RF systems
WCJB-TV’s new digital transmitter
WCJB-TV began operation in 1971 serving Gainesville, FL. Eleven years later, the station built a new transmission facility, installing a new Townsend 120kW klystron transmitter. Nearly 20 after that, the station began planning for the addition of a digital transmitter.
Because the station is owned by Diversified Communications, which owns multiple television stations throughout the eastern United States, it made sense to take advantage of the combined purchasing power of these stations. WCJB and the other Diversified stations determined that Axcera offered some of the most technologically advanced products and had a strong reputation of customer satisfaction. The station’s were able to choose from a wide selection of solid-state and IOT-based transmitter offerings. This became important, as WCJB’s DTV transmitter output power requirements changed several times throughout the project cycle.
With the primary goal of using the existing facility, it quickly became obvious that something would need to be done with the station’s aging Townsend analog transmitter to allow the building to accommodate both the analog and DTV transmitters. Being an externally diplexed design, the Townsend was actually made up of three power amplifier cabinets. WCJB determined that it would be necessary to completely replace the analog transmitter with a more space-efficient unit. The station chose an Axcera Visionary series dual-tube IOT transmitter.
In addition to requiring a much smaller footprint than the Townsend, the station saw other benefits to replacing the analog transmitter. The efficiency of the Visionary IOT transmitter was superior to that of the klystron-based unit. Additionally, after 20 years of operation, the Townsend was beginning to show its age. By installing the new analog transmitter, the station could improve both the efficiency and the reliability of its analog plant.
For DTV, the station’s initial allocation required a transmitter output power of only a couple thousand watts. WCJB felt that this power level was too low to replicate its current coverage area, so it applied for maximization, which would have required a 15kW transmitter. Unfortunately, this request was initially rejected by the FCC, so the station had to petition the FCC for approval.
The FCC finally approved the station’s application, however WCJB determined that it would be most cost effective to build out to its current power level of 344kW. At this power level, the station decided that it made sense to select a solid-state digital transmitter, and the station chose the Axcera Innovator DT solid-state unit.
Because the station had already decided to use the existing facility, the next challenge became to maintain the NTSC transmissions during the replacement of the NTSC transmitter while at the same time creating space for the DTV transmitter. To accomplish this, the Visionary power amplifier cabinets and RF system were installed in their permanent location, but the exciter/driver and NTSC monitoring racks were initially installed on wheels. When the Townsend transmitter was removed, the exciter and monitoring racks were rolled to their final location and the wheels were removed.
The site was now ready for the new Innovator solid-state digital transmitter. It was delivered, installed and plumbed into the RF system. The system was brought up to power on schedule and both transmitters are operating well.
Design TeamTechnology at Work WCJB: Axcera: Steve Ingram, CE Visionary HP80DAW Axcera: NTSC transmitter Dave Benco, dir. of sales Innovator DT-LDU2A-8 DTV transmitter Jeff Heldman, app. eng. Dielectric: Ed Ritz, sr. field service eng. Filters and combiners Walt Beaver, sr. field service eng. Transmission line and antenna MYAT filters and combiners
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