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mWatt meter -72db tot 18 db

As you can see on my site I’m building a lot of little tools that can help me out in the shack. While browsing AliExpress i ran in to an nice little power measuring module based on the AD8307. I decided to buy one and see if it would do the job.  After a few weeks I got this little module. So now starts the building. I decided to use a Arduino Nano for the calculations, I no it ain’t the best solution, but keep in mind i build Tools, not precision devices. I need to get a bearing on things. Otherwise i would have bought a A grade, Iso Certified machine. Hi.. So i ripped some code online and added my own flavor to it. Version 0.1b will be extremely basic. Just measuring the output of the module and converting it to dBm and mWatts on a 1602 I2C display. While playing with i noticed it never comes to a complete 0 in values. So i build a copper box te see if it helps. Strangely it did do much, so for now it sits in a box, but it didn’t solve my problem. Now I know there is a lot of QRM in my shack. Radio’s, WSPR, computers running. So i have to test it somewhere else. The current stage is that I have to calibrate it software wise. I need an 0dBm source and feed it to the AD8307 so i can determine the output value for 0dBm. Also a -72dBm and +18dBm source would be useful. So I guess I will bring it to a club night and do it there.


The attenuation:

As I want the input to go to as high as 25W i needed a attenuator for the input. Using the tool made bij PA1B (found here) I calculated the resistor values for 10dB and 20dB.

I only need a max of 30dB as 25Watts is 44 dB, so with 30dB attenuation it brings me back with in the scope of the module. Witch can handle up to 18dB.

So there will be 4 options to choose from:

25W 30dBm
5W 20dBm
500mW 10dBm
50mW none


I ordered 2 Watt resistors with an 1% deviation in order to keep the errors down.

The Software

The Arduino sketch will be made public on my github account.

To Do:

Calibrate it
Add a switchable rf attenuator to the input and in the software to measure larger loads. (25 Watt will be the limit tho).
Add calibration to the software.
Put it in a nice box with it’s own power supply.
Make it easy updatable.

Published 21/01/2017 => First publication on my site after some development.

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