Published: TV Technology magazine Issue: 6 April 98 Allotment Changes Cause Confusion by Joe Fedele WASHINGTON Broadcasters under the impression that their DTV coverage contour is an exact copy of their existing NTSC field contour may be in for a big shock. The recently updated DTV Table of Allotments assigned every television station a channel whose field contour was supposed to replicate their current NTSC assignment. In this update the commission also expanded the core spectrum of channels to be used for DTV to include the low band VHF frequencies (channels 2 through 6). Thus, the DTV spectrum will range from channels 2 through 51. How- ever, even with the addition of these channels, the total broadcast band remains smaller than it originally was when it extended to Channel 69. In order to cram all the DTV channels into a smaller television band — and because the commission used F50,90 propagation curves rather than the F50,50 used for NTSC — the FCC had to “tweak” every station’s antenna pattern. A NEW RESULT The result is a new set of antenna parameters that effectively turns omni-directional contours into directional patterns and transforms directional patterns into variations of the same. Perhaps the distinction lies in semantics, but broadcasters should not confuse what the FCC calls “replication of service” with “duplication of service.” One FCC official said complications broadcasters will face from changes to antenna patterns were “an unexpected wrinkle in the process. You can’t imagine the problems [the changes] created,” the FCC official continued. “The phone has been ringing constantly and people are very upset. I wish [the commis- sion] hadn’t [made changes to antenna patterns].” A LOOK AT THE FACTS Figure 1 depicts the resulting DTV contour for station WUSA in Washington, D.C., after the commission changed its contour from an omni-directional pattern to a squashed circle. A 2 dB difference in signal strength is apparent from about 100 to 155 degrees on the contour. This covers a large portion of the Chesapeake Bay area. While 2 dB may seem rather insignificant in NTSC terms, it should be remembered that with digital transmissions, a variation of only 1 dB will mean the difference between a perfect picture and snow at the edge of the contour. In gen- eral, stations that currently utilize omni-directional antennas will not likely suffer from as many contour anomalies as broadcasters using directional patterns. Figure 2 shows how a station’s directional antenna pattern was altered from its original NTSC contour. Again, a variation of about 2 dB can be found in large areas of the contour. But, more importantly, the antenna pattern is differ- ent. This means the station can not use a single broadband antenna to transmit both channels simultaneously — unless the FCC alters the transmitter’s power to ensure that coverage does not extend beyond the NTSC contour. This has serious implications for many stations around the country who are thinking of multiplexing onto master an- tennas. In some instances stations may not be able, or even wish, to alter their DTV power levels for the convenience of multiplexing. In the process of altering these contours, the FCC has even violated some of its own rules regarding antenna patterns. Front-to-back antenna ratios exceeding the 10 and 15 dB limits of VHF and UHF patterns, respectively, have been found to exist in the new table. A paper published by Dr. Oded Bendov, vice president of engineering for Dielectric, noted that “the FCC’s assigned directional patterns cannot, in general, be matched by practical antennas.” The paper also cites several example of stations that will have to reduce their power by as much as 3.5 dB to comply with FCC rules if the stations want to utilize their existing NTSC antennas for DTV. A SERIOUS DILEMMA The technical and financial ramifications of these changes may create serious problems for stations who are design- ing or implementing their DTV plans. The FCC has published the DTV antenna contours and has made them available on its Web site at www.fcc.gov/bureaus/ engineering_technology/databases/mmb/dtv/. The information contained in the file named “dadb.zip” consists of 36 relative field points for every DTV antenna pattern allocated in the new table. Using a ruler and protractor it is fairly simple to plot the points to see what your station’s DTV contour resembles. But a more comprehensive study and comparison of your NTSC and DTV station requires either some very sophisti- cated software or the services of a technical consulting firm. A copy of Dr. Oded’s paper is available on the Internet at the Association of Federal Communications Consulting Engineers (AFCCE) Web site at: www.afcce.org/understa.htm.