Antenna requirements for the Skytrax200 (ST200), as of 2021: When adding the AXP340 and the Skytrax200: You must install a sperate antenna for the ST200. Avidyne does not recommend a Duplexer, as this would not block the output power from damaging the receiver input of the ST200.
IFD Interfaced with AXP340 Transponder RS232 Config Page