Thank you to Ihar Yatsevich for writing in and sharing his open source project called "rpitx-coax-pcb" which is an expansion board for the Raspberry Pi that converts the GPIO pin used by RPiTX into a coaxial SMA connector for easily connecting the output to an antenna. He notes that there are two revisions. One includes a filter in the in the GP1212 / GP731 case and the other does not. Filters in this type of enclosure can be found from Minicircuits. Finally he notes that he has not yet fully tested the design, but believes that there will be no problems.

The GitHub contains the EasyEDA design files, schematics and gerbers which you can use to print and build the PCB yourself.

If you are unfamiliar with it, RPiTX is a program for Raspberry Pi single board computers that allows you to transmit almost any type of signal on frequencies between 5 kHz up to 1500 MHz with nothing more than a wire connected to a GPIO pin. However, it is highly recommended that appropriate filtering be used if you are transmitting with an amplifier or longer range antenna as the RPiTX contains harmonics that can cause interference with other devices.