WSPR Node with Raspberry Pi

 I had played around with creating a 10m morse beacon once which worked a treat. The problem is there is no instant way to see if anyone can hear it. However with WSPR there are quite a few websites where you can see immediately where your signal is being heard such as WSPRnet.org. I use MSHV mostly for my FT4 and FT8 work but it has no WSPR function. WSJT-X on the other hand does and it takes care of everything for you.

I had tried lots of different solutions on the Raspberry Pi that I have. It is an old one (Rpi 2B) and I found that installing an old version of Raspbian Stretch was the way to go. I eventually got it to work properly using this link to GitHub:

GitHub - JamesP6000/WsprryPi: Raspberry Pi WSPR transmitter using NTP based frequency calibration

Follow the instructions and install it from the command line. It can take a long time so don't think the RPi has given up because it can take up to half an hour to install it. Once installed you can call it from the command line with the correct parameters (see author's comments) The output should provide only around 10dB, 10mW, which is low but people have reported thousands of km contacts with it.

Now then, apparently the output from the RPi is too rough and dirty to send out over the air so we have to do a bit of cleaning up of the signal. I wanted to try it ASAP so I looked for an easy makeshift solution for a low pass filter and discovered that it can be done with just a couple of capacitors. See circuit below:

The signal is derived from the CLK output of the RPi GPIO pin 7. To prevent any dangerous DC voltages getting in or out of the RPi the first capacitor at 100nF blocks any DC in or out. The second capacitor at 110pF acts as a filter by smoothing out the imperfections in the waveform. As you can see from my scope, it hasn't done a bad job at all.

My scope is an old CRT one and struggles when getting above 20MHz.

Zoomed in a little it looks like a pretty good sine wave.

I tuned my radio to the right frequency for WSPR to catch the stray transmission and there it was.

This was good for 28MHz but it was a different story on lower frequencies as I expected. I set the RPi program to output at 14MHz to see what the waveform was like on the scope.

As you can see here the waveform is getting a bit out of shape - 'saw-toothy'.

Another check on 10MHz confirmed my suspicions even further. The trace for this is below:

You can clearly see the waveform is even more out of shape and also the formation of harmonics which is very bad for transmission. I decided to stop and not go down to 7MHz because I knew what the outcome would be. To get the shape back, further filtering would be required. I think that I can safely use this WSPR beacon on 10m though at the moment. I may purchase a proper LPF so I can use other bands.