Basic QRP Power Meter

One of my first home built projects was a basic power meter that I planned on using for future QRP work (once I got my amateur license 😊).  It is essentially a peak voltage detector consisting of a 50 Ω dummy load (three, 150 Ω, 2W resistors in parallel), a diode for rectification and a small capacitor for smoothing out the voltage.  Here is a basic circuit I found on an amateur radio club site (NorCal Power Meter).

I scratch built my own circuit board using a stencil made out of paper and fingernail polish as the resist.  The components were gathered and soldered into place and everything was then tucked away in a nice enclosure to complete the project.  Next came the calibration. The blue pots were used for this purpose so that a full scale reading on the panel meter would occur when a 1W or 5W signal was applied.

Top of board with calibration pots in blue

As an example, since P = V*V/R (where R = 50), it follows that 5W represents 15.8Vrms or 22.3Vpk.  Subtract 0.5V for the diode drop and you have 21.8VDC.  This voltage was then applied to TP1 from my power supply and the pot adjusted to achieve a full scale reading.

Completed project