High power broadcast from a transmission tower is fed by a very prominent copper component which looks like a large drain pipe, often with bolted flange at the base connections.
To transmit 10's to 100's of kilowatts, drive voltages in the thousands are present between an inner core and the outer casing. The smooth surfaces and dry nitrogen present in the interior, prevent corona discharge from initiating an arc between the two conductive components.
In theory, anything striking the tubing with sufficient momentum could ding such a component, and at the least compromise the SWR of the transmission line. A really potent hit might perforate such component; and, short the high voltage power supply. Therefore, the towers would not be appropriate for slingshot practice. ;-)
