Paint a LimeSDR with Radio waves.

The below is silly, a total waste of RF spectrum, but it is fun to do at least one.

Because I installed gnuradio with pybombs I was able to install the gr-paint block with pybombs
$ pybombs -v install gr-paint

I also downloaded a random image (of a LimeSDR board) from the Internet. Looking back at what I actually did below there probably was no need for me to convert the image into a png file. My subconscious worked out, with no actual verification, that a png would require slightly less CPU than a jpeg file to decompress is the only justification I can give for why I converted the file. You could probably skip those steps and everything should work fine. In hindsight I do not think the CPU overhead saving would be all that much.

$ wget https://www.crowdsupply.com/img/f331/limesdr-frontcrop-1_jpg_open-graph.jpg
$ sudo apt-get install imagemagick
$ convert limesdr-frontcrop-1_jpg_open-graph.jpg limesdr-frontcrop-1_jpg_open-graph.png
$ file limesdr-frontcrop-1_jpg_open-graph.png
limesdr-frontcrop-1_jpg_open-graph.png: PNG image data, 1500 x 1000, 8-bit/color RGB, non-interlaced

I used the above command because I needed to find out the exact "Image Width" parameter for the "Spectrum Painter"  block. I also tweaked the "Line Repeats" parameter by trial and error to get a waterfall that was not too squashed or stretched. It is still not perfect, but it is good enough.

 (click on the above flowgraph to see it full screen, there is not much to it)

The above multicoloured waterfall (click on for full size) is just a local screen output from GRC, no RF was actually transmitted (I don't have any LimeSDR hardware). With an actual LimeSDR I could TX, at extremely low power, from a TX antenna to a RX antenna (Or cable TX to RX though a 40dB attenuator, to avoid any actual RF emissions and damage). And then use a LimeSDR Source block tuned to the same frequency band  as a TX block and connect that block directly to a Waterfall sink in the same flow graph.