A HF coupler is pressurized usually with nitrogen to (1) prevent high voltage arcing (2) prevents moisture to prevent corrosion (3) provides uniform cooling medium and one of various controls which provide access to preset channels. Hopes this answers your question.
Hi there, Here in Brazil, there have been several cases of failures in the system of HF only in flight, while on the ground it normally operates. It was found that the most common cause is the HF coupler, the loss of pressurization causes the failure when the aircraft is at cruising altitude. sr79: can you provide the source of the information about the nitrogen? i would like to read more about this. tks guys!