Analysis and design of miniaturized lumped-distributed impedance-transforming baluns

Conventional distributed baluns employ λ/2 transmission lines or λ/4 coupled lines. By adding only one or two lumped capacitors, the required coupled-line lengths of a new balun structure can be made as short as λ/24. Design curves for these miniaturized impedance-transforming baluns are derived using a systematic even- and odd-mode analysis technique. This approach also provides valuable insight into the tradeoffs between coupled-line lengths, number of capacitors, bandwidth, and impedance-transforming ratios. Experimental results of a single-capacitor balun with λ/9 coupled lines and a two-capacitor balun with λ/24 coupled lines will be presented. Both baluns achieved 0.5-dB amplitude balance and 5° phase balance, with over 10-dB return loss from 1.1 to 1.6 GHz.