A wideband balun consisting of a transmission line transformer (TLT) and matching and phase conversion circuits is proposed and fabricated. A TLT using three 37.5-Ω coaxial cables converts the impedance from 50 to 25Ω, and a matching circuit using four resistors from a singly terminated filter improves the matching and isolation characteristics over a four-octave range. A phase conversion circuit is implemented by cross-connecting two coaxial cables. The fabricated balun provides matching (S₁₁, S₂₂) and isolation (S₂₃) characteristics of about -15dB, an output magnitude difference within 0.3dB, and an output phase difference within 180°±1.4° from 62 to 960MHz.