Fractal-Shaped Balun Using Composite Right/Left-Handed Transmission Line for Bandwidth Enhancement

This article presents a fractal-shaped broadband balun based on the lumped-element composite right/left--handed transmission line (CRLH TL). The developed balun involved there -90 degrees Koch-shaped microstrip branches and one +90 degrees CRLH branch, features compact, excellent transmission performance and bandwidth enhancement. Closed-form formulae for broadband strategy and general guide lines for fractal implementation are derived. For verification, a balun operated at 1.5 GHz is designed, fabricated and measured. Consistent results between simulation and measurement has validated this concept as well as the developed equations. The fabricated balun, with an occupied area of 17.7 x 24.8 mm(2) corresponding to 12% of the area that its conventional counterpart occupies, exhibits a relative bandwidth of 93.3% ranging from 1.03 to 2.43 GHz. Across this frequency region, the return loss is better than 10 dB while the amplitude and phase imbalance varies within 1 dB and 4.2 respectively.