A new coaxial r.f. power meter, developed for use as a primary standard for the frequency range 0–8 GHz, has an uncertainty of ≤0.2%, excluding d.c. standards and mismatch errors. The reading time is 3 min, and the effective efficiency, which can be determined without dismantling the instrument, varies smoothly from 99.89% at 1 GHz to 99.43% at 8 GHz. A novel feedback technique is used. The instrument is portable, and the r.f. head is small enough to be accommodated without difficulty in most calibration test benches.