In December the FCC released maps comparing stations' analog coverage with their DTV coverage. They also offered some tools for filling in any holes in DTV coverage. These include distributed transmission systems (rules adopted in November) and a proposed rulemaking for replacement digital low-power TV translator stations, released on Dec. 23, the same day as the maps. This month, I'll discuss how to read the maps and then look at some of the options for filling in any holes in DTV coverage.
READING THE MAPS
The maps show the analog Grade B contour and the DTV service area contour. You can find the maps at www.fcc.gov/dtv/markets. The DTV studies are based on approved DTV construction permits for post-transition operation or existing licenses, if they will be used post-transition. The maps use symbols to show cells of population that were not predicted to receive the analog signal but are predicted to receive the DTV signal (green dots), cells where the DTV signal is lost but the same network is available from another DTV station (orange diamonds), and cells predicted to receive the analog signal, but no DTV signal and no other service by the same network (red triangles). Note that many stations have applications to expand DTV coverage still pending at the FCC and the impact of these applications is not shown.
The maps are based on the horizontal plane antenna azimuth pattern in the Consolidated Database System (CDBS) and the default elevation pattern. This means the actual coverage of stations using mechanical beam tilt from high elevation antenna sites like Mount Wilson, north of Los Angeles, is likely to be different from what is shown in these maps. The maps ignore Longley-Rice Error Code 3. The OET-69 treatment of Error Code 3 considers cells as having sufficient signal and receiving no interference, even though the actual signal level may be too low or the interference too high to allow reception. Hammett and Edison have demonstrated in FCC filings that ignoring Error Code 3 improves accuracy. As with OET-69, no coverage is assumed beyond service area contours.
Fig. 1: The contour covers a significant portion of WECT analog viewers that lost DTV reception.
Low-VHF analog TV stations with UHF DTV channels experienced the biggest loss in DTV coverage. Even with 1,000 kW of effective radiated power, a UHF DTV service area contour usually will not match the low-VHF analog Grade B contour. This leads to a ring of lost coverage at the fringe of the station's contour. In most cases, coverage is available from another DTV station. The FCC analysis showed 58.3 percent of the viewers predicted to lose reception from one or more network affiliated stations can expect to receive service from other stations affiliated with the same network. UHF analog stations with UHF DTV channels, for the most part, have a ring of new coverage outside their analog Grade B contour. The green and orange/red rings on the nationwide maps of coverage for ABC, CBS, Fox and NBC highlight these differences.
If you look closer, you will see some stations have a loss of coverage inside their DTV service contour. One example is KCBS-TV, channel 2 analog; channel 43 DTV post-transition. The reason is terrain obstructions cause much greater loss at UHF than at low VHF frequencies. I discussed this in my March 10, 2004 column "DTV on Low-VHF Channels." The maps don't show whether terrain or interference is responsible for loss of reception inside the contour. One way to tell is to compare coverage for stations transmitting from the same antenna farm in the same TV band. If terrain is the problem, all stations should be affected. If only one or two stations are affected, interference is likely the problem. Compare the KTVT channel 11 DTV map with the maps for VHF DTV stations KFWD and WFAA-TV in Dallas. The loss area to the east and northeast appears to be due to interference.
It would be difficult, if not impossible, to fill in the entire ring of lost coverage just outside the DTV contour. There is simply too much area to cover. A large number of low-power transmitters would be required, each requiring a site, a program feed and a channel that doesn't cause interference. The only practical choice is picking the communities in these areas that are most important and focusing on them. There are more options for filling in areas inside the contour where terrain shielding or interference prevents reception. Stations losing coverage due to a change in transmitter location may have another option—install a second transmitter at the analog site.
If a channel is available, a replacement digital LPTV translator will be the simplest, easiest way to provide coverage to areas without DTV reception. No modifications to the main transmitter are needed, and if the right translator site is picked it may be possible to feed it with an off-air signal instead of microwave or fiber. Self-interference isn't an issue. When considering channels, your station's analog channel, which has been protected from pre-transition interference from other stations, may be the best choice. The main problem with using a translator is limited effective radiated power—only 300 watts at VHF and 15 kW at UHF.
A distributed transmission system (DTS), which can have transmitters operating at the maximum power allowed for full-power TV stations, is a better choice if the loss area is large and terrain shielded. System design is much more complicated because if signals from multiple transmitters arrive at a receiver at levels and timing the receiver's equalizer can't handle, no reception will be possible due to the self-interference.
On-channel repeaters or boosters should work in areas that are completely terrain shielded. They receive the off-air signal and re-transmit into a valley or shadowed area on the same channel. Isolation between the receive and transmit antennas has been an issue, although new echo-cancellation systems should make it easier and allow higher powers. I'm not aware of any on-channel boosters or repeaters in operation in the United States using echo-cancellation.
When comparing DTS and translators, consider the antenna the viewer is likely to be using. A UHF DTV translator at 300 feet height above average terrain (HAAT) operating at the maximum ERP 15 kW would have an FCC (50, 90) 41 dBµV/m contour extending just beyond 32 miles. At high-VHF, 300 watts ERP at the same height covers just less than 30 miles and at low-VHF, the 28 dBµV/m contour extends 34 miles. This may be sufficient coverage for viewers with high gain outdoor antennas and preamplifiers. If the goal is to reach viewers with indoor antennas, the "typical case" described by Bill Meintel's paper at IEEE BTS requires 75.5 dBµV/m at UHF and 77.3 dBµV/m at VHF. This reduces the coverage of our maximum power translator to less than 9 miles at UHF, less than 3.5 miles at high-VHF, and less than 3 miles at low-VHF!
If the interference issues to the primary site and other broadcasters can be resolved, a DTS offers the ability to transmit sufficient power to reach indoor antennas over a much wider area. Because it doesn't require a second channel, DTS or on-channel boosters (for smaller areas) will work better for mobile DTV.
In the Wilmington, N.C. early DTV transition, some viewers that had been receiving the WECT channel 6 analog signal were not able to receive the WECT channel 44 DTV transmitter located closer to Wilmington. What would happen if WECT used its existing channel 6 antenna at the analog site as a replacement digital LPTV translator? As you can see in Fig. 1, the 28-dBµV/m contour covers a significant portion of the WECT analog viewers that lost DTV reception. WTVJ in Miami is another station transmitting analog on channel 6 with a UHF DTV transmitter at a different location that would benefit from converting the analog site to a replacement digital LPTV translator. While it misses the loss area in Collier County, it covers much of the loss area in the Keys and it shouldn't be difficult to use a translator to fill in the rest of the area south to Marathon.
The proposed rules for replacement digital LPTV translators require stations to minimize coverage outside the loss area. Exceptions are allowed. Allowing the use of an existing analog antenna with a translator provides a way for low-VHF analog stations to restore coverage to at least part of the area lost due to transmitter site moves. Stations are unlikely to ask for this unless they feel the benefit to their viewers offsets the cost of tower space and maintaining a separate site. Viewers will need to make an effort to receive these weak low-VHF DTV signals, but they may have the antenna in place for that now.
I'm interested in your comments and experiences, especially any relating to weak signal low-VHF DTV reception. E-mail me firstname.lastname@example.org.
Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.
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