The way it was explained to me by an engineer who appreciated he was talking to a pilot, is that the shunt acts as an exciter which allows the whole fin to become the HF aerial. The coupler also matches the rest of the airframe to the shunt to act as the ground plane. In effect, the whole aircraft is the HF aerial. When pilots write up the HF as being poor, the engineers test them on the ground where often the aircraft is properly earthed - which is often the cause of the problem. No fault found!