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. 

qrp_pm_000

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.   

qrp_pm_001

qrp_pm_002

All components in place. 

qrp_pm_003

Sorry for not taking more pictures from the build of the main pcb. :-(

qrp_pm_004

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

qrp_pm_005

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 . . . . 

qrp_pm_006

qrp_pm_007

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.

qrp_pm_009

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

qrp_pm_008 

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.

qrp_pm_010

Just adding the cover and . . . . 

qrp_pm_011

. . . . 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.

qrp_pm_sch

 

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?