RF System Protection
A common method of protecting an ungrounded transmitting tower (used for AM broadcasting) is shown in Figure 17.6. Two forms of protection are used. The first combines the effects of a one-turn loop and lightning ball gaps to shunt lightning discharges to ground. The loop adds inductance to retard the advance of the lightning current into the antenna tuning unit until the ball gaps have fired (arced-over). The second protective element is the gas-gap device installed within the antenna tuning unit, which con- ducts whenever the voltage at its terminals exceeds a specific potential. The current-limiting fuse protects the transmission system from the effects of power follow-on and the possibility of gas-gap failure in a short-circuit. A static drain choke, also shown in the figure, serves to maintain the tower at dc ground to bleed off the buildup of static electricity on the tower structure.