RF-zero

This is an Arduino based unit that can be used from well below 10 kHz to 300 MHz. As it stands, it can be used as a beacon with 20mW output. Before today, I had not heard of it. Sadly, it is out of stock due to the worldwide shortage of semiconductors. 

I guess a PA could be added to reach a higher power out for a beacon for example.

See https://www.rfzero.net/ .