Topband: Feeding a Base Insulated Tower
rfry at adams.net
Thu Jan 16 19:04:15 EST 2014
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
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
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.
More information about the Topband