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. 

Most of the meter is based on the QRPMe design but I added the CW / SSB (PEP) circuit based on DL2LTO mod for the WM2 qrp power meter to it. Also I used a 1uA meter (used a 270 ohm resistor in parallel to it) because I bought a few for just a few Euro's at my local electronics store (they have walls full of small drawers with components and lots of NewOldStock). 

And as always I started with the case to get a idea about the available space and switches/connectors/meters/etc placement. 


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. 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 main 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). 


For testing I used 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 label time. My main source of DIY project labeling is using my Brother PT-1005FB Labelwriter with 6 or 12 mm transparent labels.


I didn't label the on/off switch on the back, but that was on purpose. 


Because I replaced the 2 light bulbs in the meter with low current white LED's. That way I can see if the meter is on.


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.



Update 2021-07-07 : after a discussing in the GQRP groups.io about a Simple QRP power meter, this page was referenced by the GQRP club on one of there technical pages. How cool is that?