Lightning struck a television transmission tower atop Farnsworth Peak about 17mi southwest of downtown Salt Lake City Sept. 13 taking eight full-power DTV stations off the air.
The incident, which occurred at about 9 p.m., knocked four of the stations off the air for about 90 minutes. DTV transmission resumed for the remaining four stations at about 1:30 a.m. on Sept. 14, said KSL chief engineer Brent Robinson.
The stations included: KSL, the Bonneville International-owned NBC affiliate in Salt Lake City; Four Points Media Group-owned CBS affiliate KUTV in Salt Lake City; Newport Television-owned ABC affiliate KTVX in Salt Lake City; PBS member station KUED in Salt Lake City; Utah State Board of Regents station KUEN in Ogden, UT; PBS member station KBYU in Provo, UT; Larry H. Miller-owned KJZZ in Salt Lake City; and High Plains Broadcasting-owned CW Television affiliate KUCW in Ogden, UT. DTV Utah, an alliance of the stations, operates a shared RF infrastructure on Farnsworth Peak, which includes two combiners — one each for four channels — and two antennas in the main structure — one of which sustained the lightning strike — as well as a backup antenna.
The lightning strike of the tower caused a failure of the waveguide switch control system, said Robinson. “The strike put one of the combiner chains into an illegal condition, which caused the output (of one of the combiners) to be fed back into the input (of the other combiner),” he said.
According to Robinson, the damage from lightning included disrupting the waveguide switch control panel, which displayed invalid information following the strike. With the assistance of an engineer familiar with the details of the combining system via phone, the on-site RF engineer was able to determine what happened, take corrective steps and begin turning on KSL, KUTV, KUED and KJZZ –the stations not directly involved in the incident, he said.
“The other remaining stations (KTVX, KUEN, KBYU and KUCW) were on the combiner that blew,” said Robinson. “Oil was leaking all over the place. We were worried about the condition of the antenna, combiner and waveguide.”
By early the next morning, engineers were satisfied that everything was in working order and it was safe to resume transmission of the remaining stations. According to Robinson, who was interviewed via phone Sept. 16, everything at the Farnsworth Peak site is working properly at full power and the situation continues to be monitored.
According to Robinson, the incident illustrates the fact that nothing is foolproof. Currently no decision has been made about what to do with the control system that failed. Replacement is an option, said Robinson. “Or, we could just leave it in the manual position with templates to guide engineers on making configuration changes when necessary,” he said.