WCAX’s 24/7-manned transmission facility powers northeastern U.S. market

The transmitter facility for WCAX-TV, the CBS affiliate for Vermont’s Burlington/Plattsburgh market, is one of only a handful of 24/7-manned facilities in the United States. The WCAX-owned facility is also unusual because it houses digital transmission systems for three other local affiliates: NBC, ABC and FOX.
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WCAX’s 24/7-manned transmission facility powers northeastern U.S. market

The transmitter facility for WCAX-TV, the CBS affiliate for Vermont’s Burlington/Plattsburgh market, is one of only a handful of 24/7-manned facilities in the United States. The WCAX-owned facility is also unusual because it houses digital transmission systems for three other local affiliates: NBC, ABC and FOX.

Located on Mt. Mansfield in the resort town of Stowe, VT, the existing WCAX analog transmission building was torn down, with the new facility planned around the concrete shell. Three distinct areas comprise the facility: one for digital main and backup transmission systems, one for older analog transmission systems and an office area that provides a quiet place for engineers to communicate with the studios and monitor the health of the transmission systems.

The Harris Power­CD UHF tube transmitter, in a dual-cabinet configuration, features a dual-exciter design with Harris Apex digital exciters for maximum redundancy. Harris custom designed a dual-cooling system to separate the cooling of the transmitter’s power amplifier chains into independent heat exchange systems.

For its analog channel, the station uses a Harris Platinum Series VHF transmitter in a dual-cabinet configuration. Installed in the older area of the facility, it will be removed after the analog shutoff, creating space for two FM stations.

Harris hung the filtering and RF systems ahead of the transmitter installation. WCAX built a combiner system for the Power­CD and the Harris Sigma UHF transmitter for the NBC affiliate. The two stations, with different directional patterns, share a dual-antenna system fed by 6-1/8in and 3-1/8in transmission lines.

The PowerCD transmitter provides several key benefits. Its multicollector IOT tube design helps to reduce the facility’s enormous electrical costs, allowing the ROI to be achieved in four to five years. It also reduces the noise level within the facility through special features. Most UHF transmitters feature a thyratron crowbar circuit. This is an overload circuit that dumps all the high voltage energy to the ground when tripped — and makes an enormous bang similar to a sledgehammer on an anvil. The PowerCD’s multicollector tube design eliminates the thyratron crowbar circuit, allowing the transmitter to protect itself in a more graceful manner.

The Harris eCDi control and monitoring system integrated into the PowerCD transmitter adds the final touch. WCAX engineers use eCDi to navigate the screens and parameters from a quiet desk in the separate office area. It also provides additional fault logging and parameters for deeper system monitoring than is possible from the front-panel GUI.

The facility is in its final stages of preparation for analog sunset. The team is moving the microwave systems from the analog to the digital side, and come early 2009, all but a few hints of the facility’s analog past will have disappeared.