New York area stations are back at full power with new Dielectric combiner
The Empire State Building once again became a hub of New York television transmission activity with the recent installation of a six-channel Dielectric UHF combiner for DTV in CBS's space on the 85th floor. This, along with the 8-bay broadband UHF omnidirectional antenna that the network had already installed on the northwest corner of the mast and had been using, is allowing full-power DTV broadcasts for five other New York City stations for the first time since 9/11.
The six DTV stations are: WNBC-Channel 28, WPIX-Channel 33, WWOR (UPN)-Channel 38, WABC-Channel 45, WCBS-Channel 56, and public broadcaster WNET-Channel 61.
In addition to the UHF combiner, a new four-channel high VHF combiner, also from Dielectric, was installed at the same time in the same, albeit expanded, room for NTSC on channels 7, 9, 11, and 13.
Prior to 9/11, most of New York's TV transmissions emanated from the World Trace Center (WTC), and all but CBS used the WTC for DTV as well. After that disaster, eyes eventually turned to Empire once again.
'VERTICAL REAL ESTATE'
CBS spearheaded the project as it already had the "vertical real estate," said Kevin Coleman, director of transmission engineering at CBS and the manager of the project.
Before the 9/11 tragedy, CBS had replaced an old 5-bay NTSC antenna with a new 2-bay NTSC antenna, and in place of the remaining three bays, a new Harris (whose antenna group was subsequently purchased by Dielectric) 8-bay omnidirectional pattern antenna, with each bay consisting of four surrounding panels.
The antenna was designed to be broadband and to accompany other stations, but "before 9/11, we thought it would be just us for a long time," Coleman said, referring to the other stations' decision to go with the WTC. CBS had proved through extensive field measurements of its DTV transmission that even though Empire wasn't as tall as the WTC, it could still provide the excellent necessary coverage, according to Coleman.
Running the 6-1/8 inch diameter Dielectric DigiTLine transmission line was a project in itself, Coleman said, with each of the 48 elbows (total in the two lines) needing to be individually tuned for the best broadband performance.
After 9/11 CBS and the other stations negotiated with Empire for a new expanded installation. Once that was set, "from a technical perspective, all the stations were a pleasure to work with," Coleman said. "Once they came on board, we hammered out the technical specs and facilities issues."
The starting point was the design that Dielectric developed for a 6-channel combiner at WTC, which was installed in the summer of 2001.
"The important thing was, we did this once for World Trade, but the one for Empire has slightly different characteristics and a different physical configuration," said Jim Senberg, director of RF Systems business development for Dielectric, based in Raymond, Maine.
The combiner is large and quite architectural looking. The overall dimensions are 144 inches wide, by 204 inches long, and 131 inches tall, and customized to fit the room.
The DTV combiner weighs in at 7,000 pounds, and had to be divided and packed into 46 over-sized cases to fit into the Empire elevators, which were not designed for freight. "We loaded in the cases over two nights," Coleman said. "Much of the cases had to ride on top of the elevator."
The 85th floor was a good location to put both the UHF combiner along with the new L-shaped 5,000 pound high-V combiner, Coleman said, because the 84th floor just below them is a convenient place to run transmission line across the building to the mooring mast.
But to accommodate both combiners, plus primary and backup air conditioning units, CBS relocated WCAA-FM from the 85th floor to the 81st, and did demolition and reconstruction work to create a larger space.
And, "we had to go through the structural analysis, to make sure the floor could take the load of the combiners," Coleman said. "We placed the combiners across the beams." Sensitive circuits and systems are on the CBS generator and protected in case of a power outage.
The combiner consists of two rows of six towers, which are the filters for the individual channels. In each row the towers descend in size due to their frequency allocation. The towers are connected to another single tower, used for switching, positioned between the two rows, via copper transmission line.
Coleman continued with the description:
"The combiner has a tower of six transfer switches, centered in the middle of the combiner, that accepts the feed from each of the six broadcasters via 4 1/16-inch transmission line. The input transfer switch then will switch the inputs between either one of two inline filter towers that process the individual stations for combining using vertically positioned, waveguide filters.
Aluminum waveguide connects the output of the individual filters to the common output feed. The composite feed is then split in two to feed both the upper and lower bays of the antenna. The output waveguide is then transitioned to two feeds of 6 1/8-inch transmission line that feed the antenna. The antenna is fed in quadature.
"Spacing between filters is super critical as is the placement of the filters," Coleman added. "This combiner is here to stay until someone changes frequencies. It's designed specifically for the frequencies that are used."
The combiner is actually a dual system, consisting of two combiners, a primary and a backup for redundancy, and connected to a control system. "If anything goes wrong with the input stage, through computer control we can automatically switch from one combiner to the other," Coleman said. "We can also go from full antenna to half, or some stations on combiner A and some on combiner B. Three waveguide switches allow the composite feed, of the combiner, to switch between any one of six output configurations. We probably have more diversity than we need, but we had to have the ability to make changes if there were problems."
Each TV station on the combiner has a computer screen to monitor status of the system, but the overall management and maintenance of the combiner rests with CBS, which has a manager controller at CBS's master control in its New York Broadcast Center.
The combiner is a manifold design to handle the high power and wide bandwidth. Coleman said that with each station operating at full power, the total effective radiated power (ERP) is 1.3 MW.
"This is one of the widest bandwidth combiners that's been built," Senberg said. "With six channels, it starts at channel 28 and goes up to 61 for a total of 33 channels of bandwidth." Or 190 MHz, as Coleman said.
As for design challenges, "the combiner itself wasn't difficult, but the splitting and switching systems were technically challenging," Stenberg said. Also the combiner design called for a waveguide size that wasn't standard, so Dielectric had to develop that as well.
"They [Dielectric] learned a lot with the WTC combiner, and the new combiner far exceeded the preliminary specifications," Coleman said. "Channel to channel isolation at WTC was typically 50 dB, but for the new combiner we measured 70 dB or more and up to 80 dB of isolation." Other key specs that had excellent performance were through-loss and delay, Coleman said.
The combiner was installed in November 2004, with all but WNET on the air by the end of December.
The transmission line will be evacuated and pressurized with nitrogen to avoid " arc over" and to allow the transmission line to work up to its design (power) specifications. "The line will be operated at 75 percent of the manufacturer's maximum power specification," Coleman said.
Still to be done is removing the oxygen in the transmission line and replacing it with nitrogen to avoid "spark-overs."
Coleman credits key people with the success of the project: antenna consultant Dean Sargent, Empire State Building contact Chris Blackman, CBS engineer Alan Parnau for his work on the transmission line, and Hatzel and Buehler electricians, with lead person Bob Mittenzwei, and Robert Seidel and Robert Ross of CBS.
"I can't say enough good things about Dielectric," Coleman said. "The combiner works as great technically as it looks mechanically."
New York area stations are back at full power with new Dielectric combiner