The lightning protection coil (Blitzschutzspule) is provided in order to prevent lightning from entering the antenna tuning hut. The principle of functioning is shown on the sketch to the right. There are two spark gaps (Funkenstrecke) before and behind the lightning protection coil. In case the quick lightning current flows down the mast towards the a.t.u., an additional voltage builds up across the lightning protection coil: u = - L di/dt. Due to the increased voltage across the spark gap F1 the lightning current ignites the spark gap and flows across it not entering the antenna tuning hut.
The second spark gap F2 is adjusted much more sensitive than F1 for the discharge of remaining overvoltages.