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).
Low pressure Nitrogen (N) is sealed within the HF coupler device
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.
Nitrogen pressurisation (shop visit)
Hello, do you have a photo of the entire servicing mechanism? I’m trying to make a unit so we can service our units when on the road. Many airports do not have maintenance available, and I am looking to make a unit that I can store a low amount of nitrogen in a bottle and be able to service our units if necessary. Sometimes we end up sitting for a month or two, and in some cases we have lost pressure in our HF’s.
Hi Bill,
I am sorry, I have no picture. I have seen it at an avionic shop during a short visit and asked many things on it just to understand the process. You might want to check what the CMM says about the servicing and whether it provides with drawings to fabricate the device in house. Or you might ask for such drawings to the OEM directly.