Doug Lung /
11.10.2004 12:00 AM
Coverage Complicates Channel Elections
Over the last few months, I've discussed some items to consider when selecting a final DTV channel and building out final DTV facilities. In September, the FCC released the Report and Order in the Second Periodic Review of the DTV transition, clearly outlining the steps stations need to take to elect their final DTV channel and to protect authorized DTV coverage areas.

Stations with at least one in-core channel (2 through 51) will have to make their first channel election before the end of the year. (See RF Report, Sept. 14, 2004, for a summary of the Report and Order and the timeline.)

The FCC posted the initial Table of Station Assignment and Service Information in early October. In it, you will find the predicted DTV population and area within the DTV service contour.

For many stations, these numbers reflect coverage quite different from what more accurate studies reflecting real-world conditions would show.

This month, I'll examine the options stations have in the channel election process. I'll also describe a filing by Hammett and Edison that pointed out FCC coverage and interference calculation simplifications that, if uncorrected, will pose a major problem for a number of TV stations.

If you've studied the Second Periodic Review Report and Order, you should have some understanding of how complex this will be. Although stations have the option of allowing the FCC to assign them a final DTV channel, I imagine most stations will want to pick their own. If your station's current DTV coverage is maximized on an in-core channel, the safest approach is likely to be to leave things as they are.

If your current DTV channel is outof- core or on a low-VHF channel (2-6), deciding what channel to use could be very complicated. Returning to an incore analog channel may not be the best option. As I pointed out in my Aug. 4 column, a station operating at maximum power on its analog channel could find interference to existing DTV stations (which have preference under the FCC's plan) will limit future DTV power on that channel.

Knowing what channels nearby stations will choose would be helpful. I've considered setting up a Web site to summarize channel elections, but realized I wouldn't have the time to keep it up to date. By the time you read this, I hope MSTV, NAB or some other broadcast organization sets up a way for stations to coordinate channel choices.

Stations with at least one in-core channel do have the choice of returning their in-core channel(s) and waiting until a later round to select a DTV channel from one of the remaining channels that may be better than what they have now. By "better" I mean the channel receives less interference or, if UHF, is closer to the center of the band.

While this may seem attractive, remember that other stations may have the same idea, leading to conflicts over desirable channels. The FCC set guidelines for evaluating competing applications for the same channel based on MSTV recommendations. In general, stations that built higher-power DTV facilities early on their current DTV channel will be given preference over stations that didn't.

If a station is unable to find a channel, it can ask the FCC to specify a channel for its use at full replication.

The FCC will select a channel that minimizes new interference to all affected stations. For UHF stations, coverage is likely to be less than what could be obtained with a maximized facility.

REAL-WORLD CONDITIONS

As I've explained before, the FCC analysis often does not reflect realworld conditions. One problem is current DTV receivers, which perform worse than the ATSC planning factors require. This will diminish as newer receivers make it to market.

Of greater concern are the simplifications used in the FCC software that made it possible to do the large number of calculations required to assign every analog TV station a DTV channel and calculate the result of one station's modifications on surrounding stations using computer hardware available in the mid-1990s. These simplifications include the use of one DTV and one analog antenna-elevation pattern per TV band (low-VHF, high-VHF and UHF), limited consideration for mechanical beam-tilt, and incorrect depression-angle calculation.

For depression-angle calculation, the FCC source code uses height above ground (AGL) rather than height above mean sea level (AMSL).

While this may offset some of the errors caused by using a default electrical beam-tilt of 0.75 degree for mountaintop facilities employing much greater beam-tilt, the results won't be the same as if the correct antenna elevation pattern and correct depression angle were used.

There is also a lot of concern about the FCC's software, which assumes cells that return Longley-Rice Error Code 3 (EC3) are receiving coverage, not receiving interference and are not creating interference. Consulting engineering firm Hammett and Edison have repeatedly asked the FCC to change their treatment of EC3.

