A system for monitoring a condition in an enclosed plasma processing space (102). The system comprises a sensor (338), arranged to be provided within the enclosed plasma processing space, for sensing a condition in the enclosed plasma processing space and a modulation circuit (342), connected to the sensor, and arranged to modulate an output of the sensor to provide a modulated signal. The system further comprises a first transmission line coupler (330) arranged to be disposed within the enclosed plasma processing space. The first transmission line coupler (546) is connected to the modulation circuit and is arranged to couple the modulated signal to a transmission line, which is arranged to deliver energy into the enclosed plasma space. The system further comprises a second transmission line coupler, arranged to be disposed outside the enclosed plasma processing space and coupled to the transmission line and a demodulator (550), connected to the second coupler, for receiving and demodulating the modulated signal.