Doug Lung /
08.24.2007 12:00 AM
FCC Third Periodic Review Comments Seek Relief for VHF DTV Stations
Reply comments in the FCC’s Third Periodic Review of the rules and policies affecting the DTV transition are due August 30. Although RF Report will have a complete review of the comments next week, licensees of stations planning to return to their VHF channels may want to file reply comments before the deadline supporting other comments seeking relief for stations returning to their analog VHF channels.

While it might seem that it would be easy for a station to return to its analog VHF channel, using the same antenna with the power level adjusted to stay within the replication contour, that isn’t always the case, as the comments point out. VHF analog stations that received UHF DTV allotments sometimes broadcast different patterns than their VHF antennas, due to differences in propagation over terrain or the FCC’s maximum 1,000 kW UHF power level. Because the FCC used the horizontal plane radiation pattern in calculating contours, stations using mechanical beam tilt sometimes ended up with unusual patterns. Most Los Angeles stations, for example, have patterns showing more energy into the mountains to the north than into Los Angeles.

The major problem facing stations returning to an analog VHF channel is that their Allotment antenna pattern won’t match their current analog antenna’s pattern. The problem is exacerbated by the FCC’s proposed limit of 0.5 percent to other allotments, since 50 percent of all VHF allotments already cause more than 0.5 percent interference to other allotments, as shown in the comments filed by du Treil, Lundin and Rackley (DLR).

DLR recommends setting the allowable interference higher and asks the FCC to clarify whether negotiated interference agreements will be allowed. If stations can’t match the FCC’s Allotment Pattern and it isn’t possible to replicate the allotment pattern in a new VHF antenna, the only option stations returning to their analog VHF channel will have is to reduce power, affecting their ability to cover their market unless interference limits are relaxed or they are allowed to change to another channel post transition.

Byron St. Clair outlined the problems with the FCC’s treatment of antenna patterns in his comments. Dr. St Clair explains, “The “Post Transition DTV Table of Allotments” includes calculated antenna patterns, for stations that are going back on their analog channel. These new patterns are actually distortions of the stations’ analog patterns resulting from calculating the analog coverage with F50/50 propagation and backing into the digital patterns using F50/90 propagation. The resulting digital patterns are not patterns which could easily be obtained in practice.” He provides an example of this in his comments.

Dr. St. Clair also asks the FCC to correct a problem I’ve written about in TV Technology and that I highlighted in my RF Delusions presentation at NAB2006. He states, “The non-use of actual antenna vertical patterns is a failure to use the best engineering practice. This failure should not be allowed to continue even though the necessity to obtain the vertical patterns for existing stations that may be involved in a particular calculation is a short tern nuisance. However, it has significant and important long term benefits.” His recommendation is to modify the rules to add the provision:

Interference calculations performed to determine whether an application to modify an existing station or to obtain an authorization for a new station shall included the manufacturer-provided vertical pattern and electrical downtilt if not already included in the pattern. This pattern will be used in performing the outgoing interference calculation.

If a protected station does not have a vertical pattern on file the standard FCC vertical pattern shall be used in establishing the stations protected area and in the Longley-Rice cell signal strength calculations. However, if necessary to establish non-interference an applicant may request that the actual vertical pattern of a protected station be provided by the potentially affected licensee, permittee or applicant. If this protected party fails to provide the actual vertical pattern the applicant may request that the manufacturer’s standard vertical pattern for the specified antenna be substituted for the FCC vertical pattern.

Comments from the Metropolitan Television Alliance. (MTVA), representing broadcasters in New York City, highlight the problems stations in New York City face returning to VHF channels. Because of the height of the Freedom Tower, where MTVA members are planning to locate their post-transition DTV antennas, the maximum ERP on high VHF channels will be limited to about 3.5 kW. While this may be sufficient to allow reception on an outdoor antenna at 30 feet in rural areas, without the use of additional beam tilt and higher effective radiated power (ERP) closer to the transmitter site, signal levels in the urban areas are likely to be too low to allow reliable reception, especially with indoor antennas and interference from computers and other electrical noise.

MTVA proposes allowing the use of beam tilt to put higher ERP below the radio horizon. Under previous FCC rules, the use of extra beam tilt was allowed on UHF channels but the maximum ERP had to be reduced by 1 dB. MTVA asks that this provision be extended to include high VHF without the 1 dB penalty.

With close to 300 stations ending up on VHF channels post-transition, the requested changes could have a significant impact on their ability to continue to serve their viewers. If you agree with the proposals, reply comments are due Aug. 30.


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