In early October, Hammett and Edison filed a Petition for Reconsideration of the FCC's Second Periodic Review of the DTV, listing several technical corrections that need to be made to the FCC rules and procedures for calculating DTV coverage and interference.

Hammett and Edison warned, "Failure to make these technical corrections will result in some final DTV assignments that will likely be fundamentally flawed, especially for DTV stations transmitting from mountaintop sites.... The laws of physics and radio-wave propagation do not respect engineering or software 'simplifications.'"

The petition "implores the commission to take these corrective steps now that it is at a new juncture, requiring recalculation of all NTSC baseline populations using 2000 census data, and then the calculation of allowable final-DTV-channel ERPs."

Hammett and Edison asks the FCC to modify OET-69 software to ignore EC3, to correctly calculate depression angles, give stations the opportunity to submit their actual elevation patterns and, if mechanical beam-tilt is used, their main beam-elevation patterns.

Once this information is available, OET-69 software and the CDBS need to be modified to include a station's main beam-azimuth pattern, actual elevation pattern, actual electrical beamtilt, actual mechanical beam-tilt and actual mechanical beam-tilt direction.

For an earlier filing, Hammett and Edison looked at the impact of OET-69's treatment of EC3 cells on DTV allocations in the contiguous United States. It found that on average, 18.2 percent of a DTV allotment's population fell in EC3 cells.

The filing notes, "It made little sense to have a 2-percent de minimis criteria for DTV stations and a 0.5-percent de minimis criteria for NTSC stations when the underlying prediction model had an average error of 18 percent. And it borders on silly to use a 0.1-percent de minimis criteria, as now proposed, while retaining a prediction model with an average uncertainty that is 182 times greater."

RECOMMENDATIONS

Hammett and Edison devotes three pages of its filing outlining the problems caused by miscalculation of depression angles and the use of generic elevation patterns. The conclusion?

"The regrettable failure to use actual elevation patterns, actual ebt [electrical beam-tilt] and, for stations employing both ebt and mbt [mechanical beamtilt] basing cell calculations on the main beam-azimuth pattern rather than the 'distorted' horizontal-plane azimuth pattern, means that a significant percentage of the commission's OET-69 studies were inaccurate, with little linkage to reality."

The petition asks, "Can the commission afford to have significant calculation errors in these final rounds of DTV calculations?" I included Figs. 3B and 3C from Hammett and Edison's filing to illustrate the severity of the problem. Note that these combinations of electrical and mechanical beam-tilt are not uncommon at high elevation sites like Mount Wilson.

(click thumbnail)

A copy of the filing should be available on Hammett and Edison's Web site, www.h-e.com The inaccuracies in the FCC's coverage software will have little if any impact on stations transmitting from

buildings or towers over relatively flat terrain and probably won't have a significant impact on maximized stations operating at the FCC's maximum allowed power (1,000 kW at UHF), except for interference in EC3 cells.

Impact on other stations could be significant. If your station transmits from a mountaintop and you are considering a different DTV channel, I'd recommend doing a Longley-Rice coverage study ignoring EC3, using the actual antenna elevation and main beam azimuth patterns and the correct depression angles to get a more realistic idea of what the actual interferencefree coverage on the channel looks like. If you can obtain actual elevation patterns for interfering stations, it would be a good idea to see how they impact the channel without the "freeparking" allowed on EC3 cells in OET 69 calculations.

If your NTSC replication pattern is based on a UHF facility using an antenna with mechanical beam-tilt, compare the FCC coverage with the coverage calculated using the antenna's actual azimuth pattern and the true elevation pattern, including the correct electrical and mechanical beam-tilt. If the numbers differ substantially, check with your FCC consulting engineer and FCC attorney for options on how to deal with this. Note that it may be possible to use a combination of electrical and mechanical beam-tilt on the DTV antenna to improve DTV realworld coverage, especially at higher signal strengths, with little or no extension of FCC calculated coverage.

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!


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