Searching the internet I came across a design A very simple to build, menu driven RF Power Meter, based on the AD8307 log amp by Loftur TF3LJ. It used a Teensy++ micro available at www.pjrc.com and a 16x2 LCD display. I already had a Teensy++ board and a 16x2 backlit LCD. All I needed was the AD8307. I while ago I had tried to build the W7IEQ SWR/Power meter published in QST Jan 2011 which used two AD8307, a PIC and an LCD. It was a disaster and I never got it working mainly due to software bugs which never got fixed in the code especially the version hosted on the ARRL website. I threw away the main PCB and box but kept the 30dB directional QRO coupler and the 4 PCB with AD8307 and BNC input connectors.
|Modified AD8307 RF detector PCB|
It did not take many minutes with a Scalpel to modify one detector PCB to match the TF3LJ detector circuitThe Teensy was mounted on a stripboard with a plug and socket , the whole unit was mounted in a 5x3x2" box.
|Innards of completed meter|
The meter also enumerates as a serial port on a computer and can be made to continually output a power value in Watts at 4800Baud using the $pcont command:-
more detailed information can be obtained on a 1 shot basis using the $ppoll command:-
Power (inst, pep, avg):
286pW, 1.84nW, 1.04nW
Total cost (with new parts) would be around $75
The meter measures from ~-70dBm to +17dBm thro 440MHz. It has provision for 3 calibrations (eg HF, VHF, UHF) to accomodate calibration issues over that range I also found the -40dB tap flat through 500MHz shown on the TF3LJ site but originally published in the W7ZOI power meter addendum article. With this I can measure up to 200W and I can have the power meter read directly as I can enter the 40dB offset
I thought of building 2 units one for forward power one for reverse power. Then I spotted that Loftur now has a Power and SWR Meter with bargraphs, using 2x AD8307 design on his website using a 20x4LCD and a Teensy++. I can see one getting built, especially as I already have two AD8307 PCB!