Dielectric has completed the design of a technically complex, stacked antenna array for San Francisco DTV broadcasters KGO-TV and KRON-TV. The stacked array is slated for installation later this year atop Sutro Tower in downtown San Francisco.
KGO-TV, the market’s ABC owned-and-operated affiliate currently broadcasting on VHF channel 7 will be repacked to channel 12. KRON-TV, the Nexstar Media Group-owned MyNetworkTV affiliate now broadcasting on UHF Channel 38 (Virtual Channel 4) will be repacked to channel 7, once vacated by KGO.
Dielectric THV Series directional pylon antennas have been customized to each station’s unique needs. The stacking of KGO’s antenna on top of KRON’s required a careful design process to factor in earthquake compliance and other critical, environmental considerations.
“This is one of our most complex repack antenna designs, in large part due to the strict earthquake code that applies to antennas installed in the San Francisco Bay Area,” said Keith Pelletier, vice president and general manager, Dielectric. “Our mechanical engineering team also ensured that KRON’s antenna could support the weight of KGO’s antenna. These design considerations called for a grade of steel that is much thicker and stronger than that which is customary for antenna construction. By design, the stack will be capable of handling the extra weight, and compliant with all earthquake restrictions.”
According to Pelletier, the final designs are currently being reviewed by a third-party structural engineering firm in San Francisco to ensure that they meet the earthquake code. The antenna array will then be constructed during the second quarter of 2019.
“Sutro Tower is scheduled for Phase 9, which means that the installation must be completed by May 2020,” said Pelletier. “We will be ready to ship in June of this year, and install the system by the end of 2019. This will ensure that the new stacked array is up and running before the strict repack deadline.”
Challenging Installation Planned
The stacked antenna array will be affixed to one of the three “candelabra” arms extending from the top of 977-foot tall Sutro Tower. In order to meet the earthquake code, the stacked array is being reinforced with guy wires which are anchored at the top of the candelabra platforms. Dielectric specializes in tower top and tower stacks but the guy wire requirement is unique to Sutro Tower. The installation grows trickier given the densely-packed downtown residential neighborhood and busy streets surrounding the tower – a job that only a handful of specialized tall tower crews can accommodate.
Each station chose to customize top-mount (slotted coaxial) designs to avoid possible interference between the antennas and the tower that could affect radiated patterns and power levels. Pelletier notes that the design offers the industry’s lowest windload – yet another critical consideration for this project.
Dielectric will also supply its auxiliary antenna (TLSV series) antenna, as part of an effective strategy to ensure a clean repack process with no off-air time.
“Since KGO is already operating in the VHF band, we will install an auxiliary antenna lower down on the tower that allows them to operate on channel 7,” Pelletier said. “That frees up the tower top and allows us to install KRON and KGO new antennas on top of the tower and allows KGO to remain on the air. Once that’s completed, KGO can move to its permanent antenna on top and Sutro Tower will remain outfitted with an auxiliary VHF antenna.”