Last month, I reviewed a petition by Hammett and Edison for reconsiderationof the FCC's Second DTV Periodic Review Report and Order. The petition pointed out errors in predicted coverage and interference due to the FCC treatment of Longley-Rice error code 3 (KWX=3) cells and the default antenna elevation patterns that didn't reflect the large amounts of electrical and/or mechanical beam-tilt often used at high-elevation sites. I discussed this with other engineers at the IEEE Broadcast Technology Symposium in Washington, D.C., in October.
ADJUSTING FOR ELEVATION
Antenna design engineers I talked with thought it would be possible to come up with a formula to calculate a generic elevation pattern based on antenna elevation gain that could then be shifted to account for different values of electrical beam-tilt. Other discussions indicated a simpler approach might be easier to implement and still provide a significant improvement over the limited OET-69 Table 8 patterns. This approach would add elevation patterns to reflect the use of low-gain and medium-gain antenna patterns, and again offset them to reflect the use of different amounts of electrical beam-tilt.
Before anyone gets upset at the idea of adding elevation patterns to OET-69, I should note that the FCC did just that in its Digital LPTV Report and Order, which doubled the relative field values of the Table 8 patterns, truncating them at 1.0 for LPTV stations in OET-69 studies.
Because the LPTV antenna-relative field at all depression angles will be equal to or higher than those predicted using the original OET-69 Table 8 elevation patterns, predicted interference based on D/U ratios will be affected.
CHANNEL ELECTION
IEEE tried something new at its Broadcast Technical Symposium this year-a Wednesday evening session that featured a panel discussion on DTV repacking transition issues. It was led by Bill Meintel, who played a key role in developing the software used to determine DTV coverage and interference.
The panel included Victor Tawil, senior vice president of MSTV, consulting engineers Charles Cooper and Dennis Wallace, and Keith Larson from the FCC.
While many questions dealt with unusual conditions related to specificstations, some were of general interest. FCC DTV software was discussed-one person asked what variations the FCC would accept in grid size. Larson said up to 1 km, but to generally use the OET-69 parameters (2-km grid with 1-km terrain extraction). LPTV stations would have to make the adjustment to the Table 8 pattern I mentioned earlier.
When asked if new software was coming for DTV, Larson indicated a new implementation was needed, but the FCC had to modify the OET-69 first.
Protected coverage was a hot topic; of particular interest were stations that received a 1,000 kW UHF facility in the DTV Table of Allotments that was smaller than their VHF Grade B coverage area.
Tawil said MSTV fought to have the VHF Grade B coverage area protected, but the FCC said the authorized facility as constructed at UHF would determine the baseline coverage. Meintel said stations could apply for a larger facility later.
One problem for some VHF stations with nondirectional antennas is that they received a directional pattern for their UHF DTV allotment. Replication coverage will be based on this directional pattern. Tawil said MSTV has recommended the FCC give stations the option to go back to their NTSC pattern if they return to their VHF channel.
The strict 0.1 percent threshold forincreased in terference to other DTV stations was explained. Tawil said the 0.1 percent threshold applied only to the channel-election process. After that, the 2 percent new interference and 10 percent total interference thresholds will return. The reason new interference was restricted to 0.1 percent was to prevent aggregation of interference. Even if there are many new interference cases, the total increase in interference should remain under 2 percent.
There were also some questions about what the protected coverage would be if a station couldn't build out full replication facilities due to available antenna height or the use of a directional pattern.
The response from the panel was that once the new table was created, the existing DTV allocations would disappear and the new allocations would be based on what was constructed or authorized. This was clarified in the NAB/MSTV Digital Decisions Webcast Nov. 4. FCC staff said they would consider protecting replication coverage in some cases where stations could not construct full replication facilities due to, among other things, limits on tower height or tower structural limits.
This could help those VHF stations that A) are allotted a 1,000 kW UHF replication facility at the same height as their analog antennas; B) have DTV antennas mounted below analog antennas, and C) cannot increase power to offset the resultant reduction height.
IMPULSE NOISE & LOW VHF
Charles Einolf presented a paper he wrote with Victor Tawil on the "Impact of Impulse Noise on DTV Reception at Low VHF." This paper is on the IEEE 2004 Broadcast Technology Symposium CD.
While I haven't seen any details on how to purchase the proceedings, copies may be available. Visit www.ieee.org/bts for information and contact information.
Einolf researched noise levels at low-VHF frequencies and concluded that the median noise from man-made sources could add 20 to 30 dB of noise within the low-VHF channels. However, the planning factors the FCC used assume only an additional 5 dB for "environmental noise" on low-VHF channels. Several tests have been done over the past 10 years with DTV on low-VHF channels, and while the results differed depending on the location, it was clear noise would be a problem in many locations.
In the Cleveland tests, the receiver was found to have a major effect on reception, with one receiver having a failure rate of 17 percent while another had a failure rate of 56 percent. This difference did not seem to carry through in the other tests. COFDM did worse than 8-VSB in this test.
In Las Vegas, sites were selectedwhere high-line noise was expected-near high-tension lines or substations. Two types of receivers were used. Five of the 20 sites measured showed some noise impairment to NTSC Channel 3, but it did not affect reception of the DTV signal on either receiver. It was suggested these very positive results could be attributed to the moderately high signal levels on DTV Channel 2 and the age and proper maintenance of the power grid there.
The results in Chicago were not as good. Fourteen of 15 sites showed some impairment from noise. At five sites with medium to high levels of noise, DTV reception could not be achieved on either of the two receivers.
Einolf and Tawil concluded that current DTV receiver technology could provide low-VHF reception under light- or medium-impulse noise conditions. The paper states, "It is possible that impulse noise, in many locations, may render the low-VHF channels undesirable for digital television."
I'll have more information on the topics presented at the IEEE Broadcast Technology Symposium next month.
Your comments and questions on any RF topic are always welcome. Drop me an e-mail at dlung@transmitter.com. Your question may become the basis for my next RF Technology column!
