Introducing the Raspberry Pi TV HAT - Raspberry Pi
JustBoom Digi HAT for the Raspberry Pi
GitHub - IgrikXD/rpitx-coax-pcb: Expansion board for Raspberry PI allowing to use coaxial SMA output instead of direct wire connection to GPIO to connect radio equipment when working with the rpitx package.
Amazon.com: Raspberry Pi DAC Audio Decoder Board HiFi Expansion Moudle Supports Coaxial Fiber I2S Out for Raspberry Pi 3B 3B+ 4B T0522 : Electronics