It's really simple to make a QRP power meter measuring RF output powers from a few mW up to half a watt. This simple project described here is half an hour's work and all components are from the junk box. I used an old "VU" meter from an old piece of consumer equipment, probably a 70's Hi Fi or something similar, long-since forgotten.

The diode is a historic old OA81. Any diode will do but a Germanium diode is better because of the lower voltage drop across the junction. The 100K trimmer potentiometer adjusts the full scale deflection of the meter.

I opened the meter, stuck sticky white labels over the old "VU" scale, and drew on my new scale. To calibrate the power meter scale I adjusted the power output of my Ultimate3S QRSS/WSPR transmitter into a 50-ohm dummy load, monitoring the accurate power measurement on my spectrum analyser. An oscilloscope could be used too, look at the peak-peak of the sinewave and calculate the output power.

The photographs above show how I built this QRP power meter right inside the RF output (antenna) connection of my Ultimate3S QRSS/WSPR transmitter in its officical QRP Labs case. The meter stays wired in during operation and doesn't significantly affect output power. Of course, the reading is only accurate if the antenna matching is correct (50 ohms). I don't really get 350mW on the 10m band from this transmitter in its current configuration but the antenna is a 30m dipole, so there's some mismatch presumably.