For long range communications (over distances of 3000 km or more), an HF vertical antenna is installed in the vertical fin of the aircraft.
The related HF coupler, located relatively near to the antenna in the vertical stabilizer, is used to balance the capacitance and inductance as required for the desired frequency and it is made of a “black box” filled with pressurised dry nitrogen (at 7±1 PSI / about 0,476ATM).
This solution mainly aims to take advantage from the nitrogen unreactive atmosphere created inside the box in order to prevent high voltage arcing, however it also benefits of other advantages, like preventing moisture (and therefore corrosion) and eventually providing with an uniform cooling medium.
The HF coupler is in fact equipped with electromechanical relays, which might produce sparks and creating some problem.
That is why we advantage of the nitrogen property of forming molecular crystals under low pressure and temperature conditions, giving to this gas the desired electric insulator characteristic.
Therefore, thanks to the pressurised nitrogen in the coupler, HF communication system saves its high reliability as long distances communication system.
The pics below show HF Antenna coupler during a shop visit, where Nitrogen is fed through a tube.