A Miniaturized Microstrip Balun Constructed With Two $\lambda /8$ Coupled Lines and a Redundant Line

This work presents a miniaturized balun consisting of two re-entrant coupled line sections with ~ λ/8 length connected by a redundant line with ~ 0.04λ length fabricated by planar PCB technology. The structure of the balun was analyzed to obtain the appropriate lengths of couplers and redundant line for the balun realization. An experimental balun operated at -band was designed and fabricated to verify the proposed structure of balun. The prototype demonstrates a miniaturized dimension of ~ 0.29 λ x 0.05 λ and less than 0.36 dB amplitude imbalance associated with 1.6° phase difference in the frequency range of 2.09 3.06 GHz (37.7%). The return loss in the operation band is less than 10 dB. The length of the prototype is almost shrunk to nearly half of the traditional balun.