Multi-hop DTV translators at work in Utah

In many parts of the country, the transmitter is not the last link before home reception; a translator (or multiple translators) extends its reach. Without television translators, part of the American viewing audience, in particular that portion in the Rocky Mountains and west, would be without free, over-the-air television.

Jim DeChant, general manager of KTVZ in Bend, OR, relates the situation that faces many broadcasters, “I cannot cover my service area with our main (transmitter) only. We'd be hard pressed if we didn't have the eight translators.” Oregon has over 500 NTSC translators extending the range of the stations in Portland, Eugene, Medford and other larger cities.

As we make the migration to digital television, this type of service becomes increasingly more important, especially the legacy issues. Full-power stations have been granted a second frequency to aid them in their transition to digital; but not so for translators. Dr. Byron St. Clair, president of the National Translator Association said, “We are extremely concerned about the present movement in Congress and the FCC to recover channels 52-59 before the transition to digital is complete. If these channels become unavailable for translator use, it will be virtually impossible to extend a significant number of DTV primary stations into rural areas traditionally served by translators.”

There have been tests and experiments to see how the currently accepted technology (8VSB) will perform in the rigorous geography of the West. Paul Burkholder, communications director of Humboldt County in Winnemucca, NV, started tests as far back as November 1999.

“We tested ATSC signals with low-power transmitters and translators for a period of six months,” Burkholder said. “I even wrote a 100-page book on it. Twelve unique receive sites were used where we compared 100W NTSC signals with 10- to 60W digital 8VSB signals. During the initial test, we didn't do any hops [translator-to-translator], we simply wanted to see if the digital signals could replicate NTSC coverage in the typical ‘west-of-the-Great-Plains’ terrain. You know, mountains and valleys.”

The tests uncovered issues that Burkholder says the translator companies are currently addressing. “Phase noise in standard heterodyne converters proved to be a big problem,” he said, adding, “The type of converters most translator operators are familiar with probably wouldn't pass a digital signal.”

The tests didn't stop with just translators and LPTV. “We did some work on microwave delivery to translators,” Burkholder continued. “We found that it wasn't necessary to convert from 8VSB to one of the other modulation schemes such as QAM and that we could get several 8VSB signals into the 25MHz-wide microwave channel. We found that the standard IF frequency of 70MHz worked just fine using off-the-shelf downconverters and downconverting to standard TV broadcast channels 3, 4 and 5. We'd inject those into the IF and let the microwave equipment do the upconverting, recovering those channels at the other end of the microwave system. Instead of being able to carry only one NTSC channel on the microwave system, we found that we could carry up to three ATSC signals on the same carrier.”

“Although we did do some translator-to-translator testing, we turned that part of our testing over to Kent Parsons,” Burkholder concluded. Kent Parsons is well known to Utah broadcasters. He has been involved in the Utah state translator system for over 45 years. In addition to this, he is the vice-president of the National Translator Association. Because Utah has about 10 percent of the total translators licensed in the United States, and with Parson's experience, it was a natural match, not to mention that about 500,000 homes, or about one-quarter of viewers in Utah depend upon translators for TV service.

“After four months of experimenting, monitoring and testing with two long-range repeaters, the conclusion reached is that DTV translators we worked with will and do deliver quality television signals to rural viewers with high reliability and at reasonable costs,” Parsons proudly stated. “The signals I'm receiving in my home after the double hop are the same quality as those being transmitted from the originating television station in Salt Lake City.”

The tests were done with the cooperation of KSL-DT (Channel 38), an NBC affiliate owned by Bonneville International Corp.

The first leg in the translator double hop is located atop Levan Peak, some 83 miles from Salt Lake City. Parsons injected an interesting factor: “One of the anomalies is that this is not line of sight. In the path, not five miles from Farnsworth Peak where KSL-DT's transmitter is located, there is a protrusion that sticks up about 180 feet above the line of sight. Other than that, the path is clear.

“At the first receive site (Levan Peak), we are getting a signal strong enough that we have 29db to spare without using a preamplifier. The received signal goes into a Zenith ATSC (8VSB “remodulator”) cable translator that converts the signal to a standard 45MHz IF frequency. We then upconvert using a General Instruments frequency-agile upconverter. This feeds, on frequency, a Larcan MX100 which produces 30W of digital-television carrier power on channel 17 that feeds a PR450 directional Paraflector antenna made by the Scala Division of Kathrein. Our experimental call sign for this location is K17FJ,” Parsons reported.

Parsons' goal was to to get a digital signal into his home in Monroe, UT, where he could track error rates etc. But for that to happen it was necessary to translate the signal one more time, which was done at the Cove Mountain translator site, about four miles east of Monroe. “Cove has a direct, unobstructed shot from Levan Peak,” Parsons said.

“In one test, we found that, with very limited power, we can transmit very long distances. For example, we reduced the power and transmitted the signal 67 miles with only 10mW of power on channel 17 and got good pictures. Because of the cliff effect, you either have a signal with digital or you don't.” Parsons also pointed out that they are receiving channel 17 from a site that is also transmitting a powerful channel 16 analog signal without any appreciable adjacent channel interference.

Those who have seen the results of this experiment are no longer skeptical that DTV can be implemented with translators. The bottom line in all this is that DTV translators work, can use modest power, will eventually cost about the same as analog translators, and produce quality service for those hundreds of thousands of rural TV viewers and the many cable systems that rely on translators.