QRP power meter

Recently I found a page which described Roy, W7EL, Lewallens Simple & Accurate Directional Wattmeter as it was published in 2/90 QST.  Searching for more information I noticed that W1REX - QRPme also has a updated version of this Simple & Accurate Directional Wattmeter.  Combined with some information from the German QRP enthusiast DL2LTO I build my own analoge QRP powermeter. 

The difference between the QRPMe design and  my build is that I added the CW / SSB (PEP) circuit to it (based on DL2LTO mod for the WM2 qrp power meter) and used a 1uA meter (used a 270 ohm resistor in parallel to it).


The funny part is that I often start with the case to get a idea about the available space.  So lets start building


When that was done, it was time to build the coupler PCB. 

Note : the schematic in the photo's has errors in the CW / SSB circuit.Use the one above. Cost me about a hour to find out what it was. Can you spot it ? hint : it's something negative an something positive. Correct circuit is at the bottom of this page.   



All components in place. 


Sorry for not taking more pictures from the build of the amp pcb. 


Lets wire it up, just to find out that every works except the CW / SSB circuit (gives immediately full scale when switched to SSB). 


Using using my FT897D set to 10W and my 36dB Attentuator , I calibrated all the ranges (so basically I didn't need the Sens variable resistor). 

After some minor rework (fixing the CW / SSB circuit) and adding a 3D printed battery holder . . . . 


. . . . 


. . . . it was labelwriter time. Using the 6mm transparent labels I labeled all the switches and sockets.


Ok, except for the on/off switch on the back, but we got a fix for that.  


because I replaced the 2 light bulbs in the meter with low current white leds. That way I can see if the meter is on (so no labels needed for the power on/off switch).


Just adding the cover and . . . . 


. . . . it's all done and ready to use.  

Correct CW / SSB circuit 

Note to myself : Never just copy a part of schematic without checking if it is really correct.