but for that to work, you need to attach an antenna, no? and where do i get such an FM transmitter? AND android does not support it in the software level, and there's no protocol for the waves?
To have an FM receiver work on a phone, you do need an antenna, the wired headphones serve that purpose perfectly. An FM transmitter is easy to find; you can use the simple 'Jack-to-FM' adapters designed for car radios, or much better, a USB SDR (which can range from a few kHz to GHz).
Regarding the 'protocol for the waves,' you'll need to play with modulation. That’s the fun part. In technical literature, there are many well-defined modulations (like AFSK or FSK) with clear suggested applications for low-SNR environments.
As for Android support, I have no idea. I understand that in this thread, 'free' sounds like 'freedom,' but freedom has a cost. The freedom of communication requires investment: in hardware, software, and the time to learn the physics of the environment.
It depends on which step of the staircase, from pure hardware to pure software, you want to position yourself. Some projects require staying closer to the metal, while others can be purely software. I move up and down this staircase depending on the specific requirements.
If the requirement is to communicate where consumer standards like Bluetooth fail, like in a ship, you have to choose the system for the environment. I evaluate these choices like an architect building a robust system, rather than just using what is available at the nearby shop.
