Coaxial microwave transistor test fixtures provide lowest insertion loss possible and include, as part of the input and output sections, transformer networks in form of single stage λ/4 segments bringing (pre-matching) the system 50Ω load closer to the conjugate internal impedance of power transistors. The transforming networks are flat, cylindrical or have oval or elliptical cross sections and have adjustable length, thus operating optimally for transistors with varying optimum frequency and internal impedance and/or capacitive or inductive impedance part. The change in length is done without affecting the overall fixture geometry and structure. The fixtures can be calibrated using standard TRL method.