditional 90° microstrip-lines of a single-band BLB. The even/odd-mode technique is employed to derive the closed-form formulas and facilitate the analysis of the quad-band properties. The proposed concept is verified through fabrication and measurements of a quad-band BLB operating at 1.0, 2.7, 4.1 and 5.8 GHz. The manufactured quad-band BLB provides a reflection coefficient better than -20dB with a magnitude imbalance smaller than 0.45dB. Furthermore, the fabricated prototype (0.051λg
²
) exhibits a size reduction of 59.2% and 84.9% when compared to a single-band balun (0.125λg
²
), and the recent quad-band BLB (0.336λg
²
) reported in the literature, respectively.