06.28.2010 10:20 AM
Linear Industries Helps Take OCT Digital
The Linear Industries AT7120 ATSC exciter can operate both as transmitter and/or translator.

OKLAHOMA CITY
Oklahoma Community Television (OCT) operates 38 television translators on six tower locations in Southwestern Oklahoma. We carry many of the broadcasters' signals from the Oklahoma City DMA and at one of our locations, from the Wichita Falls, Texas DMA.

Our typical analog translator system consists of 100 Watt transmitters, channel combiners, and transmitting antennas providing an ERP of about 1 kW. There can be as many as three transmit antennas and lines per location. Most of the sites are "daisy chained," or linked together using a high-gain receiving antenna.

CONVERSION DILEMMA

I had many concerns when it came time to convert to digital, but three items stuck out in particular. I wondered if our towers could accommodate the antennas needed to transmit both digital and analog. I needed to identify the equipment that we'd require, and I also wondered if OCT would be able to fund this equipment.

My first concern was eased after I'd researched many different antennas and manufacturers and selected a wideband omni pattern model. While I didn't feel comfortable that some of our tower sites could support this antenna, it did indicate that we could have the capability to transmit multiple channels via a single antenna.

The next issue was settled when the NTIA announced their low-power TV and translator digital upgrade program for reimbursing costs associated with going digital.

I still wasn't pleased that the antenna selected was best for the job and kept looking, finally discovering a smaller model. Its profile was what I'd been seeking, but the gain was 3 dB less , meaning I'd need to make up the power for my desired ERP with a larger transmitter.

I found the final piece to my puzzle at Linear Industries. The company offers a 120 Watt transmitter/translator (the AT7120) that fit my budget. With its 120 Watts of output and an antenna gain of 8 dB, I could come close to my desired 50 percent ratio of digital to analog power. I re-calculated the transmitter output, combiner and line loss, and antenna gain and submitted those numbers to the FCC for a minor CP change. At this point I had a system, a budget, and a deadline.

The AT7120 accepts an ASI signal from an off-air receiver, and includes a built-in ATSC modulator. The transmitter is supplied already set up on channel with linear and nonlinear corrections provided. The user interface is simple and easy to use, providing power supply voltages and currents, output RF power, level, as well as reflected power levels. It includes a port for remote monitoring and provides the ability to remotely turn off the unit. There's also a feature that mutes on loss of an input signal.

Linear Industries and I have currently built three "daisy chained" sites, using the antenna I chose, five to seven channel combiners, off-air receivers, and Linear transmitters. Each site has met or exceeded my expectations.

Jack Mills is general manager of Oklahoma Community Television LLC. He may be contacted at jack.mills@okcommtv.net.

For additional information, contact Linear Industries at 877-428-5793 or visit www.linear-tv.com.



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