Topband: Feeding a Base Insulated Tower

[Richard Fry]

Most, as in nearly all of the licensed AM broadcast stations in the U.S. use 
all three of the devices listed below (together) to reduce the probability 
of lightning damage to their transmit systems when using base-insulated 
towers.

1.  A static drain choke leading from the base of the tower to a low-Z 
"lightning" ground path buried in the earth.

2.  A conductor with a single-turn loop of ~15- to ~30-inch diameter 
connecting the bottom of the tower to the output terminal of its physically 
adjacent Z-matching network.

3.  An arc gap crossing the base insulator connecting to that good lightning 
ground of item 1 above -- the gap spacing set to flash over when the peak 
voltage there exceeds the peak voltage present during normal operation, by a 
safe percentage.

Modern, commercial AM broadcast transmitters also include SWR protection 
that will instantaneously/momentarily quench their r-f output into a load Z 
exceeding a safe value for the duration of that excessive SWR.

The end result is that the hardware of commercial AM broadcast stations 
rarely is affected by lightning events near their transmit antenna systems, 
and as far as their listeners might discern.

R. Fry