RF MiliWatt meter OZ2CPU

RF meter front

http://www.webx.dk/oz2cpu/radios/miliwatt.htm
http://f6bon.albert.free.fr/Milliwattmetre.html

Downloads:   PIC Hex file.   Note: This file is for a 20 character LCD!

In ‘Elektuur’ (Elektor) of October 2002 an article was written about the construction of a PIC controlled RF Watt meter. I found the design earlier on the site of OZ2CPU and I bookmarked it then for later use…. That moment was here and I ordered the print and software and got to work.

I must say, it’s a very nice measuring tool. It is based on a PIC 16F876(a) and an AD8307 logarithmic detector. Extension with a MAX232 offers the possibility of connecting the apparatus to your PC. The original available software has many features to make a very useful tool of it.  It is written in ‘C++’ format and should be compiled into a hex file. I used the PICC compiler. I made a small modification in the software to activate the second DC input (B) for using it, together with the already available DC input (A), for SWR measurements. I have modified the software to enable me to determine an SWR value out off two DC input voltages.

Update November 2013.
Because the need of measuring frequencies in the GHz range, I have build an extra logarithmic detector (AD8318) board, designed by F6BON. On the picture below you can see the board right below the PIC board. Selecting the AD8307 or AD8318 is done with a relay controlled by the PIC software (using RC6). Because the AD8318 has a reversed output
in comparison with the AD8307 and adding more frequency bands, I had to modify the PIC software (in MPLABX) for this as well.

RF meter view insight

RF meter view insight

 

I found that the AD8318 is sensitive for temperature. After 1 hour the measurement of the input signal increases about 1.2 dB due to warming up of the enclosure. I replaced the transformer for one with 9 Volt output in stead of 12 Volt. I also mounted the 7805 regulator on the backside of the enclosure. After that the measurement increases not more than 0.3 dB after an hour.