Send text data using FM frequencies
You can use basic radio modules like these: -





These happen to be from a company called RF solutions but there are plenty of people supplying them.

The tricky part is packaging the data (that you wish to send) into a data stream format that makes a reliable FM system (well, as reliable as radio can be). This requires transmitting and receiving/decoding: -


A identifiable preamble
An address
Data
A checksum


And you do need all of the above for best reliability because, in the absence of a proper data transmission, the FM receiver will be hunting around for anything that might be a signal and this generally means noise: -



That's where the real work lies. So choose your modules based on: -


Modulation method (assumed to be FM)
Carrier frequency (country dependent)
Transmit power (affects range that the link will work over)
Data rate - lower data speeds usually are more reliable for a given output power and range.


And good luck.