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

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.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.   

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All components in place. 

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

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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

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 . . . . 

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. . . . 

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

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

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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).

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Just adding the cover and . . . . 

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. . . . 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