SWR/Power/Return Loss monitor

SWR meter photo

I began development of this multi-functional piece of test equipment when I developed an interest in combining the power of microprocessors with the convenience and user-friendliness of an analog panel meter display. I also liked the idea of single-purpose, dedicated manual switches for the user inputs. This approach recalled some of the flavor of the ham equipment I remembered from my early days in amateur radio.

This project has also been featured in the 2011, 2012 and 2013 editions of the ARRL Handbook for Radio Communications ("A Microprocessor Controlled SWR Monitor").

The final design achieved the above-mentioned goals to my satisfaction. The merger of microprocessor technology with old-fashioned analog controls and display resulted in a device that is simple to construct and intuitive to use. This meter has been in use in my ham shack for a few years now. I find it invaluable for monitoring the performance of my rig when moving from one band to another, or trying out new antenna designs.

Design plan

My basic approach was to digitize the analog inputs from the SWR sense head and derive a pulse-width modulated (PWM) waveform at the output. A simple single-stage resistor-capacitor filter smoothes the PWM signal enough to drive a conventional moving-coil analog panel meter. In between, it's all digital.

At the heart of the hardware is a unique microprocessor -- the Propeller chip, designed and manufactured by Parallax, Inc. This chip features eight individual peripheral processors, called "cogs", and a central hub processor which cycles through the cogs and synchronizes their actions. Each cog can be programmed to carry out its own individual task, and communicate the result to other cogs. This architecture makes it very easy to write complex multi-tasking applications by breaking the overall task into smaller sub-tasks, each in its own cog, and working on them individually. There is also available an extensive library of pre-built routines which include floating point math functions, a delta-sigma analog-to-digital converter and support for serial communications through a USB port.

The power of the microprocessor is evident. True relative power is shown on a linear scale. Return Loss is shown in decibels. And these are both on a moving coil analog panel meter. It's also a real pleasure to flip the "peak hold" switch and see, within a matter of milliseconds, the fast, crisp rise of the meter, indicating peak power with only a single "dit" from the key, followed by a long, slow decay duplicating the same "acceleration" law as a falling body in a gravitational field.

The software library as well as a compiler for the Propeller are all available free on the Parallax web site.

Digging Deeper

These files give some insight into the design and function of the SWR/Power/Return Loss monitor:

Left-click on any .PDF file name to view it on-line, or you can right-click, then save the link to your hard disk. You will need Adobe Acrobat reader to view these files. Click here if you need to download the latest version of this free software from the Adobe web site.

File Name Size, kB Description
SWR_Operation.pdf 285 Operation and Features
SWR_test_proc.pdf 33 Test Procedure
SWR_sw_comments.pdf 16 Comments on Software Elements
SWR_flow_chart.pdf 16 Software Flow Chart
SWRMON_software.zip 45 Propeller Source Code
Schematics_and_Parts.zip 152 Schematics and Parts Lists
SWR_const_notes.pdf 43 Miscellaneous Notes on Construction and Usage
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NOTE: All of the Propeller library files used for this project have been released to the general public and provided under a free permissive license from the Massachusetts Institute of Technology (the "X11 License", also known as the "MIT License") and are free of any restrictions on their use. By downloading these objects, you are agreeing to the terms of the MIT License.The full terms of the license are shown below:

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