It's fun to play with, and is a great hack, but please don't try and use it for anything or any length of time; it puts out a tonne of horrible harmonics all over the radio spectrum.
I have a „Hörmann“ garage door opener. For a long time, I want to use my phone instead of the original sender. Therefore, I thought to run a raspberry and send the signal from there.
I recorded the signal with HackRF One as .wav, the problem here is that HackRF records with a sampling rate much higher than 48kHz (by default it is 10MHz!). So one has to downsample the recorded file, which I did with Matlab (downsample(signal, „fs_of_record/fs_for_rpitx“)). The resulting .wav signal is now send at 433MHz (either my garage door listens their too, or 2nd harmonic works?!)
(You do need an amateur radio license for this, but in the USA at least the technician one is very easy to get.)
the harmonics are filtered by a LPF, and the broadband noise is filtered by a BPF [...] a buffer amp is provided for isolation. This also provides a boost to the TX signal
And this project ties in an RTL-SDR for receiving: https://github.com/ha7ilm/qtcsdr mentioning AM/NFM/WFM/LSB/USB [...] although NFM/WFM works as expected, the AM/SSB modes need a much higher level of filtering
This is typically limited to a 50-100 foot radius without amplification, right?