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RF Wattmeter

Finding an affordable, portable, remote reading capable and reasonably accurate UHF or microwave wattmeter is not easy.  So build your own!

Source code for this project is on GitHub here.

A GitHUb Project Wiki with several pages covering build, setup, and operating instructions is here.

There are currently 3 versions.

1. The M5Stack (Arduino) with optional external ADS1100 AD converter.  Self contained graphics display.

2. Arduino Nano headless - bare minimum headless proof of concept, works.  Not updating for now.

3. PSoC5LP.  Uses PSoC Creator 4.3 IDE, C language, supports full hardware debugging, much improved IDE and performance and more capable hardware for similar cost and size.

      a. Optional OLED display

      b. Optional Nextion 2.4" and 3.5" touch screen intelligent displays

      c. Optionally runs on the small KitProg snap-off programming board using a bootloader component (very easy). It is the same PSoC5 chip, just fewer I/O pins exposed/  

I do the main development on the PSoC5 then merge selected feature back to the others.   The PSoC5 version is on a separate project page here.

 

Here is an Arduino based low cost easy to build DIY option. It consists of 3 hardware parts and an optional PC app to remote monitor the meter

  1. The Arduino CPU module.   $5 (n display) to $40 (with display and more)

  2.  A pair of off-the-shelf power detector modules ($10 each)

  3.  Your choice of RF coupler ($25-$150 each surplus)

  4. PC application for remote monitoring (free!)

Watt Meter - watts scale
Remote RF Power Meter Snapshot 1.png

Screen capture of Python based PC application to remote monitor the USB serial output

Arduino CPU Module

For this version I am using the M5Stack Basic Core module.  It is an ESP32 based Arduino compatible platform with built in quality TFT graphics screen, battery, Bluetooth, Wi-Fi, SD Card, USB type C port and lots of IO pins out the side of the 5cm sqaure casing. 

 

You can stack several available peripheral modules or plug in external sensors to the Grove (I2C) port.  Very nifty find. We only need the basic module for this project.

You can find these at variousl places online including teh manufacturers site at M5Stack.com

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RF Power Detector Construction

 

I found a small aluminum box just big enough to house the 2 detector modules, a coaxial DC power jack, brackets for the modules, a 6 pin cable connector, a 7805 5V regullator, and a 7809 9V regulator. 

The 5V regulator is powered from the 9V to reduce heat from a larger voltage drop. 9V supplies the detectors, 5V goes to the Arduino's Type C USB to power it continuously.

RF Power Detector

 

I am using a pair imported logarithmic power detector modules that cost under $10 each. 

For this version I chose an AD8318 based module to cover over 6GHz at +5 to -65dBm range, the more linear portion being 0 to -55dBm.

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RF Power Coupler

I use a variety of them since they are designed to cover a specific range of frequencies and at various power handling capabilities. 

 

One of my stations I use a common feedline between shack and the antennas handling all 6 bands between 50Mhz through 1296Mhz. The IF rig and a stack of transverters go to a 6 position RF switch then out the common feedline to my antenna location with another 6 position RF switch then up to various preamps and antennas.

To cover this wide range of frequencies accurately I found a relatively high power wide range RF coupler online. Mine is a used RLC #12598-M-2565 that is specified for 50Mhz through 1.0GHz at a few hundred Watts and about 20dB coupling factor. 

The coupling factor at the ends, 50MHz  and 1296MHz is about 10dB higher then the middle of the range which is just fine. 30dB is a reduction of x1000. We need 0dBM max.  So it works well at 1296Mhz also which is fortunate.  As long as you have the ability to account for the different coupling factors at each frequency of interest, most any coupler near your target frequency range can be made to work within reason. Too far off and the output becomes too small, you might have high insertion losses so do not get too crazy, go shopping!

So for a 100W transmitter output max, we need 60dB total power level reduction minimum to get 1milliwatt full scale which is about typical for these type power detectors. That means we need to add 40dB worth of additional attenuators to the forward port. We only need 20dB extra on the reflected power port since it is typically x20 less then the forward power level.

To put these couplers to best use we need calibration and correction factors. Fortunately we have a CPU to calculate that for use 24/7 and display it on a nice local screen or remotely on a smartphone or PC. A key feature that makes this power meter worth teh investment is that we can store and recall appropriate calibration tables with a button push or by sendng a remote command susch as from the companion PC application. 

Once you finish your build you enter your best guess values  then you need to calibrate against some known reference then you are good to go.  I have a default set of values to start with.  They can be edited in the Cal screen.

 

K7MDL RF Power Detector Cal Screen.jpg
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