[rm30]

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.

[deknos]

hm, than i think it's better to use the LORA stuff, no? if i need an external device with my device anyway, i can use one of them.

because the fun thing is cool, but people want some usable...

[rm30]

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.