You’re going to have to be clearer about your set up?
?
if you’re saying an unpowered or powered 50WPA connected to the QMX+ makes it oscillate (in TX) when an antenna doesn’t the first thing to check (with a VNA) is the amp input impedance
But then you say nothing on RF in it RF out (do you mean it’s oscillating when PTT active and otherwise disconnected (not entirely surprising - Put a 3dB attenuator on the open input)
Please set out the exact layout and conditions that trigger your problem?
