Multisection impedance-transforming coupled-line baluns

A new class of multisection impedance-transforming coupled-line baluns is presented in this paper. It is based on the quarter-wavelength coupled-line balun. By cascading several coupled and uncoupled-line sections, broad-band baluns with good amplitude and phase balance can be realized using simple microstrip coupled lines. The resulting circuit has a simple structure with minimal discontinuities that is highly suited for very high-frequency applications. Simple design equations and experimental verifications will be presented. Using micostrip coupled sections with even-mode impedance of 159 Omega, the fabricated balun achieved 0.5-dB amplitude balance over 30% bandwidth. With higher even-mode impedance, almost perfect amplitude balance was achieved over 100% bandwidth.