Milliwatt meter for WSPR

Having the need to measure milliwatt powers from my WSPR transceiver, I decided to build a simple measurement device that would indicate power up to 5W. The QRP milestone. I have a couple of SWR / Power meters, but nothing sensitive enough for such low levels of power. A search around the web produced many projects, almost all based on the same circuit.

This is my take on a fairly standard circuit. Note the diode has a forward voltage of only .4v. Max voltage is 30. So the device should handle up to 15W. You will need to beef up the load resistors if you build it for this higher power level. Diodes such as the ubiquitous 1N4148 can have voltage drops up to 1v, according to the datasheet. This will reduce sensitivity and accuracy at really low power levels.

Calibration can be done by applying a known 5W from a transceiver, Elecraft KX3 as I used for example, and adjusting the pot for the correct reading on the meter. My meter was salvaged from an old SWR bridge, so the scale was conveniently  marked.

 

Alternatively apply 15.82 volts to the point marked on the circuit and adjust the pot as above. The more accurate the calibrating voltage is, the more accurate your meter will be. The power reading is equivalent to (v*v)/R. Where R is usually 50R.

I scored a few pads on a piece of copper clad PCB with a sharp blade, and soldered the parts onto that, keeping all leads as short as possible. The electrolytic 4.7uF across the meter, helps to damp the movement may not be needed and is not shown on the schematic.

This version has 2 pots selectable by a switch ( not shown ) enabling 1W or 5W FSD as required.


Of course it should be installed in a shielded enclosure of some sort. This 50c tea caddy was an ideal size. I didn’t realise how battered it looked until I reviewed this pic !

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