https://github.com/F5OEO/rpitx?tab=readme-ov-file#about-rpitx

I have it on good authority that if you connect a piece of wire to pin 4 of the gpio connector (length of wire determined by frequency of course) your Pi can transmit.

This Github link will take you to the software and the video in the previous post shows you how to install and run it on a Raspberry Pi

Have fun !

🙂