"Antonio Vernucci" wrote in
:

Does anyone know of special diodes resulting in a linear wattmeter
scale?

Have a look at the diode characteristic below the knee, it is a square law
detector (meaning that the DC current is proportional to the square of the
AC voltage, and therefore power).

We are talking about millivolt levels, but the good news is that today,
stable high gain DC amplifiers are a snack (see chopper stabilised Op
Amps).

Owen