The circuit can be used till RF powers of appr. 50 Watts. When measuring larger RF powers with a diode, the max. tension of the diode can be exceeded thus introducing an inaccuracy in the measurement. This disadvantage does not exist when using a rectifier tube. Of course 6.3 Vac must be available for the tube’s filament. A Philips EZ80 or EZ81 should work fine in a similar circuit. The measuring device must be calibrated before it can be used.